CBSE Class 10 Science 2018 · Paper 31/1 · Question 12(b)

Electricity 3 marks Medium

(a)Write Joule’s law of heating.
(b)Two lamps, one rated 100 W; 220 V, and the other 60 W; 220 V, are connected in parallel to electric mains supply. Find the current drawn by two bulbs from the line, if the supply voltage is 220 V.

Answer

Joule's law gives $H = I^{2} R t$ ; the lamps draw $0.45$ A and $0.27$ A, so the line current is $0.73$ A.

(a) Joule's law of heating: the heat

H

produced in a resistor is directly proportional to the square of the current

I

through it for a given resistance, directly proportional to its resistance

R

for a given current, and directly proportional to the time

t

for which the current flows. So

H = I^{2} R t

. (b) In parallel, each lamp gets the full

220

V, its rated voltage, so each draws

I = \frac{P}{V}

. Current in the 100 W lamp:

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

\approx 0.45

A. Current in the 60 W lamp:

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

\approx 0.27

A. Total current drawn from the line:

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

\approx 0.73

🔒 Step-by-step solution