Question

(a) When does a fuse cut off current? How does it do it?
(b) What is the maximum number of 60 W bulbs that can be run from the mains supply off 220 volts if you do not want to overload a 5 A fuse?

Answer 1

(a) An electric current is cut off by a fuse when the current exceeds a safe value.
When short circuit or overloading takes place, current becomes large and heats the fuse wire too much. The melting point of fuse wire is much lower than other conducting wires; therefore, the fuse wire melts and breaks the circuit. When the fuse wire breaks, current supply is automatically switched off.

(b) Let the maximum number of bulbs be n.
Power of one bulb = 60 W
So, power of n bulbs, P = 60 $\times$ n watts
Potential difference, V = 220 volts
Current, I = 5 A
We know that:
P = V $\times$ I
So, 60 $\times$ n = 220 $\times$ 5
or, n = 18.3
Thus, 18 bulbs can be run in the given case.