Resistance of any component is the pd across the component/The current through it. (Ω)

**R=V/I**

Resistors are designed to have a certain resistance, regardless of current through it. So, ther graph of a resistor, current against pd, gives a straight line going through the origin.

Ohm's law states that the pd across a metallic conductor is proportional to the current through it, provided the physical conditions don't change.

For conductor length L, cross-sectional area A, resistance R, resistivity is:

**Resistivity, ρ (Ωm) = RA/L**

Superconductor has zero conductivity at and below a critical temperature.

When a current goes through it, there's no pd as resistance is zero, so no heating effect.

It loses it's superconductivity if it's temperature rises above the critical temperature, and they're used for high-power electromagnets for very strong magnetic fields, and power cables to transfer energy without loss.

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