NCERT CLASS 10 SCIENCE CHAPTER 11 ELECTRICITY
NCERT CLASS 10 SCIENCE
CHAPTER 11 – ELECTRICITY
IN-TEXT QUESTIONS AND ANSWERS
1. What is an electric current?
Electric current is the rate of flow of electric charge through a conductor.
Formula:
I = Q/t
Where:
I = Electric current
Q = Electric charge
t = Time
The SI unit of electric current is ampere (A).
2. Define one ampere of electric current.
One ampere is the electric current flowing through a conductor when one coulomb of charge passes through its cross-section in one second.
1 A = 1 C/1 s
3. Calculate the number of electrons constituting one coulomb of charge.
Given:
Charge, Q = 1 C
Charge on one electron, e = 1.6 × 10⁻¹⁹ C
Number of electrons = Total charge / Charge of one electron
n = 1 / (1.6 × 10⁻¹⁹)
n = 6.25 × 10¹⁸
Answer: 6.25 × 10¹⁸ electrons.
4. What is the direction of conventional electric current?
Conventional electric current flows from the positive terminal to the negative terminal of a cell through the external circuit. The actual flow of electrons is in the opposite direction.
5. What is the potential difference between two points?
Potential difference is the work done per unit charge in moving a charge from one point to another.
Formula:
V = W/Q
Where:
V = Potential difference
W = Work done
Q = Electric charge
The SI unit is volt (V).
6. Define one volt.
The potential difference between two points is one volt when one joule of work is done to move one coulomb of charge between them.
1 V = 1 J/C
7. How much energy is given to each coulomb of charge passing through a 6 V battery?
Given:
Potential difference, V = 6 V
Charge, Q = 1 C
Formula:
W = V × Q
W = 6 × 1
Answer: 6 joules.
8. What is Ohm's law?
Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided its temperature and other physical conditions remain constant.
V ∝ I
V = IR
Where:
V = Potential difference
I = Current
R = Resistance
9. What is electrical resistance?
Electrical resistance is the property of a conductor that opposes the flow of electric current through it.
Formula:
R = V/I
The SI unit of resistance is ohm (Ω).
10. Define one ohm.
A conductor has a resistance of one ohm if a potential difference of one volt across it produces a current of one ampere.
1 Ω = 1 V/A
11. On what factors does the resistance of a conductor depend?
The resistance of a conductor depends on:
Length of the conductor.
Area of cross-section.
Nature of the material.
Temperature of the conductor.
12. How does the resistance of a wire change when its length is doubled?
Resistance is directly proportional to the length of a conductor.
R ∝ L
Therefore, if the length is doubled, the resistance also becomes double, provided other factors remain unchanged.
13. How does the resistance change when the area of cross-section is doubled?
Resistance is inversely proportional to the area of cross-section.
R ∝ 1/A
Therefore, if the area of cross-section is doubled, the resistance becomes half, provided other factors remain unchanged.
14. What is resistivity?
Resistivity is a property of a material that indicates how strongly it opposes the flow of electric current.
Formula:
ρ = RA/L
Where:
ρ = Resistivity
R = Resistance
A = Area of cross-section
L = Length of conductor
The SI unit of resistivity is ohm metre (Ω m).
EXERCISE QUESTIONS AND ANSWERS
1. What is the current flowing through a circuit if 100 C of charge passes through it in 10 seconds?
Given:
Q = 100 C
t = 10 s
Formula:
I = Q/t
I = 100/10
Answer: 10 A.
2. Calculate the potential difference if 300 J of work is done to move 50 C of charge.
Given:
W = 300 J
Q = 50 C
Formula:
V = W/Q
V = 300/50
Answer: 6 V.
3. What is the resistance of a conductor if a current of 0.5 A flows through it when a potential difference of 10 V is applied?
Given:
V = 10 V
I = 0.5 A
Formula:
R = V/I
R = 10/0.5
Answer: 20 Ω.
4. Calculate the current flowing through a resistor of 20 Ω when a potential difference of 12 V is applied.
Given:
R = 20 Ω
V = 12 V
Formula:
I = V/R
I = 12/20
Answer: 0.6 A.
5. What happens to the resistance of a wire if its length is doubled and its area of cross-section is also doubled?
We know:
R = ρL/A
New length = 2L
New area = 2A
New resistance:
R' = ρ(2L)/(2A)
R' = ρL/A
Answer: The resistance remains unchanged.
6. Why are copper and aluminium commonly used for making electrical wires?
Copper and aluminium have low resistivity and good electrical conductivity. Therefore, they allow electric current to flow easily and produce less heat for a given current.
7. A wire has a resistance of 5 Ω. Find the resistance of another wire of the same material and length but having twice the area of cross-section.
Resistance is inversely proportional to area.
R' = R/2
R' = 5/2
Answer: 2.5 Ω.
8. A wire has a resistance of 10 Ω. What will be its resistance if its length is increased to three times the original length, keeping the area and material unchanged?
Resistance is directly proportional to length.
R' = 3R
R' = 3 × 10
Answer: 30 Ω.
9. Explain the difference between the resistance and resistivity of a conductor.
Resistance:
It is the opposition offered by a particular conductor to electric current.
It depends on length, area, material and temperature.
SI unit is ohm (Ω).
Resistivity:
It is a characteristic property of the material.
It depends on the nature of the material and temperature.
SI unit is ohm metre (Ω m).
10. Calculate the equivalent resistance of three resistors of 2 Ω, 3 Ω and 5 Ω connected in series.
In series combination:
R = R₁ + R₂ + R₃
R = 2 + 3 + 5
Answer: 10 Ω.
11. Calculate the equivalent resistance of three resistors of 2 Ω, 3 Ω and 6 Ω connected in parallel.
For parallel combination:
1/R = 1/R₁ + 1/R₂ + 1/R₃
1/R = 1/2 + 1/3 + 1/6
1/R = 3/6 + 2/6 + 1/6
1/R = 6/6 = 1
Answer: 1 Ω.
12. Why are household electrical appliances connected in parallel?
Household electrical appliances are connected in parallel because:
Each appliance receives the same potential difference.
Each appliance can be operated independently.
Switching off one appliance does not affect the others.
Each appliance draws the current required for its operation.
13. What is the heating effect of electric current?
When electric current passes through a resistor, electrical energy is converted into heat energy. This is called the heating effect of electric current.
According to Joule's law of heating:
H = I²Rt
Where:
H = Heat produced
I = Current
R = Resistance
t = Time
14. Calculate the heat produced when a current of 2 A flows through a resistor of 5 Ω for 30 seconds.
Given:
I = 2 A
R = 5 Ω
t = 30 s
Formula:
H = I²Rt
H = 2² × 5 × 30
H = 4 × 5 × 30
Answer: 600 J.
15. An electric bulb is rated 220 V and 100 W. Calculate the current drawn by it.
Given:
V = 220 V
P = 100 W
Formula:
P = VI
I = P/V
I = 100/220
Answer: 0.455 A approximately.
FEW IMPORTANT ADDITIONAL QUESTIONS AND ANSWERS
1. What is an electric circuit?
An electric circuit is a closed conducting path through which electric current flows. It generally consists of a cell or battery, connecting wires, a switch and an electrical device.
2. What is an electric cell?
An electric cell is a device that converts chemical energy into electrical energy and provides a potential difference to drive electric current through a circuit.
3. What is the difference between an open circuit and a closed circuit?
Open circuit:
The conducting path is broken.
No current flows through the circuit.
The electrical device does not operate.
Closed circuit:
The conducting path is complete.
Current flows through the circuit.
The electrical device can operate.
4. What is an ammeter?
An ammeter is an instrument used to measure electric current in a circuit. It is connected in series with the component through which current is to be measured.
5. What is a voltmeter?
A voltmeter is an instrument used to measure the potential difference between two points. It is connected in parallel across the component.
6. Why is an ammeter connected in series?
An ammeter is connected in series so that the same current flowing through the circuit also passes through the ammeter. It has very low resistance so that it does not significantly change the current.
7. Why is a voltmeter connected in parallel?
A voltmeter is connected in parallel because it measures the potential difference between two points. It has very high resistance so that it draws very little current from the circuit.
8. What is a resistor?
A resistor is an electrical component used to provide resistance in a circuit and control the flow of electric current.
9. State Ohm's law and write its mathematical expression.
Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided temperature and other physical conditions remain constant.
Mathematical expression:
V = IR
10. What is the V-I graph for an ohmic conductor?
For an ohmic conductor, the graph between potential difference (V) and current (I) is a straight line passing through the origin.
The slope of the V-I graph gives resistance:
R = V/I
11. What is the difference between series and parallel combinations of resistors?
Series combination:
Resistors are connected one after another.
The same current flows through each resistor.
Equivalent resistance is the sum of individual resistances.
Potential difference is divided among the resistors.
Formula:
R = R₁ + R₂ + R₃
Parallel combination:
Resistors are connected across the same two points.
The same potential difference acts across each resistor.
Current is divided among the branches.
Equivalent resistance is less than the smallest individual resistance.
Formula:
1/R = 1/R₁ + 1/R₂ + 1/R₃
12. Why is the equivalent resistance greater in series combination?
In series combination, the resistances add together.
R = R₁ + R₂ + R₃
Therefore, the equivalent resistance is greater than any individual resistance.
13. Why is the equivalent resistance smaller in parallel combination?
In parallel combination, the current has multiple paths to flow. The reciprocal of equivalent resistance is the sum of the reciprocals of individual resistances.
1/R = 1/R₁ + 1/R₂ + 1/R₃
Therefore, the equivalent resistance is smaller than the smallest individual resistance.
14. State Joule's law of heating.
Joule's law of heating states that the heat produced in a resistor is directly proportional to:
The square of the current.
The resistance of the conductor.
The time for which current flows.
Formula:
H = I²Rt
15. What are the applications of the heating effect of electric current?
The heating effect of electric current is used in:
Electric heaters.
Electric irons.
Electric kettles.
Toasters.
Electric geysers.
Incandescent bulbs.
Electric fuses.
16. What is an electric fuse?
An electric fuse is a safety device connected in series with an electrical circuit. It contains a wire with a relatively low melting point. When excessive current flows, the fuse wire melts and breaks the circuit, protecting electrical appliances.
17. What is electric power?
Electric power is the rate at which electrical energy is consumed or converted into another form of energy.
Formula:
P = W/t
Also,
P = VI
SI unit: Watt (W).
18. Define one watt.
One watt is the power of an electrical device when it consumes or converts one joule of energy per second.
1 W = 1 J/s
19. What is one kilowatt-hour?
One kilowatt-hour is the electrical energy consumed by a device of power 1 kilowatt operating for one hour.
1 kWh = 3.6 × 10⁶ J
It is commonly called one unit of electrical energy.
20. Calculate the electrical energy consumed by a 1000 W appliance operating for 2 hours.
Given:
Power = 1000 W = 1 kW
Time = 2 hours
Energy = Power × Time
Energy = 1 × 2
Answer: 2 kWh or 2 units.
21. Why does an electric bulb glow when current passes through it?
In an incandescent bulb, electric current passes through a thin tungsten filament. Due to its high resistance, the filament becomes very hot and emits light.
22. Why is tungsten used in the filament of an electric bulb?
Tungsten is used because:
It has a very high melting point.
It can withstand high temperatures.
It has suitable electrical resistance.
It can glow at a high temperature without melting easily.
23. What is resistivity? On what factors does it depend?
Resistivity is the electrical property of a material that indicates its opposition to current flow.
It depends mainly on:
Nature of the material.
Temperature of the material.
It does not depend on the dimensions of the conductor.
24. Why are alloys commonly used in heating appliances?
Alloys are used in heating appliances because:
They generally have higher resistivity than pure metals.
They can withstand high temperatures.
They usually do not oxidise easily at high temperatures.
25. What is the commercial unit of electrical energy?
The commercial unit of electrical energy is kilowatt-hour (kWh).
1 kWh = 3.6 × 10⁶ J.
IMPORTANT NUMERICAL FORMULAE
Electric current:
I = Q/t
Electric charge:
Q = It
Potential difference:
V = W/Q
Work done:
W = VQ
Ohm's law:
V = IR
Resistance:
R = V/I
Resistance of a conductor:
R = ρL/A
Resistivity:
ρ = RA/L
Equivalent resistance in series:
R = R₁ + R₂ + R₃
Equivalent resistance in parallel:
1/R = 1/R₁ + 1/R₂ + 1/R₃
Heat produced:
H = I²Rt
Electrical power:
P = VI
Electrical power using current and resistance:
P = I²R
Electrical power using voltage and resistance:
P = V²/R
Electrical energy:
E = Pt
Commercial electrical energy:
Energy (kWh) = Power (kW) × Time (hours)
IMPORTANT UNITS TO REMEMBER
Electric charge – Coulomb (C)
Electric current – Ampere (A)
Potential difference – Volt (V)
Resistance – Ohm (Ω)
Resistivity – Ohm metre (Ω m)
Electric power – Watt (W)
Electrical energy – Joule (J)
Commercial unit of electrical energy – Kilowatt-hour (kWh)
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