Question

When a fuse is rated 8A, it means.
A. It will not work if current is less than 8A
B. It has a resistance of 8 ohm
C. It will work only if current is 8A
D. It will burn if current exceeds 8A

Answer 1

Hint: Fuse is used in electrical appliances to protect them from the overflow of current through them. When a current flows through a resistor, heat gets generated in it because of its resistance to flow of current. If the heat generated will be more than the melting point of the resistor, it will burn up.

Complete answer:
We know that power generated in a resistor of resistance R carrying a current I is given by formulae $PI^2R$ and thus heat generated in time $t$ , during flow of current through it is given by $H=I^2Rt$.
Rating a fuse as 8A means that it can sustain heat generated by 8A current in 1 second through its resistor and thus if the current will exceed 8A, the heat generated will be more than the value it can sustain. Thus, causing the wire of the fuse to melt.

So, the correct answer is “Option D”.

Note:
If a wire is rated 8A, it means that resistance is set to such a value that if 8A current is exceeded through it, it will overheat and will finally burn. It should be noted that the melting of a wire is not only dependent on either the value of current flowing through it or its resistance value. Rather it depends on the product of these values. But if one value, generally resistance, is kept constant then a single factor may be used for comparison.