CBSE Class 10 Science 2020 · Paper 31/3/2 · Question 29

Electricity 5 marks Medium

(a) Define Power and state its SI unit. (b) A torch bulb is rated 5 V and 500 mA. Calculate its

Answer

Power is the rate of doing work, measured in watts; the bulb has $P = 2.5$ W, $R = 10 Ω$ and uses $22500$ J.

(a) Power is the rate of doing work, or the rate at which electrical energy is consumed (dissipated) in a circuit:

P = \frac{W}{t}

. Its SI unit is the watt (W), where

1 W = 1 J/s

. (b) With

I = 500

= 0.5

A: (i)

P = V I = 5 \times 0.5 = 2.5

W; (ii)

R = \frac{V}{I} = \frac{5}{0.5} = 10 Ω

; (iii)

E = P t = 2.5 W \times 2.5 h = 6.25

Wh, which is

22500

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