The fuse wire melts due to heating and breaks the circuit.
Explanation
An electric fuse is a safety device that operates on the principle of the heating effect of electric current (Joule’s Heating). It is inserted in series with the circuit to protect electrical appliances from damage caused by excessive current.
Mechanism of Action:
- Joule's Heating: According to Joule's Law ($H \propto I^2Rt$), heat is generated when electric current flows through a conductor. The amount of heat produced is directly proportional to the square of the current.
- Material Properties: The fuse wire is typically made of an alloy (such as lead and tin) that possesses a low melting point and relatively high resistance compared to the connecting wires.
- Breaking the Circuit: When the current exceeds the safe limit (rated value), the heat generated in the fuse wire increases rapidly. This causes the temperature of the wire to rise beyond its melting point. The wire melts and breaks, creating an open circuit and immediately stopping the flow of current.
Analysis of Incorrect Options:
- The fuse wire expands to increase resistance. Incorrect: While heating may cause minor expansion, the protective mechanism is the physical severance of the wire due to melting, not an increase in resistance through expansion.
- The fuse wire generates a counter-current. Incorrect: A fuse is a passive component and does not generate any current (counter or otherwise).
- The fuse wire absorbs the excess voltage. Incorrect: A fuse reacts to current flow, not voltage absorption. Voltage regulation is performed by devices like Zener diodes or stabilizers.
Key Takeaway:
A fuse protects an electric circuit by melting due to the heat generated by excessive current, thereby breaking the circuit and preventing damage to appliances.