resistance

Resistance is how strongly a material opposes the flow of current, and it is measured in ohms. It is the R in Ohm's law, so R = V ÷ I: for a given push, more resistance means less current. Resistance is also where electrical energy turns into heat.

Four things set it. Material matters most — copper and silver resist very little, while rubber and glass almost stop current. Length adds resistance, thickness removes it (a thicker wire carries more), and in metals a higher temperature raises it. A resistor is a part built to put a chosen amount of resistance into a circuit on purpose, and its value is read from its color bands.

The consequences are what the questions test. At a constant voltage, doubling the resistance halves the current. Adding a second path in parallel lowers the total, because the current has more room. And because power is current squared times resistance, heat climbs quickly as current rises — which is exactly how a fuse is designed to melt.