CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 21

Electricity 5 marks Medium

(a)Define electric power. Express it in terms of potential difference ‘ $V$ ’ and resistance ‘ $R$ ’.
(b)An electric motor takes 5.0A from an electric source of 220 V. Determine the power of the motor and calculate the energy consumed by the motor in kWh, when it runs for 5 hours.

Answer

Power is the rate of using electric energy, $P = \frac{V ^{2}}{R}$ ; the motor's power is 1100 W and it uses 5.5 kWh in 5 hours.

(a) Electric power is the rate at which electric energy is consumed (dissipated) in an electric circuit:

P = \frac{W}{t}

. Since

P = V I

and, by Ohm's law,

I = \frac{V}{R}

, the power in terms of

V

and

R

P = \frac{V ^{2}}{R}

. Its SI unit is the watt (W). (b) Power of the motor:

P = V I = 220 V \times 5.0 A = 1100

= 1.1

kW. Energy consumed in

5

hours:

E = P \times t = 1.1 kW \times 5 h = 5.5

kWh.

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