CoachA device is short-circuited when a conducting road joins its two terminals directly, bypassing it. The march deserts the device — a short-circuited lamp goes dark, a motor stops — while the rest of the loop carries on, now with one worker fewer opposing the march.
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Example 48.5 (Why the wires heat)
A marching current always warms its road a little — the lamp’s glow is that warming, put to work on purpose in a thread built to run white-hot. In a stampede, the warming turns menace: the short circuit’s rush can heat a copper wire past the melting point of its plastic sleeve within seconds. The pocket-scale preview: a battery shorted by a wire grows warm in your hand — the same physics that, at mains scale, chars walls. (How warming grows with the rush is measured honestly in two years, with the electric power chapter.)
Example 48.8 (Overload: the polite stampede)
Not every dangerous rush is a short circuit. A multi-socket feeding a heater, a kettle and a toaster at once asks its one branch to carry three full marches together — each appliance innocent, their sum an overload that warms the wall wiring like a slow toaster. The breaker trips on the sum, and rightly so. House rule: one hungry heat-making appliance per socket; the multi-socket is for lamps and chargers, not for a private kitchen.