Selina Concise Solutions for ICSE Class 9 Physics Chapter 9 Current Electricity

Exercise 9(A)

Question 1S. Name a source of D.C. and A.C.
Answer: Source of D.C.: Cell
Source of A.C.: Mains
In simple words: A battery (cell) provides a steady flow of electricity in one direction, while the power from your wall sockets (mains) switches direction many times a second.

📝 Teacher's Note: Use the analogy of a one-way street for D.C. and a pendulum swinging back and forth for A.C. to help students visualize the flow.

🎯 Exam Tip: Remember that "D.C." stands for Direct Current and "A.C." stands for Alternating Current.

Question 2S. Differentiate between direct current (D.C.) and alternating current (A.C.).
Answer: Direct current (D.C.) is a current of constant magnitude flowing in one direction but alternating current (A.C.) is a current which reverses its magnitude and direction with time.
In simple words: D.C. is like a steady stream of water always moving forward. A.C. is like a wave that moves up and down and back and forth.

📝 Teacher's Note: Draw a straight-line graph for D.C. and a sine-wave graph for A.C. to show the difference in magnitude and direction over time.

🎯 Exam Tip: The key difference is that A.C. "reverses" while D.C. remains "constant" in direction.

Question 3S. What is an electric cell?
Answer: An electric cell is a device which converts chemical energy into electrical energy. When connected in a circuit, it acts as a source of D.C. current.
In simple words: A cell is like a small chemical factory that produces a steady push of electricity when you plug it into a device.

📝 Teacher's Note: Emphasize that a cell doesn't store "electricity" itself, but stores chemical energy that is released as electricity when needed.

🎯 Exam Tip: Mention the specific energy conversion (chemical to electrical) to get full marks.

Question 4S. State the energy conversion that takes place in an electric cell.
Answer: Chemical energy changes into electrical energy.
In simple words: The energy inside chemicals is used up to make electricity flow.

📝 Teacher's Note: Use the example of a battery getting "drained" as the chemicals inside are used up.

🎯 Exam Tip: This is a very common one-mark question. Be concise.

Question 5S. What are the constituents of an electric cell?
Answer: Constituents of cell: Two electrodes and an electrolyte in a vessel.
In simple words: A cell needs two metal rods (electrodes) dipped in a special chemical liquid or paste (electrolyte).

📝 Teacher's Note: Explain that the chemical reaction between the electrodes and the electrolyte creates the potential difference.

🎯 Exam Tip: Ensure you name all three components: the two electrodes and the electrolyte.

Question 6S. Name the two kinds of cells with examples.
Answer: Two kinds of cells:
1. Primary cell: e.g. Leclanche cell
2. Secondary cell: Lead (or acid) accumulator
In simple words: Primary cells are "use and throw," while secondary cells can be recharged and used again.

📝 Teacher's Note: Relate this to everyday items: a TV remote uses primary cells, while a mobile phone uses a secondary cell (rechargeable battery).

🎯 Exam Tip: Memorize one specific example for each type of cell.

Question 7S. What are primary cells? Give examples.
Answer: Primary cells are cells which provide current as a result of irreversible chemical current. Examples: Simple Voltaic cell and Leclanche cell.
In simple words: These are batteries that can't be recharged. Once the chemicals inside are done reacting, the battery is dead.

📝 Teacher's Note: Focus on the word "irreversible"—this is the scientific reason they cannot be recharged.

🎯 Exam Tip: Don't forget to include the examples like Voltaic or Leclanche cells.

Question 8S. What are secondary cells? Give an example.
Answer: Secondary cells are cells which provide current as a result of reversible chemical reactions. It converts electrical energy into chemical energy when current is passed in it (i.e. during charging), while it converts chemical energy into electrical energy when current is drawn from it (i.e., during discharging). Example: Lead (or acid) accumulator.
In simple words: These are rechargeable batteries. You can "refill" their energy by plugging them into a charger.

📝 Teacher's Note: Explain the two phases: "charging" (electrical to chemical) and "discharging" (chemical to electrical).

🎯 Exam Tip: The key keyword is "reversible chemical reaction."

Question 9S. Differentiate between a primary cell and a secondary cell.
Answer:

In simple words: Primary cells are permanent chemical reactions (one-way), while secondary cells can flip their chemistry back and forth to be recharged.

📝 Teacher's Note: Internal resistance is an important practical difference. High resistance in primary cells means they can't provide very large currents compared to accumulators.

🎯 Exam Tip: Using a table is the best way to present differences in exams to score full marks.

Question 10S. Define electric current. Is it a scalar or vector? State its S.I. unit.
Answer: Current is the rate of flow of charge across a cross-section. It is a scalar quantity. Its S.I. unit is ampere (coulomb per second). If 1 ampere current flows through a conductor, it means that \( 6.25 \times 10^{18} \) electrons pass in 1 second across that cross-section of conductor.
In simple words: Current is a count of how many electrons zoom past a point in a wire every second. If that number is huge (\( 6.25 \times 10^{18} \)), we call it 1 Ampere.

📝 Teacher's Note: Even though current has a direction in a circuit, it follows the laws of scalar addition (Kirchhoff's Current Law), which is why it is a scalar quantity.

🎯 Exam Tip: Remember the value \( 6.25 \times 10^{18} \) electrons—it's often asked in objective questions.

Question 11S. What is the charge on an electron?
Answer: Charge on an electron is \( -1.6 \times 10^{-19} \) coulomb.
In simple words: Electrons carry a tiny, tiny amount of negative electrical charge.

📝 Teacher's Note: This is a fundamental constant. The negative sign represents the type of charge, while the number represents its magnitude.

🎯 Exam Tip: Be very careful with the power of ten (\( 10^{-19} \)).

Question 12S. State the formula for electric current in terms of number of electrons.
Answer: Current, \( I = \frac{ne}{t} \)
In simple words: To find the current, you multiply the number of electrons (\( n \)) by the charge of one electron (\( e \)) and divide by the time (\( t \)) they took to flow.

📝 Teacher's Note: This formula comes from combining \( I = \frac{q}{t} \) and \( q = ne \). It relates the macroscopic current to the microscopic particles.

🎯 Exam Tip: In numericals, if 'n' (number of electrons) is asked, rearrange the formula to \( n = \frac{I \times t}{e} \).

Question 13S. What is the use of a rheostat?
Answer: A rheostat is used to control current in an electric circuit.
In simple words: A rheostat is like a "dimmer switch" for current. You can slide it to make the electricity flow faster or slower.

📝 Teacher's Note: It works by changing the length of the wire in the circuit, which changes the resistance.

🎯 Exam Tip: A rheostat is also known as a variable resistor.

Question 14S. Define the functions of the following circuit components: Ammeter, Cell, Key, Load, Voltmeter, Rheostat.
Answer:
A: Ammeter – It measures the current flowing through the circuit.
B: Cell – It acts as a source of direct current for the circuit.
C: Key – It is used to put the current on and off in the circuit.
D: Load – It is an appliance connected in a circuit. It may just be a resistance (e.g., bulb) or a combination of different electrical components.
E: Voltmeter – It is used to measure the potential difference between two points of a circuit.
F: Rheostat – It is used to control the current in the circuit.
In simple words: This is like a toolkit for electricity. You have a source (cell), a switch (key), things to use the power (load), and tools to measure it (ammeter and voltmeter).

📝 Teacher's Note: Emphasize the connection type: Ammeters are always connected in series, and Voltmeters are always connected in parallel.

🎯 Exam Tip: Note that an Ammeter measures "Current" (Flow) and a Voltmeter measures "Potential Difference" (Pressure).

Question 15S. What is the function of a key or switch?
Answer: A key or switch is used to put on or off, the current in the circuit.
In simple words: It acts like a gate. When the gate is closed, electricity flows. When the gate is open, it stops.

📝 Teacher's Note: Use the term "broken" or "discontinuous" to describe an open switch.

🎯 Exam Tip: Be careful! In physics, "Closed" means the switch is ON, and "Open" means the switch is OFF.

Question 16S. Draw the symbols for: (i) Key, (ii) Cell, (iii) Rheostat, (iv) Ammeter, (v) Voltmeter.
Answer:
(i) Key symbol : It is used to switch the current on or off in the circuit.
(ii) Cell symbol : It acts as a source of direct current for the circuit.
(iii) Rheostat symbol : or It is used to control the current in the circuit.
(iv) Ammeter : It is used to measure the potential difference between two points of a circuit.
In simple words: These are the "drawings" engineers use to plan a circuit. Each one tells you exactly what kind of part to use.

📝 Teacher's Note: Have students practice drawing these symbols repeatedly until they can do them perfectly from memory.

🎯 Exam Tip: For the cell, remember the long thin line is positive (+) and the short thick line is negative (-).

Question 18S. Define conductors and insulators with examples.
Answer: The substances which allow electric current to flow through them easily are called conductors. Examples: Impure water and metals. The substances which do not allow the electric current to flow through them are called insulators. Examples: Rubber and wood.
In simple words: Conductors are like wide-open doors for electricity. Insulators are like solid brick walls that block electricity.

📝 Teacher's Note: Mention that metals are good conductors because they have lots of "free electrons."

🎯 Exam Tip: Be sure to provide at least two examples for each to get full marks.

Question 19S. Name three conductors.
Answer: Copper wire, acidulated water and human body.
In simple words: Copper (in wires), water with a little acid, and even our own bodies let electricity pass through.

📝 Teacher's Note: This is why it's dangerous to touch electrical wires—electricity can flow through you to the ground.

🎯 Exam Tip: "Acidulated water" means water mixed with a small amount of acid to make it conduct better.

Question 20S. Why do conductors allow current to flow while insulators do not?
Answer: Conductors have a large number of free electrons and they offer a very small resistance in the path of current but insulators have no free electrons and they offer a very high resistance in the path of current.
In simple words: Conductors have tiny "loose" parts called free electrons that can move easily. Insulators hold onto their parts so tightly that nothing can move.

📝 Teacher's Note: Resistance is the measure of how much a material "fights" the flow of electricity.

🎯 Exam Tip: The presence of "free electrons" is the crucial technical term to use in your answer.

Question 21S. Differentiate between a closed circuit and an open circuit.
Answer: A circuit is said to be closed when every part of it is made of a conductor and on plugging in the key or on being complete, current flows through the circuit. A circuit is said to be open when no current flows through it. It can happen when the key is not plugged in or when any one of its components is not made of a conductor or when the circuit is broken.
In simple words: A closed circuit is a perfect loop. An open circuit has a gap, so the electricity stops.

📝 Teacher's Note: Use the analogy of a bridge over a river. If the bridge is up (Open), the cars (current) cannot cross.

🎯 Exam Tip: Remember: Closed = Current flows; Open = No current.

Question 22S. What is required for an electric circuit to be closed?
Answer: For an electric circuit to be closed, every part of it must be made of conductors.
In simple words: Every piece of the loop must be a material that electricity likes, like copper wire. One piece of rubber would break the flow.

📝 Teacher's Note: This is why wire insulation is made of rubber/plastic—it ensures the conductor stays within its path.

🎯 Exam Tip: Every single component must be a conductor for the circuit to be functional.

Question 1M. The main purpose of a cell in a circuit is to:
(a) measure current
(b) provide current in a circuit
(c) increase resistance
(d) None of the options
Answer: (b) provide current in a circuit
In simple words: The cell is the "heart" or the engine that pushes the electricity through the wires.

📝 Teacher's Note: A cell provides the necessary electromotive force (EMF).

🎯 Exam Tip: A cell is a source of direct current (D.C.).

Question 2M. The unit of current is:
(a) volt
(b) ampere
(c) ohm
(d) coulomb
Answer: (b) ampere
In simple words: We count the flow of electricity in "Amps."

📝 Teacher's Note: 1 Ampere = 1 Coulomb per second.

🎯 Exam Tip: Ampere is the S.I. unit. Be careful with spelling.

Question 3M. Which of the following is an insulator?
(a) copper
(b) iron
(c) silk
(d) silver
Answer: (c) silk
In simple words: Unlike metals, silk does not allow electricity to pass through it.

📝 Teacher's Note: Silk was historically used to insulate wires before plastic was common.

🎯 Exam Tip: Most non-metals (except graphite) are insulators.

Question 1N. If a charge of 0.5 C flows through a wire in 5 seconds, find the current.
Answer: Current (I) = Charge (q)/time (t)
Or, \( I = 0.5/ 5 = 0.1 \text{ A} \)
In simple words: Just divide the total charge by the time to find out the speed of the flow.

📝 Teacher's Note: Use the formula \( I = \frac{q}{t} \). Ensure students understand that 't' must always be in seconds.

🎯 Exam Tip: Always write the formula first to secure partial marks even if the calculation is wrong.

Question 2N. A current of 1.5 A flows for 2 seconds. Calculate the total charge.
Answer: Charge (q) = Current (I) \( \times \) time (t)
Or, \( q = 1.5 \times 2 = 3 \text{ C} \)
In simple words: If 1.5 units of electricity flow every second, then in 2 seconds, a total of 3 units have passed by.

📝 Teacher's Note: Charge (\( q \)) is measured in Coulombs (C).

🎯 Exam Tip: Rearranging the formula correctly is the key step.

Question 3N. If a charge of 24 C passes in 0.8 seconds, calculate the current.
Answer: Current (I) = Charge (q)/time (t)
Or, \( I = 24/ 0.8 = 30 \text{ A} \)
In simple words: That is a lot of electricity moving very fast! The current is 30 Amperes.

📝 Teacher's Note: This is a standard calculation. Note the division by a decimal (\( 24 / 0.8 \)), which often trips up students.

🎯 Exam Tip: Check your final unit: Ampere (A).

Exercise 9(B)

Question 1S. What happens when two charged conductors are joined by a metal wire?
Answer: When both the conductors are joined by a metal wire:
1. Electrons will flow from A to B.
2. Current will flow from B to A.
In simple words: Electrons (tiny parts of electricity) move one way, but scientists historically decided to say the current flows the opposite way. It's like a mirror image.

📝 Teacher's Note: This is the concept of "Conventional Current." It flows from higher potential to lower potential.

🎯 Exam Tip: Always remember that current flow is opposite to electron flow.

Question 2S. In which direction does current always flow?
Answer: Current always flows from high potential to low potential.
In simple words: Just like water flows from high hills to low valleys, electricity flows from the "high pressure" side to the "low pressure" side.

📝 Teacher's Note: Potential is basically "electrical pressure."

🎯 Exam Tip: High potential is usually the positive (+) terminal of a battery.

Question 3S. Define electric potential difference.
Answer: Electric potential difference between two conductors is equal to the work done in transferring a unit positive charge from one conductor to other conductor.
In simple words: It is a measure of how much "work" or energy is needed to move a packet of electricity between two points.

📝 Teacher's Note: Potential difference is the reason current flows. No difference = no flow.

🎯 Exam Tip: The keyword here is "work done per unit positive charge."

Question 4S. What is the formula for electric potential difference?
Answer: Electric potential difference is the difference in electric potential (V) between the final and the initial location when work is done upon a charge to change its potential energy. In equation form, the electric potential difference is:
\[ \Delta V = V_B - V_A = \frac{\text{Work}}{\text{Charge}} = \frac{\Delta PE}{\text{Charge}} \]
In simple words: You find the potential difference by dividing the effort (work) you spent moving the electricity by the amount of electricity (charge) you moved.

📝 Teacher's Note: \( \Delta PE \) stands for change in Potential Energy.

🎯 Exam Tip: Note that \( V = \frac{W}{q} \).

Question 5S. State the S.I. unit of potential difference and define 1 volt.
Answer: S.I. unit of potential difference is volt (joule per coulomb). Potential difference between two points is said to be 1 volt if work done in transferring 1 coulomb of charge from one point to the other point is 1 joule.
In simple words: A "Volt" is the push you get when you spend 1 unit of energy (Joule) to move 1 unit of electricity (Coulomb).

📝 Teacher's Note: Use a simple 1:1:1 ratio analogy to help students memorize definitions of units.

🎯 Exam Tip: Always use the phrase "per unit charge" or "1 coulomb of charge" in your definition.

Question 6S. What does it mean when the potential difference between two points is 1 volt?
Answer: Potential difference between two points is 1 volt; it means 1 joule of work is done in transferring 1 coulomb of charge from one point to the other point.
In simple words: It means it took exactly 1 small burst of energy to move a packet of charge between those spots.

📝 Teacher's Note: This is a standard interpretive question in physics exams.

🎯 Exam Tip: Frame your answer as "amount of work done per unit charge."

Question 7S. What is electrical resistance?
Answer: The obstruction offered to the flow of current by the filament or wire is called its electrical resistance.
In simple words: Resistance is like the "friction" for electricity. It's how much a wire tries to stop the current from moving.

📝 Teacher's Note: Thinner and longer wires generally have higher resistance.

🎯 Exam Tip: Define it as the "opposition" or "obstruction" to current.

Question 8S. Explain the cause of resistance in a metal wire.
Answer: A metal wire has free electrons which move in random directions. When the ends of the wire are connected to a cell, the electrons start moving from the negative terminal of the cell to its positive terminal through the metal wire. During their movement, they collide with the free electrons and fixed ions of the wire. This causes them to lose their speed and change their direction. As a result, the electrons slow down and slowly drift towards the positive terminal. Thus, the wire offers resistance to the flow of current (or electrons) through it.
In simple words: Imagine trying to run through a crowded room. You keep bumping into people (ions), which slows you down. That "bumping" is what we call resistance.

📝 Teacher's Note: These collisions also generate heat, which is why wires get warm when used.

🎯 Exam Tip: "Collision with fixed ions" is the most important part of this explanation.

Question 9S. State the S.I. unit of resistance and define 1 ohm.
Answer: The S.I. unit of resistance is ‘ohm’ (volt per ampere). The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when the potential difference across it is 1 volt.
In simple words: An "Ohm" is the resistance you get when 1 Volt of push results in 1 Amp of flow.

📝 Teacher's Note: Show the symbol \( \Omega \) (Omega) used for Ohms.

🎯 Exam Tip: Base your definition on the Ohm's Law formula \( R = \frac{V}{I} \).

Question 10S. State Ohm’s law.
Answer: Ohm’s law states that the electric current flowing through a metallic wire is directly proportional to the potential difference V across its ends provided its temperature remains the same.
In simple words: Pushing harder (more Volts) makes the electricity move faster (more Amps), as long as the wire doesn't get too hot.

📝 Teacher's Note: Emphasize the "constant temperature" condition—Ohm's law only works if the environment stays steady.

🎯 Exam Tip: Mention "constant temperature" to get full marks. This is a very common trap.

Question 11S. Write the mathematical expression for Ohm's law.
Answer: Potential difference = Current \( \times \) Resistance
i.e., \( V = I R \)
In simple words: Pressure (V) equals Flow (I) times Resistance (R). If you know two, you can always find the third.

📝 Teacher's Note: Use the "V-I-R Triangle" to help students remember the three variations of this formula.

🎯 Exam Tip: This formula is the most important one in all of basic electronics.

Question 12S. What is the meaning of 1 ohm resistance?
Answer: The resistance of a wire is 1 ohm; it means a current of 1 ampere will flow through the wire when the potential difference across it is 1 volt.
In simple words: It means the wire is just resistive enough to let 1 unit of electricity through for every 1 unit of push.

📝 Teacher's Note: This is a standard conceptual question.

🎯 Exam Tip: Define it using values of current and voltage.

Question 13S. How does doubling the resistance affect the current in a circuit?
Answer: \( V = IR \) or, \( I = V/R \)….(i)
If R is doubled,
Then, \( I' = V/2R = I/2 \)….(ii)
From (i) and (ii), it is clear that current will be halved.
In simple words: If you make it twice as hard for electricity to move, only half as much will be able to get through.

📝 Teacher's Note: This is an "Inversely Proportional" relationship. Resistance goes up, current goes down.

🎯 Exam Tip: Clearly state that current is inversely proportional to resistance for a given voltage.

Question 14S. Upon what factors does the resistance of a wire depend?
Answer: Resistance of a wire depends upon:
1. Length of wire: Resistance is directly proportional to the length of a wire.
2. Area of cross-section of wire: Resistance is inversely proportional to the area of cross-section the wire.
3. The temperature of wire: Resistance of a wire is directly proportional to the temperature of the wire.
In simple words: Long wires, thin wires, and hot wires all have more resistance. It's harder for electricity to move through a long, narrow, hot path.

📝 Teacher's Note: Compare this to water in a pipe: a longer pipe or a thinner pipe is harder to push water through.

🎯 Exam Tip: Note the difference: length is "directly" proportional while area is "inversely" proportional.

Question 15S. Describe the effect of (a) length and (b) radius on the resistance of a wire.
Answer: (a) Resistance of a wire is directly proportional to the length of a wire; so if the length is doubled, resistance is also doubled.
(b) Resistance of a wire is inversely proportional to the area of cross-section the wire. Thus, if radius is doubled, area increases four times and hence the resistance becomes one-fourth.
In simple words: Longer wire = more resistance. Fatter wire = much less resistance.

📝 Teacher's Note: Explain that area (\( A \)) depends on the square of the radius (\( A = \pi r^2 \)), which is why doubling the radius has such a huge (4x) effect.

🎯 Exam Tip: Be careful with the radius—remember that area changes by the "square" of the radius.

Question 16S. Why does resistance increase with temperature?
Answer: The temperature of the filament increases when it glows. So, when the temperature of the wire (bulb filament) increases, ions in it vibrate violently. As a result, the number of collisions increases and hence the resistance increases.
In simple words: Hot wires jiggle more. This makes it harder for electrons to pass through without bumping into things, which slows them down and increases resistance.

📝 Teacher's Note: This is why the starting current of a bulb is different from its working current once it gets hot.

🎯 Exam Tip: Use the term "violent vibrations of ions" to explain the increase in collisions.

Question 17S. Fill in the blanks:
(i) ______ determines the flow of current.
(ii) ______ flows in a circuit.
(iii) ______ is measured in ohms.
(iv) ______ is measured in amperes.
Answer: (i) Potential difference (ii) charge (iii) resistance (iv) current.

📝 Teacher's Note: These are fundamental concept associations.

🎯 Exam Tip: Review unit-quantity pairs regularly.

Question 1M. In a circuit, current flows in the direction:
(a) from high potential to low potential
(b) from low potential to high potential
(c) from positive to positive
(d) None of the options
Answer: (a) In direction from high potential to low potential.
In simple words: Current is like a waterfall; it always drops from the high point to the low point.

📝 Teacher's Note: "High potential" refers to the positive terminal.

🎯 Exam Tip: This is a standard direction rule for conventional current.

Question 2M. S.I. unit of potential difference is:
(a) ohm
(b) ampere
(c) volt
(d) watt
Answer: (c) volt
In simple words: Volts measure the electrical "pressure" that makes current move.

📝 Teacher's Note: Named after Alessandro Volta.

🎯 Exam Tip: Do not confuse Volt (Pressure) with Watt (Power).

Question 3M. On increasing the resistance, the current:
(a) Increases
(b) Decreases
(c) Stays the same
(d) None of the options
Answer: (b) Decreases
In simple words: Resistance is a block. More blocks mean less flow.

📝 Teacher's Note: This is a direct consequence of \( I = V/R \).

🎯 Exam Tip: Always relate current to resistance inversely.

Question 1N. If 9 Joules of work is done to move 1.5 Coulombs of charge, find the potential difference.
Answer: Potential difference (V) = work done (W) / charge (q)
Or, \( V = 9/1.5 = 6 \text{ volt} \).
In simple words: Divide the total effort by the amount of charge to find the Volts.

📝 Teacher's Note: Standard plug-and-play problem for \( V = \frac{W}{q} \).

🎯 Exam Tip: Mention the unit "volt" in the final answer.

Question 2N. A wire has a resistance of 24 Ohms. If it is connected to a 12V battery, calculate the current.
Answer: Given, potential difference (V) = 12 V
Resistance, R = 24 \( \Omega \)
Therefore, current (I) = V / R
Or, \( I = 12/24 = 0.5 \text{ A} \)
In simple words: Divide the push (12) by the resistance (24) to find the flow, which is half an Amp.

📝 Teacher's Note: This uses \( I = V/R \).

🎯 Exam Tip: Ensure your arithmetic is correct; 12/24 is 0.5, not 2.

Question 3N. In a circuit, current is 1.5 A and potential difference is 6.0 V. Calculate the resistance.
Answer: I = 1.5 A
V = 6.0 V
According to Ohm's law,
\( V = IR \)
\( \therefore R = \frac{V}{I} = \frac{6.0}{1.5} = 4.0 \Omega \)
In simple words: To find the resistance, divide the voltage (6) by the current (1.5). The answer is 4 Ohms.

📝 Teacher's Note: Simple application of \( R = V/I \).

🎯 Exam Tip: Use the Ohm symbol \( \Omega \) to denote resistance.

Question 4N. Current in a circuit is 0.2 A and resistance is 15 Ohms. Find the potential difference.
Answer: I = 0.2 A
R = 15 \( \Omega \)
According to Ohm's law,
\( V = IR \)
\( \therefore V = 0.2 \times 15 = 3.0 \text{ V} \)
In simple words: Multiply the flow by the resistance to find the total push (Voltage).

📝 Teacher's Note: Third variation of Ohm's Law.

🎯 Exam Tip: Basic multiplication: \( 0.2 \times 15 = 3 \).

Exercise 9(C)

Question 1S. What is meant by efficient use of energy?
Answer: Efficient use of energy means to reduce cost and amount of energy to be used to provide us the various products and services.
In simple words: It means getting the same job done (like lighting a room) while using less power and spending less money.

📝 Teacher's Note: This is about "doing more with less."

🎯 Exam Tip: Focus on "reduction of cost" and "reduction of amount of energy."

Question 2S. State two ways to save energy.
Answer: Two ways to save energy:
1. Instead of fossil fuels, other renewable sources of energy such as the biogas prepared from animal dung should be used.
2. The use of hydroelectric energy, wind energy etc. should be given priority.
In simple words: We can save energy by using natural sources like wind, sun, and water instead of burning limited stuff like coal and oil.

📝 Teacher's Note: Discuss the difference between energy "conservation" and using "renewable" energy.

🎯 Exam Tip: Mention specific alternatives like "biogas" or "wind energy."

Question 3S. How does home insulation help in saving energy?
Answer: By properly insulating a home, it is possible to maintain a comfortable temperature inside. It will reduce the cost of heating devices in winter and cooling devices in summer.
In simple words: Insulation is like a thermos for your house. It keeps the warm air in during winter and out during summer so you don't have to use AC or heaters as much.

📝 Teacher's Note: Insulation prevents heat transfer through walls and roofs.

🎯 Exam Tip: Link insulation to the "reduction of heating/cooling costs."

Question 4S. Which lighting device is most efficient?
Answer: LED or light emitting diodes are most efficient for lighting purposes.
In simple words: LEDs use much less power to give the same bright light compared to old-fashioned bulbs.

📝 Teacher's Note: Contrast LEDs with incandescent bulbs, which waste 90% of energy as heat.

🎯 Exam Tip: LED stands for Light Emitting Diode.

Question 5S. What is the benefit of modern star-rated appliances?
Answer: Modern appliances like refrigerators make use of significantly less energy than older appliances as they have star rating according to their efficient use of electricity. Higher the star rating, higher is the efficiency.
In simple words: More stars on the sticker mean the machine is smarter about using electricity and will save you money on your bills.

📝 Teacher's Note: Use the example of a 5-star fridge vs a 2-star fridge to explain energy conservation.

🎯 Exam Tip: "Higher star rating = Higher efficiency" is the key logic.

Question 6S. List three ways to use energy efficiently.
Answer: Three ways to use energy efficiently:
1. The use of compact fluorescent lights (CFL) saves 67% energy and may last 6 to 10 times longer than the incandescent lamps.
2. The use of advanced boilers and furnaces in industry can save sufficient amount of energy in attaining high temperatures while burning less fuel. Such technologies are more efficient and less polluting.
3. The fuel efficiency in the vehicles can be increased by reducing the weight of the vehicle, using the advanced tyres and computer controlled engines.
In simple words: Use better light bulbs (CFLs), modern factory machines that burn less fuel, and lighter cars with smart engines.

📝 Teacher's Note: This covers domestic, industrial, and transportation sectors.

🎯 Exam Tip: Provide diverse examples across different fields (Lighting, Industry, Vehicles).

Question 7S. What social initiatives can help reduce electricity misuse?
Answer: The following social initiatives need to be taken:
1. Public awareness can be improved through mass-media and children’s participation in campaigns and eco-club activities.
2. Community involvement need to be done to reduce the misuse of electricity.
3. NGO’s can be used to create social awareness of the sensitive use of resources.
In simple words: We need to teach people to be careful with power using TV ads and school clubs, and groups like NGOs should help spread the word.

📝 Teacher's Note: This is the "Education and Awareness" part of energy policy.

🎯 Exam Tip: Mention "Eco-clubs" and "Public awareness" as key points.

Question 1M. The most non-polluting and efficient lighting device is:
(a) Bulb
(b) CFL
(c) LED
(d) None of the options
Answer: (c) LED
In simple words: LEDs are the champions of clean and efficient lighting today.

📝 Teacher's Note: LEDs don't contain mercury (unlike CFLs) and don't get hot (unlike bulbs).

🎯 Exam Tip: LED is the standard answer for modern efficiency.

Question 2M. IEA is the short form of:
Answer: international energy agency.
In simple words: This is a worldwide group that helps countries manage their energy needs.

📝 Teacher's Note: They are based in Paris and provide statistics and advice on energy policy.

🎯 Exam Tip: Be sure to write the full name correctly: International Energy Agency.