Chapter 12: Electricity (NCERT Class 10 Science)
Introduction to Electricity
Electricity is an essential part of modern life. It powers our homes, industries, and communication systems. This chapter explains fundamental concepts like electric current, resistance, potential difference, Ohm’s law, and electrical power.
Section 1: Electric Current and Circuit
Electric Charge and Current
Electric charge is a fundamental property of matter carried by protons (+) and electrons (-).
When electric charges move through a conductor (e.g., copper wire), they form an electric current.
The conventional direction of electric current is from the positive to the negative terminal of a battery.
Formula for Electric Current
Electric current (I) is the rate of flow of charge. It is given by:
I = Q/t
I = Current in Amperes (A)
Q = Charge in Coulombs (C)
t = Time in seconds (s)
Section 2: Electric Potential and Potential Difference
Definition of Electric Potential
Electric potential is the ability of a charge to do work. It is similar to water pressure in pipes.
Electric Potential Difference (Voltage)
The difference in electric potential between two points in a circuit is called potential difference (V).
It is responsible for moving charges in a circuit.
Formula for Potential Difference
V = W/Q
V = Potential difference in volts (V)
W = Work done in joules (J)
Q = Charge in coulombs (C)
Section 3: Ohm’s Law
Statement of Ohm’s Law
The potential difference across a conductor is directly proportional to the current passing through it, provided temperature remains constant.
V/I = R
Or, V = IR
where:
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
Section 4: Factors Affecting Resistance
Resistance and Resistivity
Resistance of a wire depends on:
1. Length (L) – More length → More resistance.
2. Cross-sectional area (A) – More area → Less resistance.
3. Material – Copper has less resistance than iron.
4. Temperature – More temperature → More resistance.
Formula for Resistance
R = ρ (rho) l/A
R = Resistance (Ω)
ρ = Resistivity (Ωm) (depends on material)
L = Length of wire (m)
A = Area of cross-section (m²)
Section 5: Series and Parallel Circuits
Resistors in Series
Total resistance (R) in series:
R_total = R_1 + R_2 + R_3 + ......
Voltage is divided across resistors.
Example: Fairy lights in series → If one bulb breaks, all go off.
Resistors in Parallel
Total resistance (R) in parallel:
1/R_p = 1/R_1+ 1/R_2 + 1/R_3+ .....
Current is divided among resistors.
Example: Home wiring → Each appliance gets full voltage.
Electric Power and Energy Consumption
Power and Its Formula
Power (P) is the rate at which electrical energy is consumed or produced.
P = VI
V = Voltage (V)
I = Current (A)
Other Forms of Power Equations
P = I^2 R
P = V^2/R
Electrical Energy and Its Calculation
Energy consumed in time t:
E = P \times t
P = Power in watts (W)
t = Time in seconds (s)
Commercial Unit of Electrical Energy
1 kilowatt-hour (kWh) = 1000 Wh
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J
Electric bills are calculated in kWh.
Special Notes:
✓ SI unit of current is Ampere (A): 1A = 1C/s
✓ A small current is measured in milliampere (mA) or microampere (µA).
✓ An ammeter is used to measure electric current and is always connected in series in a circuit.
✓ SI unit of potential difference is Volt (V): 1V = 1J/C
✓ A voltmeter measures potential difference and is connected in parallel in a circuit.
✓ A higher voltage means more energy is provided to charges.
SI unit of Resistance (R) is Ohm (Ω).
✓ Ohm (Ω): 1Ω = 1V / 1A
✓ More resistance → Less current flow.
✓ Copper and silver have low resistivity → Good conductors.
✓ Rubber and glass have high resistivity → Insulators.
✓ Series → More resistance, less current flow.
✓ Parallel → Less resistance, more current flow.
✓ 100W bulb running for 10 hours → Energy used = 1
kWh.
✓ Higher power rating → More energy consumption.
Here Multiple Choice Questions (MCQs) with answers
Chapter 12: Electricity (Class 10 Science, NCERT)
Q. The SI unit of electric current is:
A) Volt (V)
B) Ohm (Ω)
C) Ampere (A)
D) Coulomb (C)
Answer: C) Ampere (A)
Q. Electric current is defined as:
A) Flow of neutrons
B) Flow of protons
C) Flow of electric charge
D) Movement of atoms
Answer: C) Flow of electric charge
Q. The instrument used to measure electric current is:
A) Voltmeter
B) Ammeter
C) Galvanometer
D) Ohmmeter
Answer: B) Ammeter
Q. The SI unit of potential difference is:
A) Ampere (A)
B) Volt (V)
C) Ohm (Ω)
D) Joule (J)
Answer: B) Volt (V)
Q. Resistance is measured in:
A) Joule (J)
B) Ohm (Ω)
C) Watt (W)
D) Volt (V)
Answer: B) Ohm (Ω)
Q. A resistor has a resistance of 5Ω. A current of 2A flows through it. What is the voltage across it?
A) 10V
B) 5V
C) 2.5V
D) 7V
Answer: A) 10V
Q. Which material is a good conductor of electricity?
A) Rubber
B) Glass
C) Copper
D) Plastic
Answer: C) Copper
Q. In a series circuit, the total resistance is:
A) Less than the smallest resistor
B) Equal to the smallest resistor
C) The sum of individual resistances
D) The product of individual resistances
Answer: C) The sum of individual resistances
Q. What happens when resistors are connected in parallel?
A) Total resistance increases
B) Total resistance decreases
C) Voltage increases
D) Current decreases
Answer: B) Total resistance decreases
Q. What is the SI unit of resistivity?
A) Ohm (Ω)
B) Ohm-meter (Ωm)
C) Ampere (A)
D) Joule (J)
Answer: B) Ohm-meter (Ωm)
Q. If the length of a wire is doubled, its resistance will:
A) Remain the same
B) Double
C) Become half
D) Quadruple
Answer: B) Double
Q. If the cross-sectional area of a wire is doubled, its resistance will:
A) Double
B) Remain the same
C) Become half
D) Quadruple
Answer: C) Become half
Q. In which type of circuit is the voltage same across all components?
A) Series circuit
B) Parallel circuit
C) Both series and parallel
D) None of these
Answer: B) Parallel circuit
Q. The SI unit of electrical power is:
A) Joule (J)
B) Watt (W)
C) Volt (V)
D) Ampere (A)
Answer: B) Watt (W)
Q. One kilowatt-hour (kWh) is equal to:
A) 1000 joules
B) 3600 joules
C) 3.6 × 10⁶ joules
D) 1.6 × 10⁻¹⁹ joules
Answer: C) 3.6 × 10⁶ joules
Q. What is the function of a fuse in an electric circuit?
A) To store charge
B) To prevent excessive current
C) To increase voltage
D) To decrease resistance
Answer: B) To prevent excessive current
Q. Which of the following appliances works on the principle of heating effect of electric current?
A) Electric bulb
B) Electric iron
C) Water heater
D) All of the above
Answer: D) All of the above
Q. Why is tungsten used as a filament in electric bulbs?
A) It is a good conductor of electricity
B) It has a low resistance
C) It has a high melting point
D) It is a poor conductor of heat
Answer: C) It has a high melting point
Important questions with answers
Chapter 12: Electricity (Class 10 Science, NCERT)
Q. Define electric current and write its SI unit.
Answer: Electric current is the rate of flow of electric charge through a conductor. It is given by:
I = Q/t
SI unit: Ampere (A).
Q. What is the conventional direction of electric current?
Answer: The conventional direction of electric current is from the positive terminal to the negative terminal of a battery, opposite to the actual flow of electrons.
Q. Define electric potential difference and write its formula.
Answer: Electric potential difference is the work done per unit charge to move a charge between two points in a circuit.
Formula:
V = W/Q
SI unit: Volt (V).
Q. How is an ammeter connected in a circuit?
Answer: An ammeter is always connected in series in a circuit to measure electric current.
Q. State Ohm’s Law and give its mathematical expression.
Answer: Ohm’s Law states that the potential difference (V) across a conductor is directly proportional to the current (I) passing through it, provided temperature remains constant.
Mathematically,
V = IR
Q. What is resistance? Write its SI unit.
Answer: Resistance is the property of a conductor that opposes the flow of electric current. It is denoted by R.
SI unit: Ohm (Ω).
Q. List any three factors affecting the resistance of a conductor.
Answer:
1. Length of conductor (L) – More length → More resistance.
2. Cross-sectional area (A) – More area → Less resistance.
3. Material of conductor – Copper has less resistance than iron.
Q. What is resistivity? Write its SI unit.
Answer: Resistivity (ρ) is the property of a material that determines its resistance per unit length and cross-sectional area.
Formula:
R = ρ (rho) l/A
Q. What happens to the resistance of a wire if its length is doubled?
Answer:
Resistance is directly proportional to length (L).
If length is doubled, resistance also doubles.
Q. What is the total resistance in a series circuit with three resistors (5Ω, 10Ω, and 15Ω)?
Answer: In a series circuit,
R_{\text{total}} = R_1 + R_2 + R_3
= 5Ω + 10Ω + 15Ω = 30Ω ]
Q. In which type of circuit is current the same everywhere?
Answer: In a series circuit, current remains the same in all components.
Q. State two advantages of a parallel circuit over a series circuit.
Answer:
1. Each component gets the full voltage of the battery.
2. If one component fails, others keep working (used in home wiring).
14. Write the formula for electric power and its SI unit.
Answer:
P = VI
SI unit: Watt (W).
Q. Define 1 kilowatt-hour (kWh).
Answer:
1 kilowatt-hour is the energy consumed when a 1000W appliance runs for 1 hour.
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.
Q. What is the function of a fuse in an electric circuit?
Answer: A fuse protects the circuit by breaking the connection when current exceeds a safe limit, preventing overheating and fire.
Q. Why are copper and aluminum used for making electrical wires?
Answer:
Low resistivity → Less resistance, allowing easy current flow.
Good conductors of electricity.
Ductile and flexible.
Q. Why is tungsten used in electric bulbs but not in wiring?
Answer:
Tungsten has high resistance and high melting point, making it suitab
le for glowing filaments.
For wiring, we need low resistance, so copper or aluminum is used instead.
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