Advanced Study Material for Class 10 Science: Chapter 11 Electricity
Review targeted academic resources with the CBSE Class 10 Science Electricity Sure Shot Questions Set 01. Built according to official CBSE standards for the 2026-27 term, these downloadable Class 10 Science study materials support effective daily practice and deeper conceptual understanding for Chapter 11 Electricity.
Practice Class 10 Science Resources: Chapter 11 Electricity
Access the complete useful resource PDF for Class 10 Science below. Regular practice with these targeted academic notes builds familiarity with complex topics and helps secure higher marks in final school evaluations.
CBSE Class 10 Science Chapter 12 Electricity Very Short Answer Type Questions
VERY SHORT ANSWER TYPE QUESTIONS (1 MARK)
1. Define Electrostatic potential. [1 Mark]
Answer:
1. Electrostatic potential at any point in an electric field is defined as the amount of work done in bringing a unit positive test charge from infinity to that point against electrostatic forces.
2. It is denoted by V and its SI unit is volt.
Teacher's Note:
a) It represents the potential energy per unit charge at a specific point.
b) Always mention work done per unit charge to secure full credit.
2. What is potential difference? [1 Mark]
Answer:
1. Potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit positive charge from one point to the other.
2. Formula: V = \(\frac{W}{Q}\)
Teacher's Note:
a) It is the driving force that pushes electrons through a conductor.
b) Potential difference is always measured between two distinct points.
3. Define 1 volt. [1 Mark]
Answer:
1. Potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to the other.
2. Mathematically, 1 V = 1 J / 1 C.
Teacher's Note:
a) Standard definition derived directly from the formula V = W / Q.
b) Ensure units (joule and coulomb) are explicitly stated in the definition.
4. Name the SI unit of potential difference. [1 Mark]
Answer:
1. The SI unit of potential difference is volt (V).
Teacher's Note:
a) Named in honor of the Italian physicist Alessandro Volta.
b) Capital letter V is used as the symbol for the unit.
5. Is potential difference a scalar or a vector quantity? [1 Mark]
Answer:
1. Potential difference is a scalar quantity.
Teacher's Note:
a) It has magnitude only and no specific direction associated with it.
6. Name the instrument used to measure potential difference. [1 Mark]
Answer:
1. Voltmeter.
Teacher's Note:
a) Always connected in parallel across the components where potential difference is to be measured.
7. Does a voltmeter have a high or low resistance? [1 Mark]
Answer:
1. A voltmeter has a very high resistance.
Teacher's Note:
a) High resistance ensures that negligible current draws through the voltmeter so it does not alter the circuit current.
8. Write the other name of Joule/Coulomb? [1 Mark]
Answer:
1. Volt.
Teacher's Note:
a) Since V = W / Q, the unit J/C is dimensionally equivalent to volts.
9. How much work is done when one coulomb of charge moves against a potential difference of 1 volt? [1 Mark]
Answer:
1. Work done (W) = Q × V
2. W = 1 C × 1 V = 1 Joule.
Teacher's Note:
a) Direct application of the work-charge-potential relation.
10. Name the SI unit of electric current. [1 Mark]
Answer:
1. Ampere (A).
Teacher's Note:
a) Named after the French physicist André-Marie Ampère.
11. Is electric current a scalar or vector quantity? [1 Mark]
Answer:
1. Electric current is a scalar quantity.
Teacher's Note:
a) Although it has magnitude and direction, it does not obey the laws of vector addition.
12. Name the instrument used to measure electric current. [1 Mark]
Answer:
1. Ammeter.
Teacher's Note:
a) Connected in series with the circuit component to measure total current flowing through it.
13. Does an ammeter have low or a high resistance? [1 Mark]
Answer:
1. An ammeter has very low resistance.
Answer:
1. Rheostat.
Teacher's Note:
a) Used in circuits to controllably change current without changing voltage source.
15. How do we connect an ammeter in a circuit? [1 Mark]
Answer:
1. In series.
Teacher's Note:
a) Ensures the entire current passes through the ammeter.
16. Write the unit of electrical resistance. [1 Mark]
Answer:
1. Ohm (Ω).
Teacher's Note:
a) Named after German physicist Georg Simon Ohm.
17. State the factors on which the resistance of a conductor depends. [1 Mark]
Answer:
1. Length of the conductor (l), cross-sectional area (A), nature of material (resistivity, ρ), and temperature.
Teacher's Note:
a) Formula: R = \(\rho \frac{l}{A}\).
18. What is the term for the reciprocal of resistance? [1 Mark]
Answer:
1. Conductance.
Teacher's Note:
a) Its SI unit is Siemens (S) or ohm⁻¹ (mho).
19. What is the nature of the graph between I and V for a metallic conductor? [1 Mark]
Answer:
1. A straight line passing through the origin.
Teacher's Note:
a) Indicates that current is directly proportional to potential difference (Ohm's Law).
20. Does Ohm’s law hold for a copper wire? [1 Mark]
Answer:
1. Yes, provided physical conditions like temperature remain constant.
Teacher's Note:
a) Copper is an ohmic conductor under normal laboratory conditions.
21. Does Ohm's law hold for a liquid electrolyte? [1 Mark]
Answer:
1. No, Ohm's law does not hold good for a liquid electrolyte under all conditions.
2. The current-voltage relationship is non-linear for many liquid electrolytes.
Teacher's Note:
a) Ohm's law is strictly applicable only to metallic conductors at constant temperature.
b) Electrolytes show deviations due to chemical changes and polarization effects at electrodes.
22. Does Ohm's law hold for a vacuum tube? [1 Mark]
Answer:
1. No, Ohm's law does not hold for a vacuum tube.
2. Vacuum tubes are non-ohmic devices because current does not vary linearly with potential difference.
Teacher's Note:
a) Non-ohmic devices do not maintain a constant resistance value across varying voltages.
b) Child-Langmuir law governs space-charge-limited currents in vacuum tubes instead of Ohm's law.
23. Following are the I vs V graphs for a (a) metallic conductor (b) liquid electrolyte and (c) vacuum tube. In which case dows the Ohm/s law hold good? [1 Mark]
Answer:
1. Ohm's law holds good for case (a) metallic conductor.
2. Metallic conductors show a straight line I vs V graph passing through the origin.
Teacher's Note:
a) A linear I-V graph indicates constant resistance obeying Ohm's law.
b) Electrolytes and vacuum tubes yield non-linear curves.
24. What do you mean by resistance? [1 Mark]
Answer:
1. Resistance is the property of a conductor to oppose the flow of electric current through it.
2. It is defined as the ratio of potential difference applied across the ends of the conductor to the current flowing through it.
Teacher's Note:
a) The SI unit of resistance is ohm, denoted by the symbol Ω.
b) Resistance arises due to collisions of free electrons with fixed ions inside the conductor.
25. Dows the resistance of a metal change when we raise the temperature of the metal? [1 Mark]
Answer:
1. Yes, the resistance of a metal increases when its temperature is raised.
2. Higher temperature increases thermal agitation of ions, leading to more frequent collisions with electrons.
Teacher's Note:
a) The relationship is linear for moderate temperature changes given by R = R0(1 + αΔT).
b) Mentioning the increase in collision frequency is crucial for full credit.
26. Name the substance having the largest and smallest electrical resistance amongst the following: Silver, copper, nichrome, rubber, acidulated water. [1 Mark]
Answer:
1. Largest electrical resistance: Rubber.
2. Smallest electrical resistance: Silver.
Teacher's Note:
a) Rubber is an excellent insulator with extremely high resistivity.
b) Silver has the lowest resistivity among all known metals, making it the best conductor.
27. Name the best conductor of electricity. [1 Mark]
Answer:
1. Silver is the best conductor of electricity.
Teacher's Note:
a) It possesses the lowest electrical resistivity at room temperature.
b) Copper is commonly used instead of silver due to economic reasons.
28. Name an excellent insulator. [1 Mark]
Answer:
1. Rubber (or glass, ebonite) is an excellent insulator.
Teacher's Note:
a) Insulators have virtually no free electrons available for conduction.
b) They have very high resistivity values.
29. Is wood a good conductor of electricity. [1 Mark]
Answer:
1. No, wood is not a good conductor of electricity.
2. Dry wood acts as an insulator.
Teacher's Note:
a) Wet or green wood can conduct some electricity due to moisture and dissolved salts.
b) Standard dry wood has high resistance and restricts current flow.
30. When are two resistances said to be in series? [1 Mark]
Answer:
1. Two resistances are said to be in series if they are joined end-to-end consecutively.
2. The same electric current flows through each resistor when connected in series.
Teacher's Note:
a) Total equivalent resistance is the sum of individual resistances: R = R1 + R2.
b) Potential difference divides across resistors in series in direct proportion to their resistances.
31. When are two resistances said to be in parallel? [1 Mark]
Answer:
1. Two resistances are said to be in parallel if their corresponding ends are connected together at two common points.
2. The potential difference across each resistor is the same.
Teacher's Note:
a) Equivalent resistance formula is \(\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2}\).
b) Current divides inversely proportional to individual resistances.
32. How many different resistance-combinations are possible with two equal resistors, taking both of them together? [1 Mark]
Answer:
1. Two different resistance combinations are possible.
2. They are series combination and parallel combination.
Teacher's Note:
a) Series resistance equals 2R, while parallel resistance equals R / 2.
b) Taking both resistors together restricts configurations to strictly series or parallel.
33. The unit of resistance is ohm. What is the unit of conductance? [1 Mark]
Answer:
1. The SI unit of conductance is siemens (S), also known as mho (ohm⁻¹).
Teacher's Note:
a) Conductance is the reciprocal of resistance: G = 1 / R.
b) Ensure correct spelling of siemens with a capital S or lowercase when written fully.
34. Define electric power. [1 Mark]
Answer:
1. Electric power is defined as the rate at which electric energy is dissipated or consumed in an electric circuit.
2. It is also the product of potential difference and current.
Teacher's Note:
a) Formula: P = W / t = VI.
b) Emphasize energy consumed per unit time.
35. What is the SI unit of electrical power? [1 Mark]
Answer:
1. The SI unit of electrical power is watt (W).
Teacher's Note:
a) One watt is equal to one joule per second (1 J/s).
b) Larger commercial units include kilowatt (kW) and horsepower (hp).
36. Name the quantity which is the product of potential difference and current. [1 Mark]
Answer:
1. Electric power.
Teacher's Note:
a) Expressed mathematically as P = VI.
b) Derived directly from the definition of electrical work and current.
37. Express power P in terms of I and R. [1 Mark]
Answer:
1. P = I2R
Teacher's Note:
a) Derived by substituting Ohm's law V = IR into the power formula P = VI.
b) Useful for calculating power loss in transmission lines.
38. Write various formulae for electric power. [1 Mark]
Answer:
1. P = VI
2. P = I2R
3. P = \(\frac{V^2}{R}\)
Teacher's Note:
a) These three formulas are interchangeable using Ohm's law.
b) Choose the appropriate formula depending on whether current, voltage, or resistance is constant.
39. What does 220V, 100W written on an electrical appliance mean? What do you mean by power rating? [2 Marks]
Answer:
1. It means that if the appliance is operated at a potential difference of 220V, it consumes electrical energy at the rate of 100 watts.
2. Power rating refers to the maximum power consumption specification printed on an electrical appliance indicating its safe operating voltage and power consumption rate.
Teacher's Note:
a) Helps determine current draw and suitable fuse ratings.
b) Operating above rated voltage can damage the appliance.
40. Which of the following electrical appliances usually has the (a) highest (b) least, power ratings? Tube light, Electric fan, Electric heater, Immersion heater. [1 Mark]
Answer:
1. (a) Highest power rating: Electric heater / Immersion heater (typically 1000W to 2000W).
2. (b) Least power rating: Tube light (typically 40W).
3. Hence, electric/immersion heaters have the highest and tube lights have the least power ratings.
Teacher's Note:
a) Power is the rate at which electrical energy is consumed by an appliance.
b) Heating appliances draw large currents and thus have high power ratings.
c) Luminous devices like tube lights consume much less power.
41. Which of the following electrical appliances usually draws the (a) highest and (b) least, current when operated at 220V? Tube light, Electrical fan, Electrical heater, Immersion heater. [1 Mark]
Answer:
1. (a) Highest current: Electric heater / Immersion heater due to high power rating.
2. (b) Least current: Tube light due to low power rating.
3. Current is directly proportional to power for a constant voltage (\(I = \frac{P}{V}\)).
Teacher's Note:
a) Use the relation \(I = \frac{P}{V}\) to determine current drawn by each device.
b) Devices converting electrical energy entirely to heat require higher wattage.
c) Always cross-verify the voltage rating before applying the formula.
42. What do you mean by electrical energy being consumed by an electrical appliance? [1 Mark]
Answer:
1. It refers to the work done by the source of electricity in maintaining current in the circuit.
2. It represents the electrical energy converted into other forms of useful energy or heat by the appliance.
3. Given by the product of power of the appliance and the time for which it is used.
Teacher's Note:
a) Energy consumed equals power multiplied by time (\(E = P \times t\)).
b) Mention the conversion of electrical energy into other forms for full credit.
43. Write the SI unit of electrical energy. [1 Mark]
Answer:
1. The SI unit of electrical energy is Joule (J).
Teacher's Note:
a) Joule is equivalent to watt-second or volt-ampere-second.
44. Write the commercial unit of energy. [1 Mark]
Answer:
1. The commercial unit of electrical energy is Kilowatt-hour (kWh), commonly known as a 'unit'.
Teacher's Note:
a) One kWh is equal to one thousand watt-hours.
45. Define one Kilowatt-hour. [1 Mark]
Answer:
1. One Kilowatt-hour is defined as the amount of electrical energy consumed by an electrical appliance of power 1 kilowatt when it is used for 1 hour.
Teacher's Note:
a) State both the power rating (1 kW) and time (1 hour) clearly.
46. Convert 1 Kwh to MJ. [1 Mark]
Answer:
1. 1 kWh = \(1000 \text{ W} \times 3600 \text{ s}\)
2. 1 kWh = \(3,600,000 \text{ J} = 3.6 \times 10^6 \text{ J}\)
3. 1 kWh = 3.6 MJ
Teacher's Note:
a) Remember the standard conversion factor of 3.6 million joules per kilowatt-hour.
47. What is the ratio of SI units to C.G.S. unit of electrical energy? [1 Mark]
Answer:
1. The SI unit of energy is Joule (J) and the CGS unit of energy is Erg.
2. 1 Joule = \(10^7\) ergs.
3. The ratio of SI unit to CGS unit is \(10^7 : 1\).
Teacher's Note:
a) Recall that 1 J = \(10^7\) erg to establish the correct ratio.
48. What are the factors on which electrical energy consumed by an electrical appliance depends? [1 Mark]
Answer:
1. The power rating of the electrical appliance.
2. The time for which the appliance is operated.
Teacher's Note:
a) Based on the formula \(E = P \times t\).
49. What do you mean by one unit of electrical energy? [1 Mark]
Answer:
1. One unit of electrical energy means one Kilowatt-hour (kWh) of energy consumed, which equals \(3.6 \times 10^6\) Joules.
Teacher's Note:
a) Connect 'unit' directly with kWh as used in domestic electricity bills.
50. How many joules are in one watt-hour? [1 Mark]
Answer:
1. 1 Watt-hour = \(1 \text{ W} \times 3600 \text{ s}\)
2. 1 Watt-hour = 3600 Joules
Teacher's Note:
a) Multiply 1 watt by the number of seconds in one hour (3600 seconds).
51. List some important effects of electric current. [1 Mark]
Answer:
1. Heating effect of electric current.
2. Magnetic effect of electric current.
3. Chemical effect of electric current.
Teacher's Note:
a) List at least three primary effects clearly to get full credit.
52. Name the quantity whose role in mechanics is similar to the role of resistance in electrical circuits. [1 Mark]
Answer:
1. Inertia.
Teacher's Note:
a) Inertia opposes the change in state of motion of a body, just as electrical resistance opposes the flow of charge.
53. Write a formula for calculating the amount of heat (H) produced in a resistor of resistance R when current I is passed through it for time t. [1 Mark]
Answer:
1. \(H = I^2Rt\)
Teacher's Note:
a) This is Joule's law of heating.
54. List of factors on which heating effect of electric current depends. [1 Mark]
Answer:
1. Square of the current (\(I^2\)) flowing through the conductor.
2. Resistance (\(R\)) of the conductor.
3. Time (\(t\)) for which the current flows.
Teacher's Note:
a) Derived directly from Joule's heating formula \(H = I^2Rt\).
55. How is the heat produced by a current passing through a constant resistance related to the strength of the current? [1 Mark]
Answer:
1. The heat produced is directly proportional to the square of the strength of the current (\(H \propto I^2\)) for a constant resistance and time.
Teacher's Note:
a) Emphasize the square of the current in the proportionality relationship.
56. Name the Scientist who gave the formula for finding the heating produced in a conductor. [1 Mark]
Answer:
1. James Prescott Joule.
Teacher's Note:
a) The heating effect of electric current is also known as Joule heating.
57. Is joule’s heating a reversible effect? [1 Mark]
Answer:
1. No, Joule's heating is an irreversible effect.
2. The electrical energy is converted into heat energy, which cannot be completely converted back into electrical energy.
Teacher's Note:
a) Heat is dissipated into the surroundings, making the process thermodynamically irreversible.
b) Remember that energy dissipation through resistors is always one-directional in practical circuits.
58. How much can be Joule’s heating produced in an ideal insulator? [1 Mark]
Answer:
1. Zero.
2. An ideal insulator has infinite resistance, allowing no current to flow through it.
Teacher's Note:
a) Since heat produced \(H = I^{2}Rt\) and current \(I = 0\), the heat produced is zero.
b) Insulators block current completely under normal conditions.
59. Name three electrical appliances that involve the heating effect of electric current. [1 Mark]
Answer:
1. Electric iron.
2. Electric heater.
3. Electric toaster.
Teacher's Note:
a) These appliances use high-resistance heating elements like nichrome wire.
b) Electrical energy is successfully converted into thermal energy.
60. Is electric fuse an application of heating effect of current? [1 Mark]
Answer:
1. Yes.
2. An electric fuse works on the principle of the heating effect, melting when excessive current passes through it.
Teacher's Note:
a) It protects electrical circuits and appliances from overloading and short-circuiting.
b) Fuse wire has a low melting point and appropriate current rating.
61. Out of 5A fuse and 15A fuse which will you prefer to use for the lighting circuit? [1 Mark]
Answer:
1. 5A fuse.
2. Lighting circuits carry lower current compared to power circuits which use heavy appliances.
Teacher's Note:
a) A 5A fuse is meant for low current ratings typically used in bulbs and fans.
b) 15A fuses are reserved for high-power appliances like geysers and air conditioners.
62. In domestic wiring do we connect various distribution circuits in series? [1 Mark]
Answer:
1. No.
2. They are connected in parallel.
Teacher's Note:
a) Parallel connection ensures that if one appliance fails, others continue to work normally.
b) Every appliance gets the same potential difference in a parallel circuit.
63. Usually three insulated wires of different colours are used in electrical appliance. Name the three colours. [1 Mark]
Answer:
1. Red (or brown) for live wire.
2. Black (or light blue) for neutral wire.
3. Green (or yellow) for earth wire.
Teacher's Note:
a) These standard colour codes help in safe installation and identification.
b) Live carries current, neutral completes the path, and earth prevents electric shocks.
64. What do you mean by short circuiting? [1 Mark]
Answer:
1. It is a low-resistance connection between the live wire and the neutral wire.
2. This causes a sudden, dangerously high current to flow through the circuit.
Teacher's Note:
a) It often happens due to damaged insulation or a fault in the appliance.
b) It can lead to sparking, damage, and electrical fires if protection fails.
65. What do you mean by a fuse? [1 Mark]
Answer:
1. A safety device used in electrical circuits to prevent damage from overloading or short-circuiting.
2. It consists of a metallic wire with a low melting point.
Teacher's Note:
a) It melts and breaks the circuit automatically when current exceeds a safe limit.
b) Always connected in series with the live wire.
66. Should a fuse wire be connected in series or in parallel in the main circuit? [1 Mark]
Answer:
1. In series.
2. This ensures that the entire current of the circuit passes through the fuse.
Teacher's Note:
a) Series connection forces the fuse to melt and disconnect the supply during excess current.
b) Connecting it in parallel would bypass the safety protection entirely.
67. Can we use a copper wire as a fuse wire? [1 Mark]
Answer:
1. No.
2. Copper has a high melting point and low resistivity, so it will not melt easily when excessive current flows.
Teacher's Note:
a) Fuse wire requires a material with a low melting point like lead-tin alloy.
b) Using copper would defeat the safety purpose of a fuse.
68. Name the instrument used to measure current in a circuit. [1 Mark]
Answer:
1. Ammeter.
Teacher's Note:
a) An ammeter always has very low resistance to measure accurate current without altering it.
b) It is connected in series in the circuit.
69. Name the instrument used to detect weak currents in a circuit. [1 Mark]
Answer:
1. Galvanometer.
Teacher's Note:
a) A galvanometer can detect the direction and presence of very small electric currents.
b) Unlike an ammeter, its zero mark is usually at the centre of the scale.
70. Is ammeter connected in series or parallel in a circuit? [1 Mark]
Answer:
1. In series.
Teacher's Note:
a) Connecting in series ensures the entire circuit current passes through it.
b) Ideal ammeters have zero internal resistance.
71. What does the potential of a charged body determine? [1 Mark]
Answer:
1. It determines the direction of flow of charge when the body is connected to another charged body.
Teacher's Note:
a) Positive charge flows from higher potential to lower potential.
b) Electrons flow from lower potential to higher potential.
72. Is potential difference between two points in an electric field a vector quantity? [1 Mark]
Answer:
1. No.
2. Potential difference is a scalar quantity.
Teacher's Note:
a) It has magnitude only and no specific direction associated with it as a vector.
b) It represents work done per unit charge between two points.
73. What does a voltmeter measure? [1 Mark]
Answer:
1. Potential difference between two points in an electric circuit.
Teacher's Note:
a) It is always connected in parallel across the component.
b) An ideal voltmeter has infinite resistance.
74. Is a voltmeter same as a voltameter? [1 Mark]
Answer:
1. No.
2. A voltmeter measures potential difference, while a voltameter is an electrolytic cell used for electrolysis.
Teacher's Note:
a) Students often confuse these two terms due to similar spelling.
b) Voltameter deals with chemical effects of current, voltmeter with electrical measurements.
75. What are the conditions under which charges cane move in a conductor? [1 Mark]
Answer:
1. There must be a potential difference across the ends of the conductor.
2. There must be a continuous closed path (circuit) for the charges to flow.
Teacher's Note:
a) Free electrons exist in conductors, but need an electric field to drift directionally.
b) Without a potential difference, random thermal motion results in zero net current.
76. How will you maintain a potential difference between the ends of a conductor? [1 Mark]
Answer:
1. By connecting the ends of the conductor to a source of electrical energy such as a cell, battery, or power supply.
Teacher's Note:
a) Chemical reactions inside a cell maintain the potential difference.
b) It supplies the necessary energy to drive electrons continuously.
77. Name the quantity that determines the rate of flow of charge through a conductor. [1 Mark]
Answer:
1. Electric current.
Teacher's Note:
a) Current is defined as \(I = \frac{Q}{t}\).
b) Its SI unit is the Ampere (A).
78. What determines the direction of flow of charge: Potential of a body or Quantity of charge on a body? [1 Mark]
Answer:
1. The potential of a body determines the direction of flow of charge.
2. Charge flows from a body at higher potential to a body at lower potential.
Teacher's Note:
a) Electric potential acts like a level or pressure that drives charge.
b) Total quantity of charge does not dictate the direction of current.
79. When the two ends of copper wire are connected to the two terminals of a battery some potential difference is created between its ends. Do electrons start flowing from high potential end to low potential end? [1 Mark]
Answer:
1. No, electrons are negatively charged particles.
2. Electrons flow from the low potential (negative terminal) end to the high potential (positive terminal) end of the wire.
Teacher's Note:
a) Conventional current flows from high to low potential.
b) Electrons move in the exact opposite direction of conventional current.
80. In the question above, does current flow from the low potential end to the high potential end of the wire? [1 Mark]
Answer:
1. No, electric current flows from the high potential end to the low potential end of the wire.
2. The direction of conventional current is taken as the direction of flow of positive charge.
Teacher's Note:
a) Conventional current is opposite to the direction of electron flow.
b) Always remember that positive charge moves from high to low potential.
81. What is the difference between a cell and a battery? [1 Mark]
Answer:
1. A single device that converts chemical energy into electrical energy is called a cell.
2. A combination of two or more cells connected together is called a battery.
Teacher's Note:
a) A cell provides a single source of potential difference.
b) Batteries provide a higher voltage by combining multiple cells in series.
82. What do you mean by an electric circuit? [1 Mark]
Answer:
1. A continuous and closed path along which electric current flows is called an electric circuit.
2. It typically consists of a source, load, switch, and connecting wires.
Teacher's Note:
a) A break anywhere in the path stops the current flow entirely.
b) Circuits form the foundation of all electrical appliances.
83. What do you mean by a circuit diagram? [1 Mark]
Answer:
1. A diagram which indicates how different components in a circuit have been connected by using electrical symbols for the components is called a circuit diagram.
Teacher's Note:
a) Standardized symbols make circuit diagrams universally understandable.
b) They simplify complex physical wiring into clear symbolic representations.
84. How does a cell maintain some constant potential difference across its terminals? [1 Mark]
Answer:
1. A cell maintains a constant potential difference across its terminals due to chemical reactions taking place inside it.
2. These chemical reactions drive charges from lower to higher potential.
Teacher's Note:
a) Internal chemical energy is consumed to sustain the potential difference.
b) Once chemicals are exhausted, the cell stops maintaining voltage.
85. Do all substances have the same resistivity? [1 Mark]
Answer:
1. No, different substances have different resistivities.
2. Resistivity is a characteristic property of each specific material.
Teacher's Note:
a) Metals have very low resistivity, whereas insulators have very high resistivity.
b) Resistivity depends on the nature of the material and temperature.
86. Give two substances having very low resistivities. [1 Mark]
Answer:
1. Copper and Silver.
Teacher's Note:
a) Low resistivity means the material is a very good conductor of electricity.
b) Silver has the lowest known resistivity among metals.
87. Give two substances having moderate resistivities. [1 Mark]
Answer:
1. Constantan and Manganin (or Nichrome).
Teacher's Note:
a) These are typically alloy materials used in standard resistors.
b) Their resistivity varies very little with temperature changes.
88. Give two substances having high resistivities. [1 Mark]
Answer:
1. Glass and Ebonite (or Rubber).
Teacher's Note:
a) High resistivity classifies these materials as good insulators.
b) They block the flow of electric current completely under normal conditions.
89. What is the name given to substances which are used for making heating soils? [1 Mark]
Answer:
1. Alloys (such as Nichrome).
Teacher's Note:
a) Note: The question contains a slight typo ("heating soils"), which refers to heating elements or coils.
b) Alloys have higher resistivity than pure metals and do not oxidize easily at high temperatures.
90. Why do electricians wear rubber shoes or sandals or rubber hand gloves while working? [1 Mark]
Answer:
1. Rubber is an insulator with very high resistivity.
2. Wearing rubber protection prevents the flow of current through the electrician's body into the ground, avoiding electric shocks.
Teacher's Note:
a) The human body is a good conductor of electricity.
b) Insulation breaks the electrical circuit between the live wire and the earth.
91. What are the two ways in which the resistances are combined? [1 Mark]
Answer:
1. In series combination.
2. In parallel combination.
Teacher's Note:
a) Series means resistances are joined end-to-end sequentially.
b) Parallel means all terminals are connected together across common points.
92. What is meant by an equivalent resistance? [1 Mark]
Answer:
1. A single resistance that can replace a combination of resistances in a circuit to draw the exact same current under the same potential difference is called an equivalent resistance.
Teacher's Note:
a) It simplifies complex circuit analysis.
b) Also known as net or total resistance of the combination.
93. Name the SI unit of electrical energy. [1 Mark]
Answer:
1. Joule (J).
Teacher's Note:
a) One joule is equal to one watt-second.
b) Electrical energy is the total work done by a source of electricity.
94. Name the commercial unit of energy. [1 Mark]
Answer:
1. Kilowatt-hour (kWh), commonly known as a unit.
Teacher's Note:
a) One kilowatt-hour equals \(3.6 \times 10^6\) joules.
b) Electricity bills are calculated based on these commercial units consumed.
95. Name the property of a conductor by virtue of which it opposes the flow of electric current through it. [1 Mark]
Answer:
1. Resistance.
Teacher's Note:
a) Resistance arises due to collisions of free electrons with the ions in the conductor.
b) Its SI unit is the ohm (Ω).
96. Is resistance a scalar or a vector quantity? [1 Mark]
Answer:
1. Resistance is a scalar quantity.
2. It has magnitude but no fixed direction in space.
Teacher's Note:
a) Scalar quantities possess only magnitude and obey ordinary rules of algebra.
b) Resistance depends on material and dimensions, not direction.
97. Name the property of a conductor by virtue of which it allows the flow of charge through it. [1 Mark]
Answer:
1. Conductance.
2. It is the reciprocal of resistance.
Teacher's Note:
a) Conductance measures how easily current flows through a conductor.
b) The SI unit of conductance is Siemens or mho.
98. Is conductance a scalar or a vector quantity? [1 Mark]
Answer:
1. Conductance is a scalar quantity.
2. It has magnitude only.
Teacher's Note:
a) Being the reciprocal of resistance, conductance inherits its scalar nature.
b) It does not have any directional property.
99. Which has a greater resistance: thin wire or thick wire of the same material? [1 Mark]
Answer:
1. Thin wire.
2. Resistance is inversely proportional to the cross-sectional area.
Teacher's Note:
a) Formula: R = \(\rho \frac{l}{A}\), where A is area.
b) A smaller area leads to a higher resistance.
100. What happens to the resistance of a copper wire when its temperature is raised? [1 Mark]
Answer:
1. The resistance of a copper wire increases.
2. Copper is a conductor, and its resistance increases with temperature.
Teacher's Note:
a) Higher temperature causes more frequent collisions of free electrons.
b) This increases the resistivity of metallic conductors.
101. What happens to the conductance of a copper wire when its temperature is raised? [1 Mark]
Answer:
1. The conductance decreases.
2. Conductance is the reciprocal of resistance.
Teacher's Note:
a) As resistance increases with temperature, conductance drops.
b) Conductivity also decreases accordingly in metals.
102. What happens to the resistance of the following substances when its temperature is raised?
(i) Aluminium (ii) Silicon (iii) Silver (iv) Germanium [1 Mark]
Answer:
1. Aluminium: Resistance increases (metal).
2. Silicon: Resistance decreases (semiconductor).
3. Silver: Resistance increases (metal).
4. Germanium: Resistance decreases (semiconductor).
Teacher's Note:
a) Metals have positive temperature coefficient of resistance.
b) Semiconductors have negative temperature coefficient of resistance.
103. Define 1KWh. [1 Mark]
Answer:
1. One Kilowatt-hour (1 kWh) is the energy consumed by an electrical appliance of power 1 kilowatt operating for 1 hour.
2. It equals 3.6 × 106 joules.
Teacher's Note:
a) It is also known as one commercial unit of electrical energy.
b) Calculation: 1 kW × 1 h = 1000 W × 3600 s = 3.6 × 106 J.
104. Name the quantity that represents the electric work done per unit time. [1 Mark]
Answer:
1. Electric power.
2. P = W / t.
Teacher's Note:
a) The SI unit of electric power is Watt (W).
b) It measures the rate of consumption of electrical energy.
105. Express electric power (P) in terms of current (I) and resistance (R). [1 Mark]
Answer:
1. P = I2R.
2. Derived from substituting Ohm's law into the base power formula.
Teacher's Note:
a) Since V = IR, substituting V in P = VI gives I2R.
b) Used extensively in calculating heat losses in transmission lines.
106. Express electric power (P) in terms of current (I) and potential difference (V). [1 Mark]
Answer:
1. P = VI.
2. Power is the product of potential difference and electric current.
Teacher's Note:
a) Fundamental relation for electrical power.
b) Valid for all direct current and resistive alternating current circuits.
107. Name the quantity that represents the product of power rating and time. [1 Mark]
Answer:
1. Electrical energy.
2. Energy = Power × Time.
Teacher's Note:
a) Units depend on the units of power and time used.
b) Measured in watt-hours or kilowatt-hours for electricity billing.
108. What voltage for the electrical appliance is kept in India? [1 Mark]
Answer:
1. 220V.
2. Standard domestic AC supply voltage in India.
Teacher's Note:
a) The frequency of this AC supply is 50 Hz.
b) Standardized across residential households.
109. What voltage is kept for the domestic electric supply in USA? [1 Mark]
Answer:
1. 110V to 120V.
2. Standard residential voltage in the United States.
Teacher's Note:
a) The frequency of AC supply in the USA is 60 Hz.
b) Lower voltage is chosen historically for perceived safety reasons.
110. An electric heater is rated as 220V, 1KW. What does it mean? [1 Mark]
Answer:
1. When connected to a 220V supply, the heater consumes 1000 watts (1 kW) of electric power.
2. It consumes 1000 joules of electrical energy per second.
Teacher's Note:
a) Voltage rating ensures correct appliance pairing.
b) Power rating helps calculate current draw and energy consumption.
111. Name the term used to represent the values of the voltage and wattage(power) of an electrical appliance taken together. [1 Mark]
Answer:
1. Power rating.
2. Electrical rating.
Teacher's Note:
a) Stamped on a label on every electrical appliance.
b) Used to determine safe fusing and wiring requirements.
112. Which bulb has the lesser resistance: 100W or 60W? [1 Mark]
Answer:
1. 100W bulb.
2. Resistance is inversely proportional to power at constant voltage (R = V2 / P).
Teacher's Note:
a) Higher wattage means lower resistance and higher current flow.
b) A 100W bulb glows brighter due to higher power dissipation.
113. Which electrical appliance generally draws more current for the same applied voltage: Electric iron or Electric bulb? [1 Mark]
Answer:
1. Electric iron.
2. An electric iron has a much higher wattage than a normal electric bulb.
Teacher's Note:
a) Since P = VI, higher power at the same voltage requires higher current.
b) Heating appliances typically operate at 1000W to 2000W.
114. What do you understand by the heating effect of electric current? [1 Mark]
Answer:
1. The production of heat in a resistor when an electric current passes through it.
2. It is caused by the collisions of moving electrons with the atoms or ions of the conductor.
Teacher's Note:
a) Governed by Joule's law of heating: H = I2Rt.
b) Basis of operation for electric heaters, irons, and fuses.
115. A number of bulbs are to be connected to a single source. Will they provide more illumination if connected in parallel, or in series? [1 Mark]
Answer:
1. They will provide more illumination if connected in parallel.
2. In a parallel connection, each bulb gets the full source voltage and operates independently with maximum brightness.
Teacher's Note:
a) Parallel circuits ensure that the failure of one bulb does not affect the working of other bulbs.
b) In series connection, the voltage gets divided across all the bulbs, resulting in much less illumination.
116. What do you mean by the overloading of the electrical circuit? [1 Mark]
Answer:
1. Overloading refers to a situation when too many electrical appliances of high power rating are connected to a single socket.
2. This draws current from the mains that exceeds the safe limit of the connecting wires, causing overheating and potential fire hazards.
Teacher's Note:
a) Overloading can also happen due to an accidental hike in the supply voltage.
b) Always remind students that fuses and MCBs protect circuits from damage during overloading.
117. All electrical circuits consist of three wires: a live wire, a neutral wire and an earth wire, what colours are assigned to these wires? [1 Mark]
Answer:
1. Live wire is assigned red (or brown) colour.
2. Neutral wire is assigned black (or light blue) colour.
3. Earth wire is assigned green (or yellow) colour.
Teacher's Note:
a) International colour coding uses brown for live, light blue for neutral, and green-yellow for earth.
b) Standardizing wire colours prevents dangerous connection mistakes during installation and repair work.
118. What do you understand by short circuiting? [1 Mark]
Answer:
1. Short circuiting is a low-resistance path created when the live wire and the neutral wire come in direct contact with each other.
2. This causes a sudden, massive surge of electric current through the circuit, leading to sparks and potential fire.
Teacher's Note:
a) Damaged insulation on wires is the most common cause of short circuits.
b) The resistance during a short circuit is almost zero, making the current rise to extremely high values.
119. What will happen when the live and the neutral wires in a circuit touch each other due to defective wiring? [1 Mark]
Answer:
1. A short circuit will take place in the electrical circuit.
2. The current in the circuit will increase abruptly to a very high value, causing sparking and heating.
Teacher's Note:
a) Direct contact between live and neutral bypasses the electrical load entirely.
b) Protective devices like fuses melt and break the circuit to prevent severe accidents.
120. What is the use of electrical fuse in an electrical circuit? [1 Mark]
Answer:
1. An electric fuse is a safety device used to protect electrical circuits and appliances from damage due to overloading and short-circuiting.
2. It melts and breaks the circuit whenever current exceeds the safe limit.
Teacher's Note:
a) Fuse wire is made of an alloy with low melting point and high resistivity, such as lead-tin alloy.
b) It is always connected in series with the live wire of the circuit.
121. Name the effect of current on which a fuse works. [1 Mark]
Answer:
1. Heating effect of electric current.
Teacher's Note:
a) When excess current flows, Joule heating (\(I^2Rt\)) melts the fuse wire.
b) Mentioning only heating effect is sufficient for full marks.
122. Where do we connect a fuse in an electrical circuit? [1 Mark]
Answer:
1. The fuse is connected in series with the live wire of the electrical circuit.
Teacher's Note:
a) Connecting it to the live wire ensures that power to the entire appliance is cut off when the fuse blows.
b) Never connect a fuse to the neutral or earth wire.
123. By what symbol is earthing represented? [1 Mark]
Answer:
1. Earthing is represented by a symbol consisting of three parallel horizontal lines of decreasing length connected to a vertical line.
Teacher's Note:
a) The symbol signifies safe discharge of electrical charge into the deep ground.
b) Students should draw the standard schematic representation neatly in diagrams.
124. Why do we do the earthing of an electrical appliance? [1 Mark]
Answer:
1. Earthing is done to protect the user from severe electric shocks in case of live wire accidentally touching the metallic case of the appliance.
Teacher's Note:
a) It provides a low-resistance path for leakage current to flow safely into the earth.
b) The appliance fuse blows immediately due to the heavy current surge during a fault.
125. What does an electric meter in the household electric circuit measure? [1 Mark]
Answer:
1. It measures the total electrical energy consumed by the household in kilowatt-hours (kWh) or units.
Teacher's Note:
a) One kilowatt-hour is commonly known as one 'unit' of electricity.
b) Do not confuse it with power meters; energy is power multiplied by time.
126. Are distribution circuits connected to each other in series or in parallel? [1 Mark]
Answer:
1. In parallel.
Teacher's Note:
a) Parallel connection ensures that every appliance receives the same voltage and can be switched on or off independently.
b) If distribution circuits were in series, turning off one appliance would switch off everything in the house.
127. Are switches connected in the neutral wire? [1 Mark]
Answer:
1. No, switches are connected in the live wire, not the neutral wire.
Teacher's Note:
a) Connecting the switch to the live wire ensures that when the switch is off, the appliance is completely disconnected from high potential.
b) If connected in the neutral wire, the appliance would remain live even when switched off, posing a shock hazard.
128. What do you mean by earthing of an electrical appliance? [1 Mark]
Answer:
1. Earthing refers to connecting the metallic body of an electrical appliance to the earth wire which goes deep into the ground.
Teacher's Note:
a) It ensures any leakage of current goes harmlessly into the earth instead of shocking the user.
b) Essential safety feature for all metallic body appliances like refrigerators and irons.
SHORT ANSWER TYPE - I QUESTIONS (2 MARKS)
1. Why can't we use a copper wire as a fuse wire? [2 Marks]
Answer:
1. Copper has very low resistivity and a high melting point.
2. Due to this, a copper wire will not melt easily when a large or unsafe current flows through the circuit, failing to protect appliances from damage.
Teacher's Note:
a) A proper fuse wire requires low melting point and high resistivity.
b) Alloys like lead-tin are used instead of pure copper.
2. What is usual colour code followed for connecting live, neutral and earth wires? Why is it so important? [2 Marks]
Answer:
1. Live wire is red, neutral wire is black, and earth wire is green.
2. This colour code is important because it helps electricians easily identify each wire during installation, maintenance, or repair, preventing dangerous short circuits and electric shocks.
Teacher's Note:
a) Standard coding is vital for safety compliance in residential wiring.
b) Modern international standards use brown, light blue, and green-yellow respectively.
3. Define Electric current. How can you measure the magnitude of electric current? [2 Marks]
Answer:
1. Electric current is defined as the rate of flow of electric charge through any cross-section of a conductor, given by \(I = \frac{Q}{t}\).
2. The magnitude of electric current is measured using an instrument called an ammeter, which is always connected in series in the circuit.
Teacher's Note:
a) SI unit of electric current is Ampere (A).
b) An ammeter must have very low resistance so it does not alter the current it measures.
4. What is the direction of conventional current? [2 Marks]
Answer:
1. The direction of conventional current is taken as the direction of flow of positive charge.
2. It flows from the positive terminal of a cell or battery to the negative terminal through the external circuit.
Teacher's Note:
a) Conventional current flows in the direction opposite to the flow of electrons.
b) This convention was established historically before electrons were discovered.
5. State the law, which relates the current in a conductor to the potential difference across its ends. [2 Marks]
Answer:
1. Ohm's law states that the electric current flowing through a conductor is directly proportional to the potential difference across its ends, provided physical conditions like temperature remain constant.
2. Mathematically, V = IR, where V is potential difference, I is current, and R is resistance.
Teacher's Note:
a) Mentioning the constant temperature condition is mandatory for full marks.
b) The graph of V versus I for an ohmic resistor is a straight line passing through the origin.
6. Differentiate between conductors and insulators. [2 Marks]
Answer:
| Properties | Conductors | Insulators |
|---|---|---|
| Definition | Materials that allow electric current to pass through them easily. | Materials that do not allow electric current to pass through them. |
| Free Electrons | They contain a large number of free electrons. | They have virtually no free electrons. |
| Examples | Copper, Aluminium, Iron | Rubber, Wood, Plastic |
Teacher's Note:
a) Conductors possess free charge carriers that facilitate the flow of electricity.
b) Insulators have tightly bound electrons preventing charge mobility.
7. What is the law of combination of resistances in series? [2 Marks]
Answer:
1. When two or more resistors are connected end-to-end, they are said to be in series.
2. The total equivalent resistance in a series combination is equal to the sum of the individual resistances: R = R1 + R2 + R3 + ...
3. The current flowing through each resistor is the same, while the potential difference gets divided.
Teacher's Note:
a) Equivalent resistance in series is always greater than the largest individual resistance.
b) The same current passes through every resistor connected in series.
8. What is the law of combination of resistances in parallel? [2 Marks]
Answer:
1. When resistors are connected between two common points, they are said to be in parallel.
2. The reciprocal of the equivalent resistance of a parallel combination is equal to the sum of the reciprocals of the individual resistances: \(\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...\)
3. The potential difference across each resistor is the same, while the current gets divided.
Teacher's Note:
a) Equivalent resistance in parallel is always smaller than the smallest individual resistance.
b) Household appliances are connected in parallel to ensure independent functioning and equal voltage.
9. What do you mean when we say that the electric appliance is earthed? What is its importance? [2 Marks]
Answer:
1. Earthing means connecting the metallic body of an electric appliance to the earth via a low-resistance wire.
2. Importance: It protects the user from severe electric shocks by safely channeling any accidental leakage of current into the ground.
Teacher's Note:
a) The earth wire is usually connected to a copper plate buried deep underground.
b) It prevents the potential of the appliance body from rising to dangerous levels.
10. What causes electric resistance? What is the physical significance of resistance? [2 Marks]
Answer:
1. Electric resistance is caused by the collisions of moving free electrons with the fixed ions and atoms present inside the conductor.
2. Physical significance: It determines the magnitude of current flowing through a conductor for a given potential difference, acting as opposition to current flow.
Teacher's Note:
a) Greater collisions result in higher resistance and less current flow.
b) Resistance is measured in ohms (Ω).
11. What are the limitations of Ohm's law? What are ohmic conductors? [2 Marks]
Answer:
1. Limitations: Ohm's law is not applicable to non-linear devices like diodes, transistors, and electrolytes where current does not vary linearly with voltage, and it also fails when temperature changes significantly.
2. Ohmic conductors: Conductors that obey Ohm's law at a constant temperature, maintaining a constant resistance for varying potential differences (e.g., metallic wires).
Teacher's Note:
a) The V-I graph for an ohmic conductor is always a straight line passing through the origin.
b) Non-ohmic devices yield a curved V-I graph.
12. A graph is plotted between V(potential difference) and I(current) for a metal at two different temperatures T1 and T2. What is the relationship between T1 and T2? [2 Marks]
Answer:
1. The slope of the V-I graph represents the resistance (R) of the conductor.
2. Resistance of a metal increases with an increase in temperature (R ∝ T).
3. If the slope for T1 is greater than T2, then T1 > T2.
Teacher's Note:
a) The slope is calculated as \(\frac{\Delta V}{\Delta I}\), which equals R.
b) Higher slope signifies higher resistance, which corresponds to a higher temperature for metals.
13. How many different resistances are possible with two equal resistors? [1 Mark]
Answer:
1. Three different resistances are possible: individual resistor (R), series combination (2R), and parallel combination (\(\frac{R}{2}\)).
Teacher's Note:
a) The possible combinations are single, series, and parallel.
14. Draw a diagram used for a fuse to be used in an electrical appliance. Write the symbol for an electrical fuse in circuit diagram. [2 Marks]
Answer:
1. A fuse consists of a thin wire of low melting point enclosed in a cartridge or porcelain holder.
2. Symbol in a circuit diagram is represented as a zigzag line enclosed in a box or a continuous broken zigzag line: ∿∿∿.
Teacher's Note:
a) A fuse melts and breaks the circuit when current exceeds the safe limit.
b) It is always connected in series with the live wire.
15. One billion electrons pass from a point A towards another point B in 10−4 s. What is the current in amperes? What is its direction? [2 Marks]
Answer:
1. Number of electrons (n) = 1 billion = 109, Charge of an electron (e) = 1.6 × 10−19 C, Time (t) = 10−4 s.
2. Total charge (Q) = ne = 109 × 1.6 × 10−19 = 1.6 × 10−10 C.
3. Current (I) = \(\frac{Q}{t}\) = \(\frac{1.6 \times 10^{-10}}{10^{-4}}\) = 1.6 × 10−6 A.
4. Direction of current: Since electrons move from A to B, the conventional current flows from B to A.
Teacher's Note:
a) Conventional current is always opposite to the direction of flow of electrons.
b) Apply the formula I = \(\frac{ne}{t}\) directly for step-by-step calculation.
SHORT ANSWER TYPE - II QUESTIONS (3 MARKS)
1. Given five equal resistances; each of the value 5 ohms. (a) What is the maximum resistance that can be obtained from them? (b) What is the minimum resistance that can be obtained from them? [3 Marks]
Answer:
1. Given: n = 5, R = 5 Ω.
2. (a) Maximum resistance is obtained when all resistors are connected in series: Rmax = R1 + R2 + R3 + R4 + R5 = 5 × 5 = 25 Ω.
3. (b) Minimum resistance is obtained when all resistors are connected in parallel: \(\frac{1}{R_{min}}\) = \(\frac{1}{5}\) + \(\frac{1}{5}\) + \(\frac{1}{5}\) + \(\frac{1}{5}\) + \(\frac{1}{5}\) = \(\frac{5}{5}\) = 1 Ω, so Rmin = 1 Ω.
Teacher's Note:
a) Series combination maximizes equivalent resistance by adding individual values.
b) Parallel combination minimizes equivalent resistance.
c) Always check units in ohms (Ω).
2. How many different resistance combinations are possible with three equal resistors taken all of them together? [3 Marks]
Answer:
1. With three equal resistors R, taking all of them together, there are four possible combinations.
2. Combination 1: All three in series (Rs = 3R).
3. Combination 2: All three in parallel (Rp = \(\frac{R}{3}\)).
4. Combination 3: Two in parallel, one in series with them (Req = \(\frac{3R}{2}\)).
5. Combination 4: Two in series, one in parallel with them (Req = \(\frac{2R}{3}\)).
Teacher's Note:
a) Consider all possible circuit topologies using three identical components.
b) Calculate each equivalent resistance using standard series and parallel formulas.
c) This tests understanding of mixed grouping of resistors.
3. Write symbols for the following:
(a) Cell (b) Battery (c) Fixed resistance (d) Wires crossing without contact
(e) Variable resistance (f) A wire joint [3 Marks]
Answer:
1. (a) Cell: Represented by a long parallel thin line and a short thick parallel line representing positive and negative terminals.
2. (b) Battery: A combination of two or more cells connected end-to-end.
3. (c) Fixed resistance: A zigzag line.
4. (d) Wires crossing without contact: Two intersecting lines where one wire has a small bridge-like curve over the other.
5. (e) Variable resistance (Rheostat): A zigzag line with an arrow pointing across it.
6. (f) A wire joint: Two intersecting wires with a solid dot at the intersection point.
Teacher's Note:
a) Standard circuit symbols are universally accepted conventions in physics.
b) Ensure correct distinction between crossing wires with and without contact.
c) The longer vertical line in a cell symbol denotes the positive terminal.
4. Write symbols for the following: (a) Ammeter (b) Voltmeter (c) Galvanometer (d) Open switch (e) Closed switch [3 Marks]
Answer:
1. (a) Ammeter: A circle with the letter 'A' inside, having + and - terminals.
2. (b) Voltmeter: A circle with the letter 'V' inside, having + and - terminals.
3. (c) Galvanometer: A circle with the letter 'G' inside.
4. (d) Open switch (Plug key open): Two terminal dots with an open bracket or gap between them.
5. (e) Closed switch (Plug key closed): Two terminal dots with a dot inside the bracket or a closed bridge.
Teacher's Note:
a) Measuring instruments are always represented by a circle enclosing their respective measuring symbol.
b) Switches control the flow of current in a circuit.
c) Proper terminal markings are essential for accurate circuit diagrams.
5. Draw a labelled circuit diagram for the verification of Ohm’s law. Plot a I-V graph for an ohmic conductor. [3 Marks]
Answer:
1. Circuit diagram components: A resistor R, an ammeter A connected in series, a voltmeter V connected in parallel across R, a variable resistor or rheostat Rh, a cell or battery, and a plug key.
2. When the key is closed, current flows through the resistor and the ammeter and voltmeter readings are noted.
3. The I-V graph for an ohmic conductor is a straight line passing through the origin, showing that current is directly proportional to potential difference.
Teacher's Note:
a) The voltmeter must always be connected in parallel across the resistor.
b) The slope of the I-V graph gives the reciprocal of resistance.
c) Ohm's law holds true only at a constant temperature.
6. With the help of a diagram, derive the formula for the equivalent resistance of three resistances connected in series. [3 Marks]
Answer:
1. Let three resistances \(R_1\), \(R_2\) and \(R_3\) be connected in series across a source of potential difference V.
2. In a series combination, the same current I flows through each resistor, but the potential difference across each is different (\(V_1\), \(V_2\), \(V_3\)).
3. Total potential difference V = \(V_1\) + \(V_2\) + \(V_3\).
4. By Ohm's law, V = IR, \(V_1 = IR_1\), \(V_2 = IR_2\), and \(V_3 = IR_3\).
5. Substituting these values: \(IR_s = IR_1 + IR_2 + IR_3\), which gives \(R_s = R_1 + R_2 + R_3\).
Teacher's Note:
a) Current remains the same across all resistors in a series circuit.
b) The equivalent resistance is greater than the largest individual resistance.
c) This arrangement is used when a high resistance is required from smaller resistors.
7. With the help of a diagram, derive the formula for the equivalent resistance of three resistances connected in parallel. [3 Marks]
Answer:
1. Let three resistances \(R_1\), \(R_2\) and \(R_3\) be connected in parallel between two common points across a source of potential difference V.
2. In a parallel combination, the potential difference V across each resistor is the same, but the total current I divides into \(I_1\), \(I_2\) and \(I_3\).
3. Total current I = \(I_1 + I_2 + I_3\).
4. By Ohm's law, \(I = \frac{V}{R_p}\), \(I_1 = \frac{V}{R_1}\), \(I_2 = \frac{V}{R_2}\), and \(I_3 = \frac{V}{R_3}\).
5. Substituting these values: \(\frac{V}{R_p} = \frac{V}{R_1} + \frac{V}{R_2} + \frac{V}{R_3}\), which gives \(\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}\).
Teacher's Note:
a) Potential difference remains the same across all branches in parallel.
b) The equivalent resistance is smaller than the smallest individual resistance.
c) Household appliances are connected in parallel so they operate independently at the same voltage.
8. Write the SI and commercial unit of energy. Derive the relation between them. [3 Marks]
Answer:
1. The SI unit of electric energy is Joule (J) or watt-second.
2. The commercial unit of electric energy is kilowatt-hour (kWh), commonly known as a unit.
3. Derivation of relation: 1 kWh = 1 kilowatt × 1 hour = 1000 W × 3600 seconds = 3,600,000 joules = \(3.6 \times 10^6\) J.
Teacher's Note:
a) 1 watt is equal to 1 joule per second.
b) Always express the final relation in standard scientific notation.
c) Electricity bills are calculated based on kilowatt-hours consumed.
9. What do you mean by resistivity of a conductor? What are the factors on which the resistance of a conductor depends? [3 Marks]
Answer:
1. Resistivity is defined as the resistance offered by a conductor of unit length and unit cross-sectional area. Its SI unit is ohm-metre (\(\Omega \cdot m\)).
2. Factors on which resistance depends are: length of the conductor (directly proportional), area of cross-section (inversely proportional), nature of the material, and temperature.
3. The formula linking them is R = \(\rho \frac{l}{A}\).
Teacher's Note:
a) Resistivity is a characteristic property of the material and does not depend on its shape or size.
b) Metals have very low resistivity, whereas insulators have very high resistivity.
c) Temperature variation affects both resistance and resistivity.
10. State Joule’s law of heating. Derive the formula for the Heat produced due to current flowing in a conductor. [3 Marks]
Answer:
1. Joule's law of heating states that the heat produced in a resistor is directly proportional to the square of current for a given resistance, directly proportional to resistance for a given current, and directly proportional to the time for which the current flows: H = \(I^2Rt\).
2. Derivation: Work done W in moving a charge Q through a potential difference V is W = V × Q.
3. Since Q = I × t and by Ohm's law V = IR, substituting these gives W = (IR) × (It) = \(I^2Rt\).
4. Assuming all electrical work is converted into heat energy H, we get H = \(I^2Rt\).
Teacher's Note:
a) The unit of heat energy produced is Joule (J).
b) This law is applied in electric irons, heaters, and fuses.
c) Ensure proper substitution of Ohm's law terms during derivation.
LONG ANSWER TYPE QUESTIONS (5 MARKS)
1. Discuss series and parallel combinations of resistors with their salient features. [5 Marks]
Answer:
1. Series Combination: Resistors are connected end-to-end such that the same current flows through each, and total resistance is the sum of individual resistances (\(R = R_1 + R_2 + R_3\)).
2. Salient features of series: If one component fails, the entire circuit is broken; the same current passes through all devices regardless of their resistance; potential difference divides among them.
3. Parallel Combination: Resistors are connected across two common points so the potential difference across each is the same, and the reciprocal of equivalent resistance equals the sum of reciprocals (\(\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}\)).
4. Salient features of parallel: If one appliance fails, others continue to work; total resistance decreases; each appliance draws current independently according to its requirement.
5. Household wiring uses parallel circuits exclusively to avoid voltage drop and single-point failures.
Teacher's Note:
a) Structure comparison clearly with definitions, formulas, and practical implications.
b) Common mistake is confusing current and voltage behavior across combinations.
c) Emphasize why household appliances are never wired in series.
d) Mention the effect on total resistance in both cases.
2. What is electric energy and electric power? Derive their expressions and define their units. [5 Marks]
Answer:
1. Electric energy is the total work done in maintaining an electric current in a circuit for a given time. Expression: E = V × I × t = \(I^2Rt\) = \(\frac{V^2}{R}t\). Its SI unit is Joule (J).
2. Electric power is the rate at which electric energy is consumed or dissipated in an electric circuit. Expression: P = \(\frac{E}{t}\) = V × I = \(I^2R\) = \(\frac{V^2}{R}\). Its SI unit is Watt (W).
3. Definition of Joule: One joule of electric energy is consumed when 1 ampere of current flows through a circuit having a potential difference of 1 volt for 1 second.
4. Definition of Watt: Electric power is 1 watt when 1 joule of energy is consumed per second, or when 1 ampere of current flows under a potential difference of 1 volt.
5. Commercial unit relation: 1 kilowatt-hour equals \(3.6 \times 10^6\) joules.
Teacher's Note:
a) Clearly distinguish between energy (total work) and power (rate of work).
b) State units and their formal definitions precisely as expected in board marking schemes.
c) Include alternative formulas using Ohm's law substitutions.
d) Verify dimensional consistency across derivations.
3. What is Ohm’s law? How is it represented graphically? Derive an expression for the resultant resistance of series combinations of resistors. [5 Marks]
Answer:
1. 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 constant: V ∝ I or V = IR.
2. Graphical representation: A plot of potential difference (V) on the x-axis and current (I) on the y-axis yields a straight line passing through the origin. The slope of this line represents the reciprocal of resistance (\(1/R\)).
3. Series derivation - step 1: Let three resistors \(R_1\), \(R_2\) and \(R_3\) be connected in series across a voltage source V.
4. Series derivation - step 2: The current I is the same in all resistors, and total voltage V is the sum of individual voltage drops: V = \(V_1 + V_2 + V_3\).
5. Series derivation - step 3: Applying Ohm's law to each part and combining yields \(IR_s = IR_1 + IR_2 + IR_3\), leading to the final result \(R_s = R_1 + R_2 + R_3\).
Teacher's Note:
a) Mention the constant temperature condition explicitly when stating Ohm's law.
b) Explain graph axes clearly: V versus I gives resistance as the inverse slope, while I versus V gives resistance directly.
c) Show clear substitution steps during the series derivation.
d) Ensure correct labeling of circuit diagrams and graphs.
4. State Ohm’s law? Derive an expression for the equivalent resistance of parallel combinations of resistors. [5 Marks]
Answer:
1. Ohm's law states that the potential difference V across the ends of a given metallic wire in a circuit is directly proportional to the current flowing through it, provided its temperature remains constant (V = IR).
2. Parallel derivation - step 1: Let three resistors \(R_1\), \(R_2\) and \(R_3\) be connected in parallel between two points A and B across a potential difference V.
3. Parallel derivation - step 2: The total current I entering the combination splits into branch currents \(I_1\), \(I_2\) and \(I_3\), such that I = \(I_1 + I_2 + I_3\).
4. Parallel derivation - step 3: Since potential difference V is identical across each parallel branch, applying Ohm's law gives \(I_1 = \frac{V}{R_1}\), \(I_2 = \frac{V}{R_2}\), and \(I_3 = \frac{V}{R_3}\).
5. Parallel derivation - step 4: Substituting these into the total current equation gives \(\frac{V}{R_p} = \frac{V}{R_1} + \frac{V}{R_2} + \frac{V}{R_3}\), which simplifies to \(\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}\).
Teacher's Note:
a) State the constant temperature condition clearly in Ohm's law.
b) Highlight that voltage remains constant while current divides in parallel circuits.
c) Write each intermediate substitution step explicitly to secure full credit.
d) Conclude with the final reciprocal formula clearly boxed or highlighted.
5. What is Joule’s heating effect? How can it be demonstrated experimentally? List its four applications in daily life. [5 Marks]
Answer:
1. Joule's heating effect: When an electric current is passed through a high resistance wire, the wire becomes very hot and produces heat. This phenomenon of conversion of electrical energy into heat energy is called the heating effect of electric current.
2. Experimental demonstration: Take a battery, a plug key, a rheostat, an ammeter, and a nichrome wire coil immersed in a calorimeter containing a known mass of water with a thermometer inserted. Switch on the current, note the initial temperature of water, and observe the temperature rise over a fixed time interval, proving that electric current generates heat.
3. Application 1: Electric heating appliances such as electric irons, water heaters, and toasters operate on the heating effect of current.
4. Application 2: Electric bulbs use tungsten filaments that heat up to high temperatures and emit light.
5. Application 3: Electric fuses use wire with a low melting point to protect household circuits from overloading and short-circuiting.
Teacher's Note:
a) Define the heating effect clearly in terms of energy conversion.
b) Describe experimental setups logically with standard laboratory apparatus.
c) Give common, practical examples for daily life applications.
d) Explain the working principle of safety devices like fuses briefly.
6. What is electrical resistivity of a material? What is its unit? Describe an experimentally to study the factors on which the resistance of conducting wire depends? [5 Marks]
Answer:
1. Electrical resistivity: The resistance offered by a conductor of unit length and unit cross-sectional area is called resistivity. Its SI unit is ohm-metre (\(\Omega \cdot m\)).
2. Experiment - setup: Set up a circuit with a cell, an ammeter, a plug key, and a nichrome wire connected across terminals. Use wires of different lengths and uniform cross-sectional areas one by one.
3. Experiment - length factor: Connect varying lengths of nichrome wire of the same thickness into the circuit and record corresponding ammeter readings. Resistance increases directly with length.
4. Experiment - area factor: Connect nichrome wires of the same length but different cross-sectional areas (different thicknesses) and record current readings. Resistance decreases as cross-sectional area increases.
5. Conclusion: Resistance is directly proportional to length (R ∝ l) and inversely proportional to cross-sectional area (R ∝ \(\frac{1}{A}\)).
Teacher's Note:
a) Define resistivity precisely as a material constant independent of dimensions.
b) Outline the experimental procedure methodically, separating length and area variations.
c) Mention safety precautions and control variables like wire material and temperature.
d) Relate findings directly to the formula R = \(\rho \frac{l}{A}\).
Question. How is the resistivity of alloys compared with those of pure metals from which they may have been formed?
Answer. The resistivity of an alloy is generally higher than that of its constituent metals.
Question. Write the relation between resistance (R) of filament of a bulb, its power (P) and a constant voltage V applied across it.
Answer. P=V2/R
Question. Power of a lamp is 60 W. Find the energy in joules consumed by it in 1 s.
Answer. Here, power of lamp, P = 60 W
time, t = 1 s
So, energy consumed = Power × time
= (60× 1)J = 60 J
Question. Why are filaments of incandescent lamps made of thin tungsten wire ?
Answer. Tungsten has low resistivity and as such only a thin wire filament will have high resistance necessary to produce large amount of heat to light the bulb brilliantly. Further, tungsten has very high melting point (3300 K) and as such the filament will not burn at the high temperature (2400 K) at which it becomes incandescent.
Question. Why is an ammeter placed in series of a conductor/resistor in a circuit?
Answer. Because ammeter is a low resistance instrument. So, the current flowing through the circuit remains virtually unaffected.
Question. Three resistors of 3 W, 6 W and 4 W are connected in series. Calculate the total resistance of the combination.
Answer. Resistance in a series circuit is given by the expression
R = R1 + R2 + R3
Total resistance R = 3 W + 6 W + 4 W = 13 W.
Question. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 W resistor, a 8 W resistor, and a 12 W resistor, and a plug key, all connected in series.
Answer. The schematic circuit diagram is shown here:
Question. Name a device that helps to maintain a potential difference across a conductor.
Answer. A battery consisting of one or more electric cells is used to maintain a potential difference across a conductor.
Question. Name the material with the least resistivity.
Answer. Silver metal is best conductor of electricity and has lowest resistivity.
CBSE Class 10 Science Chapter 12 Electricity Short Answer Type Questions
Question. Name a device that you can use to maintain a potential difference between the ends of a conductor. Explain the process by which this device does so.
Answer. A cell or a battery can be used to maintain a potential difference between the ends of a conductor. The chemical reaction within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it. When it is connected to a conductor, it produces electric current and maintain the potential difference across the ends of the conductor.
Question. (i) List three factors on which the resistance of a conductor depends.
(ii) Write the SI unit of resistivity.
Answer. (i) Resistance of a conductor depends upon the following factors:
(1) Length of the conductor : Greater the length (l) of the conductor more will be the resistance (R).
R ∝ l
(2) Area of cross-section of the conductor: Greater the crosssectional area of the conductor, less will be the resistance.
R∝1/A
(3) Nature of conductor.
(ii) SI unit of resistivity is Ω m.
Question. Calculate the resistance of a metal wire of length 2 m and area of cross section 1.55 × 10–6 m2, if the resistivity of the metal be 2.8 × 10–8 W m.
Answer. For the given metal wire,
length, l = 2 m
area of cross-section, A = 1.55 × 10–6 m2
resistivity of the metal, r = 2.8 × 10–8 W m
Since, resistance, R=p l/A
Question. List the advantages of connecting electrical devices in parallel with an electrical source instead of connecting them is series.
Answer. (a) When a number of electrical devices are connected in parallel, each device gets the same potential difference as provided by the battery and it keeps on working even if other devices fail. This is not so in case the devices are connected in series because when one device fails, the circuit is broken and all devices stop working.
(b) Parallel circuit is helpful when each device has different resistance and requires different current for its operation as in this case the current divides itself through different devices. This is not so in series circuit where same current flows through all the devices, irrespective of their resistances.
Question. A cylinder of a material is 10 cm long and has a cross section of 2 cm2. If its resistance along the length be 20 ohm. What will be its resistivity?
Answer. Here, l = 10 cm, A = 2 cm2
R = 20 ohm
Using, R=p l/A
p =RA/l =20 ×2/10 = 4 ohm cm
= 0.04 ohm m
Question. In the circuit shown below, the ammeter and the voltmeter readings are 3 A and 6 V respectively. Then find the value of resistance R.
Answer. Case I : If ammeter A and voltmeter V are ideal, then
R= = 6V/3A=2 Ω
Case II : If ammeter A and voltmeter V has some finite
resistance, then R < 2 W.
Question. Which uses more energy, a 250 W TV set in 1 h or a 1200 W toaster in 10 minutes?
Answer. Energy consumed by TV set
= 250 W × 1 h = 250 J s–1 × 60 × 60 s
= 900,000 J
Energy consumed by toaster
= 1200 W × 10 min = 1200 J s–1 × 10 × 60 s
= 720,000 J.
Thus, the TV Set will use more energy.
Question. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply.
What current is drawn from the line if the supply voltage is 220 V?
Answer. Since both the bulbs are connected in parallel and to a 220 V supply, the voltage across each bulb is 220 V. Then
Current drawn by 100 W bulb,
I1 = Powerrating/Voltageapplied-100w/220v = 0.454A
Question. (a) List the factors on which the resistance of a conductor in the shape of a wire depends.
(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity ? Give reason.
(c) Why are alloys commonly used in electrical heating devices ? Give reason.
Answer. (a) Resistance of a conductor depends upon the following factors:
(1) Length of the conductor : Greater the length (l) of the conductor more will be the resistance (R).
R ∝ l
(2) Area of cross-section of the conductor: Greater the crosssectional area of the conductor, less will be the resistance. R∝1/A
(3) Nature of conductor.
(b) Metal have very low resistivity and hence they are good conductors of electricity.
Whereas glass has very high resistivity so glass is a bad conductor of electricity.
(c) Alloys are commonly used in electrical heating devices due to the following reasons
(i) Alloys have high melting point
(ii) Alloys have higher resistivity than metals
(iii) Alloys do not get oxidised or burn readily.
Question. Three resistors of 3 W each are connected to a battery of 3 V as shown. Calculate the current drawn from the battery.
Answer. As given in circuit diagram, two 3 W resistors are
connected in series to form R1, so
R1 = 3 W + 3 W = 6 W
And, R1 and R2 are in parallel combination,
Hence, equivalent resistance of circuit (Req) is given by
1/Req= 1/R1+ 1/R2
1/Req= 1\ 6+1\3 = 1+2/6=3/6=1/2
or Req = 2 W
Using Ohm’s law, V = IR
Electricity
We get,
3V = I × 2 W
or I = 3/2A =1.5 A
Current drawn from the battery is 1.5 A.
Question. An electric iron has a rating of 750 W, 200 V. Calculate:
(i) the current required.
(ii) the resistance of its heating element.
(iii) energy consumed by the iron in 2 hours.
Answer. Here, P = 750 W, V = 200 V
(i) As P = VI
I = P/V = (750/200) A = 3.75 A
(ii) By Ohm’s law V = IR or R = V/I
R = 200/3 .75=53 .3
(iii) Energy consumed by the iron in 2 hours
= P × t = 750 W × 2 h = 1.5 kWh
or E = (750 × 2 × 3600) J = 5.4 × 106J
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CBSE Class 10 Science Study Material: Chapter 11 Electricity
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