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The ratio of voltage across to the current in a component.

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1. Resistance and Ohm's Law

V is always equal to IR, but V is not always proportional to I. If that doesn't make sense then you need to see this video and the next one on IV characteristics.

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2. Resistance and Temperature

Why does the resistance of a conductor increase with temperature? This model of the vibrating ions is one way to visualise what is happening inside the lattice.

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3. Conductance

Just like resistance is an indication of how much current is impeded, conductance tells us how well something conducts electricity.

(This is not needed for all exam boards - check with your specification)

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4. Liquid Nitrogen and High Voltages

As you cool a substance its resistance decreases (if you go extremely low you get into the realms of superconductivity where R = 0). 

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