NCERT Solutions Class 10 Science Chapter 11 Electricity: Questions, Answers and PDF

1

What an electric circuit means

A closed, continuous path through which an electric current flows

2

SI unit of current

The ampere: 1 A means 1 coulomb of charge flows in 1 second

3

Number of electrons in 1 coulomb

$n = \dfrac{Q}{e} = \dfrac{1}{1.6 \times 10^{-19}} \approx 6.25 \times 10^{18}$

4

A device that maintains a potential difference

A cell or a battery

5

What a potential difference of 1 V means

1 joule of work is done to carry 1 coulomb of charge between the two points

6

Energy given to each coulomb by a 6 V battery

$W = QV = 6\ \text{J}$

7

Factors on which resistance depends

Length, area of cross-section, the material (its resistivity) and the temperature

8

Current in a thick or a thin wire of the same material

A thick wire, because its resistance is less ($R \propto 1/A$)

9

Effect of halving the potential difference at constant resistance

The current also becomes half

10

Why alloys are used in toasters and irons

An alloy has a higher resistivity and does not oxidise easily at high temperature

11

Iron or mercury as a conductor, and the best conductor in the table

Iron (lower resistivity), and silver is the best conductor

12

Circuit diagram of 3 cells of 2 V, 5 Ω, 8 Ω and 12 Ω resistors and a key in series

A series circuit with a 6 V battery (draw it with the key)

13

Ammeter and voltmeter readings in that circuit

Ammeter 0.24 A, and voltmeter across the 12 Ω resistor 2.88 V

14

Equivalent resistance in parallel of 1 Ω and $10^6$ Ω

Slightly less than 1 Ω, and the same for 1 Ω, $10^3$ Ω and $10^6$ Ω

15

Lamp 100 Ω, toaster 50 Ω, filter 500 Ω in parallel on 220 V: the iron that takes the same current

Resistance 31.25 Ω, and current 7.04 A

16

Advantages of parallel connection

If one device fails the others work, and each device gets the full voltage

17

Three resistors of 2 Ω, 3 Ω and 6 Ω to get 4 Ω and 1 Ω

4 Ω: the 3 Ω and 6 Ω in parallel, in series with 2 Ω. 1 Ω: all three in parallel

18

Highest and lowest resistance with 4 Ω, 8 Ω, 12 Ω and 24 Ω

Highest 48 Ω (series), lowest 2 Ω (parallel)

19

Why the cord of a heater does not glow

The cord has a much lower resistance, so it produces much less heat than the element

20

Heat from 96,000 C through 50 V

$H = VQ = 4.8 \times 10^6\ \text{J}$

21

Heat in an iron of 20 Ω and 5 A in 30 s

$H = I^2Rt = 1.5 \times 10^4\ \text{J}$

22

What determines the rate at which energy is delivered by a current

Electric power

23

Motor taking 5 A from 220 V for 2 h

Power 1100 W, and energy $7.92 \times 10^6$ J

1

A wire of resistance R cut into 5 equal parts, joined in parallel: $R/R'$

25, option (d)

2

The term that is not electric power

$IR^2$, option (b)

3

A 220 V, 100 W bulb run on 110 V

25 W, option (d)

4

Heat in two identical wires in series and in parallel

1 : 4, option (c)

5

How a voltmeter is connected

In parallel across the two points

6

Copper wire (0.5 mm diameter) for a 10 Ω coil, and the effect of doubling the diameter

Length about 122.7 m, and the resistance becomes one-fourth

7

Resistance from the V–I data

About 3.3 Ω (the slope of the V–I graph)

8

12 V battery and a current of 2.5 mA

$R = 4800\ \Omega = 4.8\ \text{k}\Omega$

9

9 V battery and 0.2, 0.3, 0.4, 0.5 and 12 Ω in series

Current 0.67 A (the same in every resistor)

10

Number of 176 Ω resistors in parallel to carry 5 A on 220 V

4

11

Three 6 Ω resistors for 9 Ω and for 4 Ω

9 Ω: two in parallel, in series with the third. 4 Ω: two in series, in parallel with the third

12

Number of 10 W bulbs on 220 V with a limit of 5 A

110

13

Currents in a hot plate with two 24 Ω coils

9.2 A (one coil), 4.6 A (series) and 18.3 A (parallel)

14

Power used in the 2 Ω resistor in two circuits

8 W in each circuit, so the ratio is 1

15

Current from the mains for 100 W and 60 W lamps in parallel on 220 V

About 0.727 A

16

A 250 W TV for 1 h or a 1200 W toaster for 10 min

The TV: 250 Wh against 200 Wh

17

Rate of heat in an 8 Ω heater drawing 15 A

1800 W

18

Five “explain why” questions

Tungsten has a high melting point. Alloys have a high resistivity. Series is not used at home, because one failure stops all. $R \propto 1/A$. Copper and aluminium have a low resistivity

11

Electricity

11.1

Electric current and circuit

11.2

Electric potential and potential difference

11.3

Circuit diagram

11.4

Ohm’s law

11.5

Factors on which the resistance of a conductor depends

11.6

Resistance of a system of resistors (resistors in series, resistors in parallel)

11.7

Heating effect of electric current (practical applications)

11.8

Electric power

Electric current

$I = \dfrac{Q}{t}$

ampere (A)

Potential difference

$V = \dfrac{W}{Q}$

volt (V)

Ohm’s law

$V = IR$

Resistance

$R = \rho\dfrac{l}{A}$

ohm (Ω)

Resistivity

$\rho = \dfrac{RA}{l}$

ohm-metre (Ω m)

Resistors in series

$R_s = R_1 + R_2 + R_3$

ohm (Ω)

Resistors in parallel

$\dfrac{1}{R_p} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3}$

ohm (Ω)

Heat produced

$H = I^2Rt$

joule (J)

Electric power

$P = VI = I^2R = \dfrac{V^2}{R}$

watt (W)

Electrical energy

$E = Pt$

joule (J) or kWh

Current

Same through every resistor

Divides among the branches

Potential difference

Divides among the resistors

Same across every branch

Equivalent resistance

The sum, so it is larger than the largest resistor

Smaller than the smallest resistor

If one device fails

All the devices stop

The other devices keep working

Use at home

Not used

Used for the appliances

Writing that the current divides in a series circuit

In series the current is the same, and the voltage divides

Writing that the voltage divides in a parallel circuit

In parallel the voltage is the same, and the current divides

Taking the equivalent resistance in parallel as the sum

In parallel, use $\dfrac{1}{R_p} = \sum \dfrac{1}{R}$, and the answer is smaller than the smallest resistor

Not converting units (mA, cm, hours)

Convert to A, m and s before you substitute

Using the wrong power formula

$P = VI = I^2R = \dfrac{V^2}{R}$. Use $\dfrac{V^2}{R}$ when the voltage is fixed, and $I^2R$ when the current is fixed

Connecting an ammeter in parallel or a voltmeter in series

An ammeter is connected in series, and a voltmeter in parallel