Question

what you know now about electricity, explain the old adage that a light burns brightest just before it fails.

Answer 1

Incandescent lights have a thin filament wire that heats up when current passes through it. The amount of heat and light it produces is proportional to the resistance of the wire, which is proportional to the thickness of the wire. As the bulb ages, the wire gets thinner, so its resistance rises, so its light output rises. Initially this is a fairly gradual process, but as the wire gets ever thinner, it begins to become a runaway effect, where as the wire gets thinner, it gets hotter, and as it gets hotter so the rate at which it gets thinner increases, which then makes the wire hotter yet; and towards the end this happens so rapidly as to become obviously noticeable.