A fuse wire made of tin is designed to melt when the current exceeds 10 A. What change should be made to the fuse wire (of the same material) so that it safely carries a current of up to 30 A to an electrical appliance?
- AIncrease the cross-sectional area (thickness) of the fuse wire and connect it in series with the appliance
- BIncrease the cross-sectional area (thickness) of the fuse wire and connect it in parallel with the appliance
- CDecrease the cross-sectional area (thickness) of the fuse wire and connect it in series with the appliance
- DDecrease the cross-sectional area (thickness) of the fuse wire and connect it in parallel with the appliance
Show answer and explanation
Correct answer: (a) Increase the cross-sectional area (thickness) of the fuse wire and connect it in series with the appliance
Explanation
A fuse melts when the heat produced in it (H = I²Rt) raises it to its melting point. To let it carry 30 A instead of 10 A without melting, its resistance must be made smaller:
- R = ρL/A, so for the same material, a thicker wire (larger cross-sectional area) has lower resistance and a higher current rating.
A fuse must always be connected in series with the appliance, so that when it melts it breaks the circuit and stops the current. In parallel, the current would simply bypass a blown fuse.