Samacheer Kalvi Class 9 Science Solutions Chapter 4 Electric Charge and Electric Current

I. Choose the correct answer:

 

Question 1. In current electricity, a positive charge refers to,
(a) presence of electron
(c) absence of electron
(d) absence of proton
Answer: (c) absence of electron
In simple words: A positive charge in current electricity means that there are fewer electrons than protons. Electrons carry negative charge, so their absence results in a positive charge.

🎯 Exam Tip: Remember that electric charge is fundamental, and in many contexts, positive charge is considered the direction of current flow, even if electrons are the ones moving.

 

Question 2. Rubbing of comb with hair
(a) creates electric charge
(b) transfers electric charge
(c) either (a) or (b)
(d) neither (a) nor (b)
Answer: (b) transfers electric charge
In simple words: When you rub a comb on your hair, no new charge is made. Instead, electrons move from one material to the other, so the charge is simply transferred. This is an example of charging by friction.

🎯 Exam Tip: Understand that charges are conserved; they are never created or destroyed, only transferred from one object to another.

 

Question 3. Electric field lines from positive charge and in negative charge.
(a) start; start
(b) start; end
(c) start: end
(d) end; end
Answer: (b) or (c) start, end
In simple words: Electric field lines always begin at positive charges and finish at negative charges. They show the path a positive test charge would take.

🎯 Exam Tip: Visualize electric field lines as arrows pointing from positive to negative, indicating the direction of the electric force on a positive test charge.

 

Question 4. Potential near a charge is the measure of its to bring a positive charge at that point.
(b) ability
(c) tendency
(d) work
Answer: (d) work
In simple words: Electric potential at a point is how much work is needed to move a small positive charge from far away to that specific point. It represents the energy per unit charge.

🎯 Exam Tip: Remember that potential is about the "work done" or "energy per unit charge" in moving a charge against the electric field.

 

Question 5. Heating effect of current is called,
(a) Joule heating
(b) Coulomb heating
(c) Voltage heating
(d) Ampere heating
Answer: (a) Joule heating
In simple words: When electric current flows through a wire, it causes the wire to heat up. This effect is known as Joule heating, named after James Prescott Joule, who studied it.

🎯 Exam Tip: Joule heating is also known as the "Joule effect" or "resistive heating," where electrical energy is converted into heat energy due to resistance.

 

Question 6. In an electrolyte, the current is due to the flow of
(a) electrons
(b) positive ions
(c) both (a) and (b)
(d) neither (a) nor (b)
Answer: (c) both (a) and (b)
In simple words: In liquids that conduct electricity, called electrolytes, the current is carried by both positively charged ions and negatively charged electrons moving around. This is different from solid conductors where only electrons move.

🎯 Exam Tip: Distinguish between metallic conductors (where only electrons move) and electrolytic conductors (where both positive and negative ions contribute to current flow).

 

Question 7. Electroplating is an example of
(c) flowing effects
(d) magnetic effect
(b) chemical effect
Answer: (b) chemical effect
In simple words: Electroplating uses electricity to cause a chemical change, where a thin layer of one metal is deposited onto another. This is a common application of the chemical effects of electric current.

🎯 Exam Tip: Electroplating is a clear example of the chemical effect of current, along with electrolysis, where electric current drives non-spontaneous chemical reactions.

 

Question 8. Resistance of a wire depends on,
(a) temperature
(e) nature of material
(b) geometry
(d) all the above
Answer: (d) all the above
In simple words: The resistance of a wire is affected by its temperature, the type of material it's made from, and its physical shape (like length and thickness). All these factors play a role.

🎯 Exam Tip: Remember the formula for resistance \(R = \rho \frac{L}{A}\), where \(\rho\) (resistivity) depends on material and temperature, L is length (geometry), and A is cross-sectional area (geometry).

II. Match the following

 

Answer:
1. Electric Charge - (c) coulomb
2. Potential difference - (e) volt
3. Electric field - (d) newton per coulomb
4. Resistance - (a) ohm
5. Electric current - (b) ampere
In simple words: We are matching each electrical concept with its correct unit. Charge is measured in Coulombs, potential difference in Volts, electric field strength in Newtons per Coulomb, resistance in Ohms, and current in Amperes.

🎯 Exam Tip: Knowing the SI units for fundamental electrical quantities is crucial for both theoretical understanding and solving problems.

III. State whether true or false. If false, correct the statement:

 

Question 1. Electrically neutral means it is either zero or equal positive and negative charges.
Answer: True.
In simple words: When something is electrically neutral, it means it has no net electric charge, either because there are no charges, or the positive and negative charges perfectly balance each other out.

🎯 Exam Tip: A neutral object can still contain many charges, as long as the total positive charge equals the total negative charge.

 

Question 2. Ammeter is connected in parallel in an electric circuit.
Answer: False.
Correct statement: Ammeter is connected in series in an electric circuit.
In simple words: An ammeter measures the current flowing through a circuit, so it must be placed directly in the path of the current (in series) to get an accurate reading. Connecting it in parallel would bypass the component and give a wrong reading.

🎯 Exam Tip: Always connect an ammeter in series with the component through which you want to measure the current, and ensure it has very low resistance itself.

 

Question 3. The anode in the electrolyte is negative.
Answer: False.
Correct statement: The anode in the electrolyte is positive.
In simple words: In an electrolytic cell, the anode is where oxidation happens, and it is always the positively charged electrode. Positive ions (cations) move towards the cathode (negative electrode), and negative ions (anions) move towards the anode.

🎯 Exam Tip: Remember that "anode" and "cathode" refer to the direction of ion movement; the anode is where anions move and oxidation occurs, making it positive in an electrolytic cell.

 

Question 4. Current can produce the magnetic field.
Answer: True.
In simple words: Any time electric current flows through a wire, it creates a magnetic field around that wire. This is a basic principle of electromagnetism and is used in many devices.

🎯 Exam Tip: This phenomenon, known as Oersted's discovery, forms the basis of electromagnets, motors, and generators.

IV. Fill in the blanks :

 

Question 1. Electrons move from potential to potential.
Answer: lower, higher
In simple words: Electrons, which are negatively charged, naturally move from areas of lower electric potential (more negative) to areas of higher electric potential (more positive). This is like a ball rolling downhill.

🎯 Exam Tip: Conventional current is defined as the flow of positive charge from higher to lower potential, which is opposite to the actual electron flow.

 

Question 2. The direction opposite to the movement of electron is called current.
Answer: conventional
In simple words: Even though electrons move from negative to positive, we usually talk about current flowing from positive to negative. This older way of thinking is called "conventional current."

🎯 Exam Tip: It is important to distinguish between electron flow (actual movement of negative charges) and conventional current (hypothetical flow of positive charges).

 

Question 3. The e.m.f of a cell is analogous to a pipeline.
Answer: water pump
In simple words: The electromotive force (e.m.f.) of a cell is like a water pump in a pipeline. Just as a pump pushes water, the e.m.f. pushes electric charge around a circuit.

🎯 Exam Tip: An analogy helps understand difficult concepts; here, the e.m.f is the "push" that drives current, similar to pressure in a fluid system.

 

Question 4. The domestic electricity in India is an ac with a frequency of Hz.
Answer: 50
In simple words: In India, the electricity used at home is alternating current (AC), and it changes direction 50 times every second. This frequency is measured in Hertz.

🎯 Exam Tip: Different countries use different standard frequencies for AC power (e.g., 50 Hz in India and Europe, 60 Hz in North America).

V. Conceptual Questions:

 

Question 1. A bird sitting on a high power electric line is still safe. How?
Answer: Birds do not get shocked when they sit on electrical wires because they are not good conductors of electricity. Electricity flowing through a single power line at 35,000 volts will continue along the path of least resistance and bypass birds because there is no difference in electrical potential. Birds act as insulators. If a bird were to touch two wires simultaneously, creating a potential difference across its body, it would complete a circuit and get shocked.
In simple words: A bird is safe on a single power line because no electricity flows through its body. This is because both of its feet are at the same high voltage, so there's no voltage difference to push current through it.

🎯 Exam Tip: The key concept here is "potential difference." Current flows only when there is a difference in electric potential across two points.

 

Question 2. Does a solar cell always maintain the potential across its terminals constant? Discuss.
Answer: A solar cell delivers a constant current for any given illumination level, while the voltage is determined by the load resistance. Potential in a solar cell depends on the intensity of solar radiation. Since the intensity of solar radiation is not always constant, the potential across its terminal is also not constant. Solar cells produce voltage based on the amount of light they receive, converting light energy into electrical energy.
In simple words: No, a solar cell does not always keep its voltage the same. The voltage it produces changes depending on how much sunlight is hitting it. More sun means more voltage.

🎯 Exam Tip: Remember that solar cell output (both current and voltage) is highly dependent on environmental factors like light intensity and temperature.

 

Question 3. Can electroplating be possible with the alternating current?
Answer: Electroplating is a process where there is a continuous flow of ions for the deposition of copper, which is not possible in an Alternating current. Therefore, electroplating is possible with DC only, for the sake of perfectness and homogeneity of the electroplating. Using AC would cause the metal ions to move back and forth, preventing a smooth, even coating.
In simple words: No, electroplating cannot be done with alternating current (AC). It needs direct current (DC) because AC constantly changes direction, which would stop the steady layering of metal.

🎯 Exam Tip: Electroplating requires a unidirectional flow of charge (DC) to ensure consistent deposition of metal ions onto the desired surface.

VI. Answer the following:

 

Question 1. On what factors does the electrostatic force between two charges depend?
Answer: The electrostatic force between two charges depends on the following factors:

• value of charges on them,
• distance between them, and
• nature of medium between them.

This force is described by Coulomb's Law, which shows its direct relationship to charge magnitudes and inverse square relationship to distance.
In simple words: The push or pull between two electric charges depends on how big each charge is, how far apart they are, and what kind of material is between them.

🎯 Exam Tip: Clearly state all three factors (magnitude of charges, distance, and medium) and mention Coulomb's Law as the governing principle.

 

Question 2. What are the electric lines of force?
Answer: The lines representing the electric field are called electric lines of force. These imaginary lines show the direction and strength of the electric field around a charged object.
In simple words: Electric lines of force are make-believe lines that show which way an electric force would push a tiny positive charge. They also show how strong the force is.

🎯 Exam Tip: Emphasize that these lines are imaginary but are a useful tool to visualize electric fields; they never cross each other.

 

Question 3. Define electric field.
Answer: The electric lines of force are straight or curved paths along which a unit positive charge tends to move in the electric field. An electric field is a region around a charged object where another charged object would experience an electric force.
In simple words: An electric field is the space around an electric charge where other charges would feel a push or pull. It's like an invisible force zone.

🎯 Exam Tip: Remember that an electric field exists even before a test charge is placed in it; it's a property of space due to the source charge.

 

Question 4. Define electric current and give its unit.
Answer: The electric current is defined as the rate of flow of electric charge through any section of a conductor. In simpler terms, it is how much charge passes a point in a wire every second.
Electric current \( I = \frac{Q}{t} \)
Its unit is \( \text{Cs}^{-1} \)
Its SI unit: ampere (A).
In simple words: Electric current is how fast electric charge moves through something. Its main unit is the ampere, often just called "amp."

🎯 Exam Tip: Clearly state both the definition (rate of charge flow) and the SI unit (Ampere), relating it to Coulombs per second.

 

Question 5. State Ohm's law.
Answer: Ohm's law states that electric potential difference across two points in an electrical circuit is directly proportional to the current passing through it. That is, the voltage across a conductor is proportional to the current flowing through it, assuming constant temperature and other physical conditions.
\( V \sim I \)
The proportionality constant is the resistance (R) offered between the two points.
Hence, Ohm's law is written as \( V = RI \) (or) \( V = IR \)
Where, V is the potential difference in volt (V), I is the current flow in ampere (A), R is the resistance in ohm (\( \Omega \))
In simple words: Ohm's law says that the voltage across a wire is directly linked to the current flowing through it, as long as its temperature stays the same. The harder it is for current to flow, the higher the resistance.

🎯 Exam Tip: State Ohm's law clearly, include its mathematical form \( V=IR \), and importantly, mention the condition of constant temperature and other physical conditions.

 

Question 6. Name the appliances which work under the principle of heating effect of current.
Answer: Electric heating appliances like an iron box, water heater, toaster, etc., work on the principle of the heating effect of current. In these devices, the electrical energy is converted into heat energy.
In simple words: Many common household items use electricity to make heat, like electric irons, water heaters, and toasters.

🎯 Exam Tip: Recognize that any appliance designed to produce heat (e.g., electric kettle, room heater, hair dryer) uses the heating effect of electric current.

 

Question 7. How are the home appliances connected in general, in series or parallel? Give reasons.
Answer: The home appliances are connected in parallel. This is because, when the appliances are connected in parallel, each of them can be switched on and off independently. This is a feature that is essential in house wiring. Also, if the appliances were wired in series, the potential difference across each appliance would vary depending on the resistance of the appliance. Furthermore, if one appliance in a series circuit stops working, the entire circuit would break, and no other appliance would function.
In simple words: Home appliances are connected in parallel. This allows each appliance to work separately and to get the full voltage needed to operate correctly. If they were in series, turning one off would turn them all off, and they might not get enough power.

🎯 Exam Tip: The two main reasons for parallel connection are independent operation of each appliance and maintaining a constant voltage across each device.

 

Question 8. List the safety features while handling electricity.
Answer: Safety features while handling electricity include:
(i) Ground connection: The metal bodies of all electrical appliances are to be connected to the ground by means of a third wire apart from the two wires used for electrical connection. This safely channels stray currents into the earth.
(ii) Trip switch: It is an electromechanical device which does not allow a current beyond a particular value by automatically switching off the connection. This protects against overcurrents.
(iii) Fuse: A fuse is another safety mechanism which works on the Joule heating principle. It contains a thin wire that melts and breaks the circuit if the current becomes too high.
In simple words: Important safety features for electricity are ground connections for appliances, trip switches that turn off power when too much current flows, and fuses that melt to break the circuit if current gets too high.

🎯 Exam Tip: Always prioritize safety when working with electricity; ground connections, fuses, and trip switches (or circuit breakers) are vital for preventing electrical hazards.

 

Question 1. Rubbing a comb on hair makes the comb get – 0.4C.
(a) Find which material has lost electron and which one gained it.
(b) Find how many electrons are transferred in this process.
Answer:
a. Comb gained electrons. Dry hair lost an electron. Since the comb gets a negative charge, it must have gained electrons.
b. No. of electrons transferred \( = -0.4 \, \text{C} \)
We know that 1 coulomb \( = 6.25 \times 10^{18} \) electrons.
So, \( -0.4 \, \text{C} = -0.4 \times 6.25 \times 10^{18} \) electrons
\( = -2.5 \times 10^{18} \) electrons
In simple words: (a) The comb gained electrons to become negative, meaning the dry hair lost electrons. (b) To find the number of electrons, we use the fact that 1 Coulomb is a certain number of electrons, and then multiply by the given charge.

🎯 Exam Tip: Remember that a negative charge indicates an excess of electrons, while a positive charge indicates a deficit of electrons. The charge of an electron is fundamental.

 

Question 2. Calculate the amount of charge that would flow in 2 hours through an element of an electric bulb drawing a current of 2.5A.
Answer:
Current \( I = 2.5 \, \text{A} \)
Time \( t = 2 \, \text{hours} = 2 \times 3600 \, \text{seconds} \)
\( t = 7200 \, \text{s} \)
We need to find Charge \( q = ? \)
Using the formula \( q = I \times t \)
\( q = 2.5 \times 7200 \)
\( q = 18,000 \, \text{C} \)
The total charge that flows is 18,000 Coulombs.
In simple words: To find the total charge, multiply the current by the time. First, change the time from hours into seconds. Then, multiply 2.5 Amperes by 7200 seconds to get the total charge in Coulombs.

🎯 Exam Tip: Always ensure that all quantities are in their SI units before performing calculations (current in Amperes, time in seconds, charge in Coulombs).

 

Question 3. The values of the current (I) flowing through a resistor for various potential differences V across the resistor are given below. What is the value of resistor?
I (ampere) 0.5 1.0 2.0 3.0 4.0
V (volt) 1.6 3.4 6.7 10.2 13.2
[Hint: plot V-I graph and take slope]
Answer:
Answer: (c) absence of electron
In simple words: A positive charge in electricity means that there are no extra electrons, or there is a lack of electrons. When something loses electrons, it becomes positively charged.

🎯 Exam Tip: Remember that charge is carried by electrons and protons. A positive charge indicates a deficit of electrons, while a negative charge indicates an excess of electrons.

 

Question 2. Rubbing of comb with hair
(a) creates electric charge
(b) transfers electric charge
(c) either (a) or (b)
(d) neither (a) nor (b)
Answer: (b) transfers electric charge
In simple words: When you rub a comb on your hair, you are not creating new electricity. Instead, electrons move from your hair to the comb, making the comb charged and your hair oppositely charged. This process is called charging by friction.

🎯 Exam Tip: Understand that electric charge is conserved; it can only be transferred, not created or destroyed.

 

Question 3. Electric field lines from positive charge and in negative charge.
(a) start; start
(b) start; end
(c) start; end
(d) end; end
Answer: (b) or (c) start, end
In simple words: Electric field lines always start from a positive charge and finish on a negative charge. They show the direction a small positive test charge would move.

🎯 Exam Tip: Always remember that electric field lines never cross each other and are closer where the field is stronger.

 

Question 4. Potential near a charge is the measure of its to bring a positive charge at that point.
(a) energy
(b) ability
(c) tendency
(d) work
Answer: (d) work
In simple words: Electric potential at a point is how much work you need to do to move a tiny positive charge from far away to that specific point. It shows the energy stored per unit charge at that location.

🎯 Exam Tip: Work done is a key concept in understanding potential difference and energy in electric fields.

 

Question 5. Heating effect of current is called,
(a) Joule heating
(b) Coulomb heating
(c) Voltage heating
(d) Ampere heating
Answer: (a) Joule heating
In simple words: When electricity flows through a wire, it often makes the wire hot. This effect is called Joule heating, named after the scientist James Prescott Joule. Many appliances use this effect to produce heat.

🎯 Exam Tip: Recall that Joule's Law quantifies this heating effect with the formula \( H = I^2 Rt \), where H is heat, I is current, R is resistance, and t is time.

 

Question 6. In an electrolyte, the current is due to the flow of
(a) electrons
(b) positive ions
(c) both (a) and (b)
(d) neither (a) nor (b)
Answer: (c) both (a) and (b)
In simple words: In a liquid that conducts electricity (an electrolyte), current moves because both electrons and charged atoms (ions) are flowing. This is different from solid wires where only electrons move.

🎯 Exam Tip: Distinguish between metallic conductors (electron flow only) and electrolytes (both ion and electron flow) for current conduction.

 

Question 7. Electroplating is an example of
(a) heating effects
(b) chemical effects
(c) flowing effects
(d) magnetic effect
Answer: (b) chemical effect
In simple words: Electroplating uses electricity to cause a chemical reaction, where a thin layer of metal is deposited onto another object. This is a practical example of the chemical effect of electric current, making objects look better or last longer.

🎯 Exam Tip: Remember that electricity can produce heat, magnetic fields, and chemical changes. Electroplating falls under chemical effects.

 

Question 8. Resistance of a wire depends on,
(a) temperature
(e) nature of material
(b) geometry
(d) all the above
Answer: (d) all the above
In simple words: How much a wire resists electricity depends on several things: its temperature, what material it's made of (like copper or iron), and its shape (how long and thick it is). All these factors change how easily current flows.

🎯 Exam Tip: The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area, and also depends on the material's resistivity and temperature.

 

Question 9. 1 volt =
(a) \( \frac {1 \text{ Joule}}{1 \text{ Coulomb}} \)
(b) \( \frac {1 \text{ Coulomb}}{1 \text{ Joule}} \)
(c) \( \frac {1 \text{ Joule}}{1 \text{ Coulomb}^2} \)
(d) 1 Joule coulomb
Answer: (a) \( \frac {1 \text{ Joule}}{1 \text{ Coulomb}} \)
In simple words: One volt means that one Joule of energy is used to move one Coulomb of electric charge. It's like how much "push" is needed to move a certain amount of electricity.

🎯 Exam Tip: Always remember the definition of voltage: work done per unit charge, so \( V = \frac{W}{Q} \), where W is work (Joule) and Q is charge (Coulomb).

 

Question 10. The resistance of a conductor is R. If Its length is doubled, then its new resistance will be.
(a) R
(b) 2R
(c) 4R
(d) 8R
Answer: (C) 4R
In simple words: If you make a wire twice as long, its resistance becomes double. If you also make it twice as thin (which doubles its length if volume is constant), its resistance will become four times the original. Resistance depends on length and cross-sectional area.

🎯 Exam Tip: For a given volume of conductor, if the length is doubled, the area is halved. Since \( R = \rho \frac{L}{A} \), the new resistance \( R' = \rho \frac{2L}{A/2} = 4 \rho \frac{L}{A} = 4R \).

 

Question 11. The following is not a safety device.
(a) Fuse
(b) Trip switch
(c) Ground connection
(d) Wire
Answer: (d) wire
In simple words: Fuses, trip switches (or circuit breakers), and ground connections are all designed to keep electrical systems safe. A simple wire, however, is meant to carry current, not to protect against electrical faults.

🎯 Exam Tip: Safety devices like fuses and trip switches automatically break a circuit during overcurrent or short-circuit to prevent damage or fire.

 

Question 12. In India the frequency of alternating current is,
(a) 220 Hz
(b) 50 Hz
(c) 5 Hz
(d) 100 Hz
Answer: (b) 50 Hz
In simple words: In India, the electric current coming to homes changes direction 50 times every second. This rate of change is called frequency, and it's 50 Hertz. Different countries use different frequencies.

🎯 Exam Tip: Know the standard AC frequency for your region, as it's a fundamental parameter for electrical appliances.

II. Fill In The Blanks:

 

Question 1. The number of electrons constituting 1-coulomb charge is...............
Answer: \( 6.25 \times 10^{18} \)
In simple words: One Coulomb of electric charge is made up of a very large number of electrons, specifically about 6.25 million million million electrons. Each electron carries a tiny bit of charge.

🎯 Exam Tip: Remember the elementary charge value of an electron (\( 1.6 \times 10^{-19} \text{ C} \)) and how to calculate the number of electrons for a given charge (\( N = \frac{Q}{e} \)).

 

Question 2. Resistors are connected in series if the resistance of the electric circuit is to be...............
Answer: increased
In simple words: When you connect resistors one after another in a line (in series), it makes the total resistance of the circuit higher. This is like making a longer path for the electricity, which makes it harder for the current to flow.

🎯 Exam Tip: For series connections, the total resistance is the sum of individual resistances (\( R_{total} = R_1 + R_2 + \dots \)).

 

Question 3. Electric fuse is a wire made up of a material having ...............melting point.
Answer: low
In simple words: An electric fuse is made from a special wire that melts very easily. If too much electricity flows through it, the wire quickly melts and breaks the circuit, protecting appliances from damage.

🎯 Exam Tip: Fuses are safety devices designed to blow (melt) and break the circuit when current exceeds a safe limit, preventing overheating and fire.

 

Question 4. ...............is the only non-metal that is a good conductor of electricity.
Answer: Graphite
In simple words: Most non-metals don't conduct electricity well, but graphite is special. It's a form of carbon that can conduct electricity because of its unique atomic structure. It is used in pencils and batteries.

🎯 Exam Tip: Remember graphite as an exception to the rule that non-metals are typically poor conductors of electricity.

 

Question 5. If the area of cross section of the conductor is doubled its resistance gets ...............
Answer: halved
In simple words: If you make a wire thicker (double its cross-sectional area), electricity has more space to flow, so its resistance becomes half. It's like making a wider road for cars, allowing them to pass more easily.

🎯 Exam Tip: Resistance is inversely proportional to the cross-sectional area of the conductor (\( R \propto \frac{1}{A} \)).

 

Question 6. A negative charge will move from ...............potential to ...............potential.
Answer: lower, higher
In simple words: A negative charge, like an electron, is attracted to positive areas and pushed away from negative areas. So, it naturally moves from a place where there's less positive "pull" (lower potential) to a place with more "pull" (higher potential). This is opposite to how positive charges move.

🎯 Exam Tip: Conventional current (positive charge flow) moves from higher to lower potential, while electron flow (negative charge flow) moves from lower to higher potential.

 

Question 7. ...............is work done per unit charge.
Answer: Electric potential
In simple words: Electric potential tells you how much work is needed to move a single unit of electric charge from one point to another. It's measured in volts.

🎯 Exam Tip: This definition is fundamental to understanding voltage and its role in circuits.

 

Question 8. An electrochemical cell converts ...............energy into ...............energy.
Answer: chemical, electrical
In simple words: An electrochemical cell, like a battery, uses chemical reactions to produce electricity. It changes the stored chemical energy directly into usable electrical energy.

🎯 Exam Tip: Remember that electrochemical cells are energy conversion devices, transforming chemical energy into electrical energy through redox reactions.

 

Question 9. Three resistors are connected in series with a cell. If the current in each resistor is 1.5A, then the current through the cell will be ...............
Answer: 1.5A
In simple words: In a series circuit, the electricity flows along a single path. This means the same amount of current goes through every single part of the circuit, including each resistor and the cell.

🎯 Exam Tip: A key characteristic of a series circuit is that the current is uniform throughout all components.

 

Question 10. Three resistors are connected in parallel with a battery. If the current in each resistor is 2A, then the current through the battery will be ...............
Answer: 6A
In simple words: When resistors are connected side-by-side (in parallel), the total current from the battery splits up to go through each one. So, to find the total current from the battery, you add up the current flowing through each separate resistor.

🎯 Exam Tip: In a parallel circuit, the total current supplied by the source is the sum of the currents in each parallel branch (\( I_{total} = I_1 + I_2 + \dots \)).

 

Question 11. As electrons are revolving in the ...............of an atom they can be easily removed from an atom and also added to it.
Answer: orbits
In simple words: Electrons spin around the center of an atom in specific paths called orbits or shells. The electrons in the outermost orbits are not held as tightly, making them easier to add or remove from the atom. This movement helps create electricity.

🎯 Exam Tip: Understand the concept of valence electrons in the outermost shell, which are responsible for chemical bonding and electrical conduction.

 

Question 12. If an electron is added in excess to an atom then the atom is ...............charged.
Answer: negatively
In simple words: Atoms are usually balanced with equal numbers of positive protons and negative electrons. If an extra electron (which is negative) joins an atom, the atom will have more negative charge than positive charge, making it negatively charged overall.

🎯 Exam Tip: The charge of an atom is determined by the balance between its protons (positive) and electrons (negative).

 

Question 13. The excess of electrons make an object negative and ................of electrons make it positive.
Answer: deficiency
In simple words: If an object has more electrons than protons, it becomes negatively charged. If it has fewer electrons than protons, it means there is a lack of electrons, making it positively charged.

🎯 Exam Tip: Charge imbalance, whether excess or deficiency of electrons, is the basis of static electricity and charge phenomena.

 

Question 14. Electric charge is ...............in nature.
Answer: additive
In simple words: Electric charge is additive, meaning you can simply add up all the positive and negative charges in a system to find the total charge. For example, +2 units and -1 unit combine to make +1 unit of charge.

🎯 Exam Tip: The principle of additivity of charge is crucial for calculating net charge in a system, treating positive and negative charges algebraically.

 

Question 15. Electric lines of force are ...............lines.
Answer: imaginary
In simple words: Electric lines of force are not real, physical lines you can see. They are a way for us to visualize and understand the direction and strength of an electric field around charged objects.

🎯 Exam Tip: While imaginary, electric field lines are powerful tools for visualizing electrostatic interactions; they originate from positive charges and terminate on negative charges.

 

Question 16. For an isolated positive charge the electric lines of force are radial...............and for an isolated negative charge they are radial...............
Answer: outwards, inwards
In simple words: For a single positive charge, electric field lines point straight away from it, like spokes on a wheel. For a single negative charge, the lines point straight towards it. This shows how they attract or repel other charges.

🎯 Exam Tip: Always draw electric field lines originating from positive charges and terminating on negative charges, or extending to/from infinity for isolated charges.

 

Question 17. ...............at a point is a measure of force acting on a unit positive charge placed at that point.
Answer: Electric field
In simple words: The electric field at a certain point tells you how strong the electric force would be if you placed a tiny positive charge there. It's a way to describe the space around charged objects.

🎯 Exam Tip: Electric field strength is defined as force per unit charge (\( E = \frac{F}{q_0} \)) and is a vector quantity, having both magnitude and direction.

 

Question 18. Electric potential is a measure of the ...............on the unit positive charge to bring it to that point against all electrical forces.
Answer: work done
In simple words: Electric potential measures the amount of work needed to move a single positive charge to a particular point from a very far distance. This work is done against the existing electrical forces.

🎯 Exam Tip: The concept of electric potential helps quantify the "energy landscape" for charges in an electric field.

 

Question 19. The movement of positive charge is called as ...............
Answer: conventional current
In simple words: Historically, scientists imagined electricity flowing as if positive charges were moving. Even though we now know electrons (negative charges) are usually the ones moving in wires, we still call the direction positive charges would move "conventional current."

🎯 Exam Tip: Always remember that conventional current flows from higher potential to lower potential, which is opposite to the direction of electron flow.

 

Question 20. The ...............is the measure of opposition offered by the component to the flow of electric current through it.
Answer: resistance
In simple words: Resistance is how much a material fights against the flow of electricity. A high resistance means it's hard for current to pass through, while low resistance means it flows easily.

🎯 Exam Tip: Resistance is measured in Ohms (\( \Omega \)) and is a key property of conductors that affects current flow according to Ohm's Law (\( V = IR \)).

 

Question 21. The process of conduction of electric current through solution is called...............
Answer: electrolysis
In simple words: When an electric current passes through a liquid solution and causes a chemical change, this process is known as electrolysis. It's used to split compounds or coat objects with metal.

🎯 Exam Tip: Electrolysis is a fundamental process in chemistry, used for industrial applications like metal refining and electroplating.

 

Question 22. The device used to convert AC to DC is called...............
Answer: rectifier
In simple words: A rectifier is a device that changes alternating current (AC), which constantly reverses direction, into direct current (DC), which flows in only one direction. This is important for many electronic devices that need DC power.

🎯 Exam Tip: Rectifiers are essential components in power supplies, converting the AC from the wall outlet into the DC required by most electronic circuits.

 

Question 23. Trip switch is a ...............safety device.
Answer: electro mechanical
In simple words: A trip switch, also known as a circuit breaker, is a safety device that uses both electrical and mechanical parts. It automatically turns off the electricity if it detects too much current, preventing damage and hazards.

🎯 Exam Tip: Trip switches (circuit breakers) offer more reliable and reusable protection than fuses, as they can be reset after tripping.

III. True Or False:

 

Question 1. Electrically neutral means it is either zero or equal positive and negative charges.
Answer: True.
In simple words: An object is electrically neutral if it has no net charge. This can happen if it truly has no charge, or more commonly, if it has an equal amount of positive and negative charges that cancel each other out.

🎯 Exam Tip: Most objects in everyday life are electrically neutral due to the balance of positive protons and negative electrons.

 

Question 2. Ammeter is connected in parallel in an electric circuit.
Answer: False.
Correct statement: Ammeter is connected in series in an electric circuit.
In simple words: An ammeter is used to measure the electric current flowing through a circuit. To do this accurately, it must be placed directly in the path of the current, connected in series, so that all the current flows through it.

🎯 Exam Tip: Ammeters have very low resistance and must always be connected in series to measure current; connecting them in parallel can short-circuit the component and damage the ammeter.

 

Question 3. An ammeter is always placed in parallel with the circuit.
Answer: False.
Correct statement: An ammeter is always placed in series with the circuit.
In simple words: An ammeter always needs to be connected directly in the line of the circuit to measure current correctly. If it were connected in parallel, it would act like a shortcut, drawing too much current and giving a wrong reading or damaging the circuit.

🎯 Exam Tip: Ensure you clearly understand the connection methods for ammeters (series) and voltmeters (parallel) to avoid common mistakes.

 

Question 4. The resistance of a dry human body is high.
Answer: True.
In simple words: Dry human skin has high resistance to electric current, which can somewhat protect against electric shocks. However, wet skin significantly lowers this resistance, making electric shocks much more dangerous.

🎯 Exam Tip: Emphasize safety around electricity, especially in damp conditions, due to the reduced resistance of wet skin.

 

Question 5. For current to flow, one needs an open circuit.
Answer: False.
Correct statement: For current to flow, one needs a closed circuit.
In simple words: For electricity to flow, it needs a complete and unbroken path from the power source and back. An "open circuit" means there's a break in this path, so current cannot flow.

🎯 Exam Tip: A continuous, closed loop is essential for current to flow in any electrical circuit.

 

Question 6. A comb rubbed with hair and brought near pieces of paper attracts them, because both comb and paper get similarly charged.
Answer: False.
Correct statement: A comb rubbed with hair and brought near pieces of paper attracts them, because both comb and paper get oppositely charged.
In simple words: When a comb is rubbed, it becomes charged. It then attracts uncharged pieces of paper by inducing an opposite charge on the side of the paper closest to it. Opposite charges attract. If they were similarly charged, they would push each other away.

🎯 Exam Tip: Remember the fundamental rule of electrostatics: opposite charges attract, and like charges repel. This principle explains static cling.

 

Question 7. Overloading of electric circuits can lead to short-circuiting.
Answer: True.
In simple words: Overloading happens when too many electrical devices are plugged into one circuit, making it draw more current than it can handle. This can cause wires to overheat and sometimes lead to a short-circuit, which is a very dangerous situation.

🎯 Exam Tip: Overloading can cause wires to heat up, melt insulation, and lead to fire hazards. Always use proper circuit protection like fuses or circuit breakers.

 

Question 8. Electrons in outer orbits are called free electrons.
Answer: True.
In simple words: Electrons that are far from the center of an atom, in its outermost energy levels, are called free electrons. They are not strongly attached to any one atom and can easily move between atoms, allowing electricity to flow.

🎯 Exam Tip: Free electrons are responsible for electrical conduction in metallic conductors and explain why metals are good electrical conductors.

 

Question 9. Electric fuse works on the Joule heating principle.
Answer: True.
In simple words: An electric fuse protects circuits by using the Joule heating effect. When too much current flows, the fuse wire heats up, melts, and breaks the circuit, preventing damage to appliances or fires.

🎯 Exam Tip: The melting point of the fuse wire is carefully selected to ensure it breaks the circuit at a specific safe current limit.

IV. Match The Following:

 

Question I. Match the following:

Answer:

• 1. Electric Charge - (c) coulomb
• 2. Potential difference - (e) volt
• 3. Electric field - (d) newton per coulomb
• 4. Resistance - (a) ohm
• 5. Electric current - (b) ampere

In simple words: This list matches different electrical quantities with their correct units. Electric charge is measured in coulombs, potential difference in volts, electric field in newtons per coulomb, resistance in ohms, and electric current in amperes.

🎯 Exam Tip: Thoroughly memorize the SI units for all fundamental electrical quantities, as they are often tested in objective questions.

 

Question II. Match the following:

Answer:

• 1-c (Electric power - Watt)
• 2-d (missing item - Joule)
• 3-c (Electric charge - Watt)
• 4-a (missing item - Volt)

In simple words: This section lists electrical terms and their corresponding units or definitions. For instance, Electric power is correctly associated with Watt. While the unit for Electric charge is Coulomb, the given answer shows a different pairing.

🎯 Exam Tip: Always double-check your knowledge of fundamental units for power (Watt), energy (Joule), charge (Coulomb), and potential difference (Volt).

V. Assertion And Reason Type Questions:

 

Question 1. Assertion (A): Electric current will not flow between two charged bodies when connected if their charges are the same.
Reason (R): Current is the rate of flow of charge.
Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
Answer: (d) Assertion is false but the reason is true
Reason: Current will not flow when two bodies are at the same potential. When their charges are the same, their potential may be different. Hence current may flow in this case.
In simple words: The assertion is wrong because having the same charge doesn't mean current won't flow; current depends on potential difference, not just charge amount. The reason is correct because current is indeed about how fast charge moves.

🎯 Exam Tip: Understand that current flow is driven by potential difference, not just the quantity of charge. Two objects can have the same charge but different potentials.

 

Question 2. Assertion (A): A bird perches on a high power line and nothing happens to the bird.
Reason (R): The level of the bird is very high from the ground.
Answer: (c) Assertion is true but reason is false
Reason: Electric shock is due to the electric current flowing through a living body. When the bird perches on a single high power line, no current passes through its body. Because its body is at an equipotent surface (i.e.) there is no potential difference. While when a man touches the same line, standing bare foot on the ground the electrical circuit is completed through the ground. The hands of man are at high potential and his feet are at low potential. Hence the large amount of current flows through the body of the man and the person, therefore, gets a fatal shock.
In simple words: The bird is safe because both its feet are on the same wire, so there's no voltage difference across its body for current to flow. The reason given is wrong; its height from the ground is not why it's safe. A person would get a shock because they complete a circuit from the wire to the ground, creating a path for current.

🎯 Exam Tip: Emphasize that current flows when there is a potential difference across two points. A bird is safe because it doesn't create a potential difference across its body when on a single wire.

VI. Answer In One Word:

 

Question 1. Name the force which acts between two point charges obey Newton's third law.
Answer: Electrostatic force.
In simple words: The push or pull between two charged objects is called electrostatic force. Like all forces, it follows Newton's third law, meaning if one charge pushes another, the second charge pushes back with equal strength.

🎯 Exam Tip: Recall that electrostatic force, like gravity, is a fundamental force, and always acts as an action-reaction pair between two charges.

 

Question 2. What is the SI unit for the current?
Answer: ampere (A).
In simple words: The standard unit for measuring electric current is the ampere, often shortened to "amp." It tells us how much electric charge is flowing past a point each second.

🎯 Exam Tip: Ampere is a base SI unit. Ensure consistent use of units in all calculations.

 

Question 4. What is the rate at which charges flow past a point on a circuit?
Answer: Current.
In simple words: The speed at which electric charges move through a circuit is called current. It's like measuring how many water molecules pass a point in a river each second.

🎯 Exam Tip: The definition of current is \( I = \frac{Q}{t} \), where I is current, Q is charge, and t is time. This is a fundamental concept.

 

Question 5. Name a device that helps to maintain a potential difference across conductors.
Answer: Cell or battery.
In simple words: A cell or battery acts like a pump, constantly pushing charges to maintain a difference in electrical "pressure," or potential, across the circuit. This difference keeps the current flowing.

🎯 Exam Tip: Cells and batteries convert chemical energy into electrical energy to provide the electromotive force (e.m.f.) needed to sustain current.

 

Question 6. What does the circuit symbol \( \longrightarrow \!\! \text{----} \!\! \longleftarrow \) represent?
Answer: Wire crossing without touching each other.
In simple words: This specific symbol in a circuit diagram means that two wires cross paths, but they are not electrically connected. One wire simply passes over the other.

🎯 Exam Tip: Correctly interpreting circuit symbols is crucial for understanding and drawing electrical diagrams.

 

Question 7. How many electrons accumulate to make 1C of electric charge?
Answer: \( 1\text{C} = 6.25 \times 10^{18} \) electrons.
In simple words: To get a total charge of one Coulomb, you need about 6.25 million million million individual electrons. Each electron carries a tiny, fixed amount of negative charge.

🎯 Exam Tip: The charge of a single electron is \( 1.6 \times 10^{-19} \) C. Remember how to convert between total charge and number of electrons using this value.

 

Question 8. What is the charge of one electron?
Answer: \( -1.6 \times 10^{-19} \text{ C} \)
In simple words: A single electron carries a very small amount of negative electric charge. This exact value is \( -1.6 \times 10^{-19} \) Coulombs, and it's a fundamental constant in physics.

🎯 Exam Tip: This value (the elementary charge) is a constant you should know, as it forms the basis for all electric charge calculations.

VII. Subjective Problems:

 

Question 1. Calculate the charge passing through a lamp in 2 minutes if the current is 3A.
Answer: First, convert the time to seconds. 2 minutes is equal to \( 2 \times 60 = 120 \) seconds. The current \( I \) is 3A, and the time \( t \) is 120 seconds. We can find the charge \( q \) using the formula \( q = I \times t \). So, \( q = 3 \text{ A} \times 120 \text{ s} = 360 \text{ C} \). Therefore, a charge of 360 Coulombs passes through the lamp. This calculation helps us understand the total amount of electricity moving through the circuit.
In simple words: We multiply the current (how fast electricity flows) by the time (how long it flows) to find the total electric charge that went through the lamp.

🎯 Exam Tip: Always remember to convert time into seconds before using it in physics formulas, as the standard unit for time is seconds.

 

Question 2. Calculate the current in a wire if a charge of 1500 coulomb flows through it in 5 minutes.
Answer: First, we convert the time from minutes to seconds. 5 minutes is equal to \( 5 \times 60 = 300 \) seconds. The given charge \( q \) is 1500 Coulombs, and the time \( t \) is 300 seconds. We can calculate the current \( I \) using the formula \( I = \frac{q}{t} \). So, \( I = \frac{1500 \text{ C}}{300 \text{ s}} = 5 \text{ A} \). The current flowing through the wire is 5 Amperes. This shows how much charge moves past a point each second.
In simple words: To find the current, we divide the total electric charge by the time it took to flow. Make sure the time is in seconds.

🎯 Exam Tip: Knowing the relationship between charge, current, and time \( (I = q/t) \) is fundamental in understanding electricity. Practice unit conversions to avoid common errors.

 

Question 3. A charge of 400C flows through a conductor for 13 minutes and 20 seconds. Find the magnitude of the current flowing through the conductor.
Answer: First, we need to convert the total time into seconds. 13 minutes and 20 seconds is \( (13 \times 60) + 20 \) seconds, which equals \( 780 + 20 = 800 \) seconds. The given charge \( q \) is 400 Coulombs, and the total time \( t \) is 800 seconds. We use the formula for current: \( I = \frac{q}{t} \). Substituting the values, \( I = \frac{400 \text{ C}}{800 \text{ s}} = 0.5 \text{ A} \). So, the current flowing through the conductor is 0.5 Amperes. This calculation shows the average current over the given period.
In simple words: First, change all the time into seconds. Then, divide the total charge by this total time to get the current.

🎯 Exam Tip: Always convert all time measurements to the base unit (seconds) before calculations to ensure consistency and accuracy in your results.

 

Question 4. \( 10^{20} \) electrons, each having a charge of \( 1.6 \times 10^{-19} \) C, flows in a circuit V is 0.1s. What is the current in ampere?
Answer: First, calculate the total charge \( q \) from the number of electrons and the charge of a single electron. The number of electrons \( n = 10^{20} \) and the charge of one electron \( e = 1.6 \times 10^{-19} \) C. So, total charge \( q = n \times e = 10^{20} \times (1.6 \times 10^{-19} \text{ C}) = 1.6 \times 10^{20-19} \text{ C} = 1.6 \times 10^{1} \text{ C} = 16 \text{ C} \). The time \( t \) is given as 0.1 s. Now, calculate the current \( I \) using the formula \( I = \frac{q}{t} \). Substituting the values, \( I = \frac{16 \text{ C}}{0.1 \text{ s}} = 160 \text{ A} \). Thus, the current in the circuit is 160 Amperes. This demonstrates how a large number of tiny charges combine to create a significant current.
In simple words: First, find the total electric charge by multiplying the number of electrons by the charge of each electron. Then, divide this total charge by the time to find the current.

🎯 Exam Tip: Remember that the total charge is the number of charge carriers multiplied by the charge of one carrier. Pay close attention to powers of 10 in scientific notation during calculations.

VIII. Long Answer Questions

 

Question 1. a State Ohm's law. b. Draw a circuit diagram for the verification of ohm's law. Also, plot graphically the variation of current with a potential difference.
Answer:
a. Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided that physical conditions like temperature and density remain constant. Mathematically, this is expressed as \( V \propto I \) or \( V = IR \), where \( R \) is the resistance of the conductor.
b. To verify Ohm's law, a circuit can be set up using a resistor, an ammeter (in series), a voltmeter (in parallel across the resistor), a battery, and a rheostat (variable resistor) to change the current and voltage. A graph plotting potential difference (V) on the y-axis against current (I) on the x-axis will show a straight line passing through the origin, confirming the direct proportionality. This straight line represents constant resistance.

In simple words: Ohm's law says that current and voltage are directly linked if other things stay the same. If you draw a graph of voltage versus current, you will get a straight line.

🎯 Exam Tip: When stating Ohm's law, always include the condition "provided physical conditions like temperature remain constant" to get full marks.

 

Question 2. a. Why is series arrangement not used for domestic circuits? b. Explain why fuse wire is always connected in series arrangement. c. Why are copper and aluminium wires usually employed for electricity transmission?
Answer:
a. Series arrangement is not used for domestic circuits for several reasons. In a series circuit, the same current flows through all appliances. If one appliance breaks or is switched off, the entire circuit is broken, and all other appliances stop working. Also, the voltage gets divided among the appliances, so each appliance might not get the full required voltage to operate efficiently. This makes parallel connections much more practical for homes. For example, if your television and lights were in series, turning off the TV would also turn off the lights.
b. Fuse wire is always connected in series in a circuit to protect it from excessive current. When a current higher than the safe limit flows through the circuit, the fuse wire, having a low melting point, heats up quickly and melts, breaking the circuit. This prevents damage to the appliances and wiring. By being in series, the fuse acts as a gate for the entire current in that part of the circuit.
c. Copper and aluminium wires are commonly used for electricity transmission because they are excellent electrical conductors, meaning they allow current to flow through them easily with very little resistance. They are also relatively inexpensive, strong, and can be drawn into thin wires without breaking. These properties make them ideal for carrying electricity over long distances without much energy loss. Additionally, they are ductile and malleable, allowing them to be easily shaped and installed.
In simple words: a. Household wiring uses parallel connections so each appliance works on its own and gets enough power. b. A fuse is put in series to break the circuit if too much electricity flows, protecting other devices. c. Copper and aluminium are good for wires because they carry electricity well and are affordable.

🎯 Exam Tip: When explaining series vs. parallel circuits, always emphasize the implications for current, voltage, and independent operation of appliances.

 

Question 3. Explain the dangers of electricity and precautions to be taken while handling electricity.
Answer: Electricity can be very dangerous if not handled carefully, leading to shocks, burns, or even fires. Here are some dangers and precautions:

• **Damaged insulation:** Wires with broken insulation can expose live parts, causing electric shock if touched. To prevent this, always use safety gloves and stand on an insulating stool or wear rubber slippers when working with electricity. Never touch bare wires.
• **Overheating of cables:** Using low-quality wires or overloading circuits can cause cables to overheat, potentially leading to fires. Use only ISI-certified cable wires for domestic wiring to ensure they meet safety standards.
• **Overload of power sockets:** Connecting too many electrical devices to a single socket can draw excessive current, causing overheating and potential fire hazards. Always connect only a few devices to one socket and avoid overloading.
• **Inappropriate use of electrical appliances:** Using appliances that are not rated for a specific AC point, TV point, or microwave oven point can lead to damage or hazards. Always use appliances according to their power rating and intended use.
• **Environment with moisture and dampness:** Water is a good conductor of electricity. Keeping electrical setups in moist or wet environments can lead to leakage of electric current and increase the risk of shock. Ensure the area around electrical devices is dry.
• **Beyond the reach of children:** Electrical sockets and appliances should be kept out of the reach of small children who might not understand the dangers of electricity. Installing safety covers on unused sockets is also a good idea.

It's important to be cautious and aware of these risks to ensure safety.
In simple words: Electricity is dangerous. Always wear rubber shoes, check wires for damage, do not plug too many things into one socket, keep water away from electric items, and make sure children cannot reach electrical points.

🎯 Exam Tip: Organize your answer with clear headings or bullet points for dangers and corresponding precautions. Using simple, direct language for safety instructions is essential.

 

Question 4. Write the difference between resistance in series and in parallel.
Answer: The key differences between connecting resistances in series and in parallel are:

When resistances are in series, they add up to make a larger total resistance, which reduces the total current. In parallel, the total resistance is smaller than the smallest individual resistance, allowing more current to flow. This makes parallel connections useful for providing power to many devices at once.
In simple words: Series connections add up resistance, making the total resistance bigger, and the current is the same everywhere. Parallel connections reduce total resistance, and the voltage is the same across each path.

🎯 Exam Tip: When comparing series and parallel connections, focus on how total resistance, current, and voltage behave differently in each configuration.

IX.

 

Question 1. Distinguish e.m.f and potential difference.
Answer: Both electromotive force (e.m.f) and potential difference are measured in volts, but they refer to different concepts in an electrical circuit.

• **e.m.f (Electromotive Force):** This is the voltage developed across the terminals of an electrical source (like a battery or generator) when it is not producing any current in the circuit (i.e., when the circuit is open). It is the maximum potential difference a source can provide. Think of it as the total "push" available from the source.
• **Potential Difference:** This refers to the voltage developed between any two points in an electric circuit when current is flowing. It is the work done per unit charge to move a charge between those two points. Potential difference can be measured across electrical devices (like resistors or bulbs) or any two points in a closed circuit. It is always less than or equal to the e.m.f due to internal resistance in the source.

The e.m.f is the cause, and potential difference is the effect. Understanding their distinction is key to analyzing real-world circuits.
In simple words: e.m.f is the total electrical "push" from a power source when nothing is connected. Potential difference is the actual "push" measured between two points when electricity is flowing.

🎯 Exam Tip: Remember that e.m.f is the cause (energy supplied by the source), and potential difference is the effect (energy consumed across a component).

 

Question 2. What are the common electrical symbols and their meanings?
Answer: Electrical circuits are represented using standard symbols to make them easy to understand. Here are some common electrical symbols and their meanings:

These symbols help engineers and electricians easily read and understand circuit diagrams without needing to draw realistic pictures of each component. Knowing these symbols is fundamental for anyone working with electronics.
In simple words: Circuit symbols are like a special alphabet for drawing electrical diagrams. Each symbol stands for a different part, like a battery or a light bulb.

🎯 Exam Tip: Memorize the common circuit symbols and their functions as they are frequently used in circuit diagrams and questions.

 

Question 3. Write a note on direct current.
Answer: Direct current (DC) is a type of electric current where the flow of electric charge is always in one direction. This means that the electrons move from the negative terminal of a power source, through the circuit, and back to the positive terminal. Batteries, solar cells, and DC generators are common sources of direct current. Many electronic circuits, like those in cell phones, radios, and electric vehicles, use DC because it provides a steady and constant flow of power. The voltage and current in a DC circuit typically remain constant over time, as shown in the waveform below, which is a straight horizontal line.This constant flow is essential for many sensitive electronic components.
In simple words: Direct current (DC) flows in only one direction, like from a battery. It is steady and does not change direction. Most small electronics use DC.

🎯 Exam Tip: When describing DC, emphasize its unidirectional flow and common sources. A simple graph showing constant current over time can enhance your answer.

 

Question 4. Write a note on alternating current.
Answer: Alternating current (AC) is a type of electric current where the flow of electric charge periodically reverses direction. This means that the electrons in the wire move back and forth rather than flowing continuously in one direction. The voltage also regularly changes polarity (positive to negative and back). AC is the type of electricity typically supplied to homes and businesses from power plants because it can be easily generated, transmitted over long distances with minimal energy loss, and transformed to different voltage levels. The variation in AC happens in a wave-like pattern, usually a sine wave, and is characterized by its frequency, which is the number of complete cycles of variation per second (measured in Hertz).This oscillating nature makes it versatile for power distribution.
In simple words: Alternating current (AC) is electricity that changes direction many times each second. This is what comes from power lines to our houses. It is good for sending power over long distances.

🎯 Exam Tip: When explaining AC, highlight its ability to reverse direction and its importance in power transmission. Mentioning frequency as a key characteristic is also valuable.

 

Question 5. Explain the advantages of Ac over Dc.
Answer: Alternating current (AC) has several significant advantages over direct current (DC), particularly for large-scale power transmission and distribution:

• **Voltage Transformation:** AC voltage can be easily increased (stepped up) or decreased (stepped down) using transformers. This is crucial for transmitting electricity over long distances at high voltages to minimize energy loss, and then stepping it down for safe use in homes and industries. DC voltage cannot be easily transformed.
• **Reduced Energy Loss during Transmission:** AC can be transmitted at very high voltages and low currents, which significantly reduces the energy loss (as heat) during long-distance transmission.
• **Ease of Generation:** AC can be generated more easily and efficiently than DC using devices like alternators.
• **Conversion to DC:** AC can be easily converted to DC using rectifiers, making it versatile for both AC-only and DC-required applications.
• **Electromagnetic Induction:** AC facilitates electromagnetic induction, which is the principle behind many electrical devices like motors and generators, and is useful in various ways.

These advantages make AC the preferred choice for electricity grids.
In simple words: AC is better than DC for many things because its voltage can be easily changed with transformers, which helps send electricity far away with less power loss. It is also easier to make.

🎯 Exam Tip: Focus on the transformability of voltage and reduced transmission losses as the primary advantages of AC for power distribution.

X. Define the following:

 

Question 1. Define Electric force.
Answer: Electric force is the attractive or repulsive force that exists between any two charged particles or objects. This force is fundamental to how charges interact. Like charges repel each other, while opposite charges attract each other.
In simple words: Electric force is the push or pull between things that have an electric charge.

🎯 Exam Tip: Remember to mention both attraction and repulsion when defining electric force, as it covers both types of interactions between charges.

 

Question 2. Define Electric potential.
Answer: Electric potential at a point in an electric field is defined as the work done per unit positive charge to move that charge from infinity to that specific point, against all electrical forces. It measures the potential energy per unit charge. The unit for electric potential is the volt.
In simple words: Electric potential is the amount of work needed to bring a tiny positive charge from a very far distance to a specific spot in an electric field.

🎯 Exam Tip: Key terms for defining electric potential are "work done per unit positive charge," "from infinity," and "against electrical forces."

 

Question 3. Define Conventional current and electron current.
Answer:

• **Conventional Current:** This is the flow of positive charge. Historically, it was assumed that positive charges flow from the positive terminal to the negative terminal of a battery. Even though we now know electrons are the actual charge carriers, conventional current direction is still used in circuit diagrams.
• **Electron Current:** This refers to the actual flow of electrons. Since electrons are negatively charged, they flow from the negative terminal to the positive terminal of a battery.

It is important to note that both definitions describe the same physical phenomenon, just from different perspectives.
In simple words: Conventional current is imagined to flow from plus to minus, while electron current is the real flow of electrons from minus to plus.

🎯 Exam Tip: Clearly state the direction of flow for both conventional and electron currents, and highlight that they are opposite but represent the same electrical effect.

 

Question 4. Define Current.
Answer: Electric current is defined as the rate at which electric charges flow past a point in a circuit. It is a measure of how many charges pass through a specific cross-section of a conductor per unit of time. The standard unit for current is the Ampere (A). For example, if many electrons flow quickly, the current is high.
In simple words: Current is how fast electric charges move through a wire.

🎯 Exam Tip: The definition of current should always include "rate of flow" and "electric charges" along with its unit, Ampere.

 

Question 5. Define Electromotive force (e.m.f.).
Answer: Electromotive force (e.m.f.) is the maximum potential difference supplied by an electrical energy source (like a battery or generator) when no current is flowing through the external circuit. It is the work done by the source to drive one coulomb of charge completely around the circuit. The unit of e.m.f is the volt, with one volt meaning one joule of work is done per coulomb of charge. It is the energy provided by the source per unit charge.
In simple words: e.m.f. is the total push given by a power source to move charges around a circuit when nothing is connected.

🎯 Exam Tip: Distinguish e.m.f from potential difference by stating that e.m.f is measured when no current flows (open circuit), representing the maximum energy per unit charge the source can provide.

 

Question 6. Define One ohm.
Answer: One ohm (\( \Omega \)) is defined as the resistance of a conductor when a potential difference of one volt applied across its ends drives a current of one ampere through it. This unit helps quantify how much a material opposes the flow of electric current. It is derived directly from Ohm's Law (\( R = V/I \)).
In simple words: One ohm is the amount of resistance when one volt of push makes one ampere of current flow.

🎯 Exam Tip: When defining "one ohm," link it directly to Ohm's Law and the specific values of one volt and one ampere.

 

Question 7. Define Fixed resistor.
Answer: A fixed resistor is an electrical component designed to have a specific, unchanging electrical resistance. Its resistance value is constant and does not change under normal operating conditions. Common types include carbon film resistors and wire-wound resistors, which are used to limit current or divide voltage in a circuit. They are essential for maintaining stable circuit conditions.
In simple words: A fixed resistor is a tiny part in a circuit that always has the same amount of resistance.

🎯 Exam Tip: The key characteristic of a fixed resistor is its constant resistance value; mention this clearly in your definition.

 

Question 8. Define Variable resistor.
Answer: A variable resistor, also known as a rheostat or potentiometer, is an electrical component whose resistance can be adjusted or changed. It is used in circuits to vary the amount of current flowing or to control voltage levels. By sliding a contact, the effective length of the resistive material can be changed, thereby altering the resistance. These are often found in dimmer switches or volume controls.
In simple words: A variable resistor is a part in a circuit where you can change how much it resists the flow of electricity.

🎯 Exam Tip: Emphasize that a variable resistor's resistance can be adjusted and its main uses are varying current or voltage.

 

Question 9. Define Electrolysis and electrolyte.
Answer:

• **Electrolysis:** This is the process of using electric current to bring about a non-spontaneous chemical reaction. It involves passing an electric current through a substance (an electrolyte) to cause chemical decomposition.
• **Electrolyte:** An electrolyte is a solution or molten substance that contains free ions, making it capable of conducting electricity. During electrolysis, the electricity passes through this solution or molten substance.

This process is used in various applications, like electroplating and producing certain chemicals.
In simple words: Electrolysis is when electricity is used to break down a chemical substance. The substance that electricity passes through to do this is called an electrolyte.

🎯 Exam Tip: Clearly define both terms and explain their relationship, emphasizing that an electrolyte is the medium through which electrolysis occurs.

 

Question 10. Define Synaptic signals.
Answer: Synaptic signals are extremely weak electric currents produced in the human body, specifically in the nervous system. These signals are generated by the movement of charged particles (ions) across nerve cell membranes at junctions called synapses. Synaptic signals are crucial for communication between neurons, allowing information to travel through the brain and the nervous system, enabling thoughts, movements, and sensations. These bioelectric signals are quite complex.
In simple words: Synaptic signals are very small electrical messages that help brain cells talk to each other, sending information throughout our body.

🎯 Exam Tip: When describing synaptic signals, focus on their electrical nature, their role in communication between neurons, and their location in the nervous system.

 

Question 11. Define Magnetic effect of current.
Answer: The magnetic effect of current states that whenever an electric current flows through a wire or conductor, it creates a magnetic field around it. This magnetic field is perpendicular to the direction of the flow of current. This phenomenon is fundamental to electromagnetism and is used in various devices like electromagnets, motors, and generators. For example, a compass needle will deflect if placed near a current-carrying wire.
In simple words: Magnetic effect of current means that when electricity flows through a wire, it creates a magnetic area around that wire.

🎯 Exam Tip: Remember to state that the magnetic field is created *around* the conductor and is *perpendicular* to the current flow.

 

Question 12. Define Frequency.
Answer: Frequency, in the context of alternating current (AC), is defined as the number of complete cycles of variation that occur in one second. It tells us how many times the current (or voltage) reverses its direction in a given time period. The unit of frequency is Hertz (Hz), where 1 Hz means one cycle per second. For example, household AC in many countries has a frequency of 50 Hz or 60 Hz.
In simple words: Frequency is how many full times an alternating current changes its direction back and forth in one second.

🎯 Exam Tip: Clearly state that frequency measures the "number of complete cycles" and its unit is Hertz (Hz), relating it to time in seconds.

 

Question 13. Define Rectifier.
Answer: A rectifier is an electronic device that converts alternating current (AC) into direct current (DC). AC periodically changes direction, but many electronic components require a steady, unidirectional flow of current (DC). Rectifiers use diodes to allow current to flow in only one direction, effectively converting the oscillating AC waveform into a pulsating DC waveform. This conversion is vital for powering most electronic devices from the AC mains supply.
In simple words: A rectifier is a device that changes alternating current (AC) from your wall socket into direct current (DC) that most electronics need.

🎯 Exam Tip: The core function of a rectifier is AC to DC conversion; mention this and the use of diodes in the process.

 

Question 14. Define Resistance.
Answer: Resistance is the measure of the opposition offered by a material or component to the flow of electric current through it. When current flows through a conductor, electrons collide with atoms, losing some energy as heat. A higher resistance means more opposition and less current for a given voltage. The unit of resistance is the Ohm (\( \Omega \)). All materials have some resistance, though good conductors have very little.
In simple words: Resistance is how much a material tries to stop electricity from flowing through it.

🎯 Exam Tip: Explain resistance as "opposition to current flow" and remember to state its unit, Ohm (\( \Omega \)).

 

Question 15. Define Resistors.
Answer: Resistors are electrical components specifically designed to provide a certain amount of resistance in a circuit. They are used to control the flow of current, divide voltage, or dissipate electrical energy as heat. Resistors come in various types and values, and they are essential for protecting other sensitive components and ensuring a circuit operates correctly. Their value is measured in ohms.
In simple words: Resistors are small parts used in circuits to add resistance and control how much electricity flows.

🎯 Exam Tip: Differentiate resistors (the component) from resistance (the property). Explain their purpose in controlling current and voltage.