Final answer:
Cutting a wire in a bomb's timer opens the circuit and stops the flow of electricity, similar to how cutting the wire in a fuse breaks the circuit when excessive current causes the wire to melt and interrupt the flow of electricity.
Step-by-step explanation:
When a character in a movie cuts a wire to stop a bomb's countdown timer, they are altering the circuit in which the timer operates. The wire that is cut likely forms part of a complete circuit that allows electric current to flow and power the timer. Cutting the wire opens the circuit, stopping the flow of electricity, and thereby deactivating the timer. This is similar to how a fuse functions in a circuit; when an excessive current flows through a fuse, it heats up the wire within the fuse until it melts, which breaks the circuit and prevents the flow of electricity. By cutting the wire, the circuit is interrupted immediately rather than being delayed as with a fuse which requires the wire to heat up before breaking.
Devices such as circuit breakers and fuses are designed to protect circuits from the dangers of excessive currents. When a circuit overheats from a high current, it poses thermal hazards which fuses and circuit breakers are meant to prevent. Fuses and circuit breakers interrupt excessive currents to safeguard against these hazards by breaking the circuit connection, just as cutting the wire in a movie bomb scenario would do.