MDCAT Physics · Physics · Current electricity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Current is the rate of flow of charge. In a metal the charge is carried by free electrons. Ohm's law says that the current in a conductor is proportional to the potential difference across it, as long as the temperature stays constant. A filament lamp does not obey it, because the filament heats up and its resistance rises.
The resistance of a wire depends on its length, its cross-sectional area and its material. Resistivity belongs to the material: it changes with temperature but not with the size or shape of the wire.
Resistors in series carry the same current and their resistances add. Resistors in parallel have the same potential difference, and the total is less than the smallest one.
Treating area as proportional to diameter. Area depends on diameter squared, so a wire with twice the diameter has four times the area and one quarter of the resistance.
Answer: 32 Ω
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