Fig. 32.1 shows a circuit in which a short thick wire is accidentally connected across the terminals of a lamp. (a) State what happens to the lamp when the ...

(a) When the short wire is connected across the lamp, the lamp goes out (stops working). [1]

The short wire has a very low resistance compared to the lamp. In a parallel arrangement, current takes the path of least resistance. Since nearly all the current flows through the short wire rather than through the lamp filament, the lamp receives essentially no current and produces no light. [1]

(b) The current from the battery increases greatly. [1]

The short wire effectively reduces the total resistance of the circuit to a very small value (just the wire's own tiny resistance plus the internal resistance of the battery). By \( I = \frac{V}{R} \), a much smaller R at the same voltage produces a much larger current.

(c) In a household mains circuit, the large current caused by a short circuit can make the wires overheat. This can melt the insulation surrounding the wires and potentially start a fire. [1]

Mains circuits carry much higher voltages (230 V or 240 V), so the currents during a short circuit can be extremely large and dangerous.

(d) A fuse or circuit breaker would protect the circuit from damage during a short circuit. [1]

A fuse contains a thin wire that melts and breaks the circuit when the current exceeds a safe value. A circuit breaker is an automatic switch that trips open when excessive current is detected, and can be reset after the fault is fixed.