CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 19(a)

Electricity 5 marks Medium

(a)Define electric power. An electrical device of resistance $R$ is connected across a source of voltage $V$ and draws a current $I$ . Derive an expression for power in terms of current and resistance.
(b)Two electric bulbs rated 100 W; 220 V and 60 W; 220 V are connected in parallel to an electric mains of 220 V. Find the current drawn by the bulbs from the mains.

Answer

Power is the rate of energy consumption, $P = V I = I^{2} R$ ; the two bulbs draw $\frac{8}{11}$ A $\approx 0.73$ A from the mains.

(a) Electric power is the rate at which electric energy is consumed (dissipated) in an electric circuit; its SI unit is the watt. The work done in moving a charge

Q

through a potential difference

V

W = V Q

, so

P = \frac{W}{t} = \frac{V Q}{t} = V I

, since

I = \frac{Q}{t}

. By Ohm's law

V = I R

, hence

P = (I R) I = I^{2} R

. (b) In parallel, each bulb gets 220 V:

I_{1} = \frac{100}{220} = \frac{5}{11}

A and

I_{2} = \frac{60}{220} = \frac{3}{11}

A. Current drawn from the mains

I = I_{1} + I_{2} = \frac{8}{11}

\approx 0.73

🔒 Step-by-step solution

CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 19(a)

Answer

From the board papers

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