Question

The p.d. across a lamp is 12 V. How many joules of electrical energy are changed into light and heat when i. a charge of 1 C passes through it, ii. a charge of 5 C passes through it, iii. a current of 2 A flows in it for 10 s?

Answer 1

i. 12 J
ii. 60 J
iii. 240 J

Explanation:

i. Note that `1 "ampere" = (1 C)/s`. That is the definition of the ampere.

If a 1 amp current flowed through the lamp for a time of 1 s, the charge that passed through it was 1 C. During that 1 s, the power would have been

`P = V*I = 12 V*1 A = 12 W`

The definition of the Watt is `(1 "Joule")/s`. Therefore 12 joules of electrical energy are changed into light and heat during that second.

ii. A 5 A current would send 5 C through the lamp each second. Allow that current to flow for 1 second. The power during that second would have been

`P = V*I = 12 V*5 A = 60 W`

Therefore 60 Joules of electrical energy are changed into light and heat during that second.

iii. A 2 A current would send 2 C through the lamp each second. Allow that current to flow for 10 second. The power during that amount of time would have been

`P = V*I = 12 V*2 A = 24 W`

Therefore 24 joules of electrical energy are changed into light and heat during each second. In 10 s, 240 J are changed into light and heat during those 10 seconds.

I hope this helps,
Steve