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Resistance is measured in ohms. It can be calculated from the potential difference across a component and the current flowing through it. The total resistance of a series circuit is the sum of the resistances of the components in the circuit.

Resistors, filament lamps and diodes produce different current-potential difference graphs. The resistance of thermistors depends on the temperature, while the resistance of light-dependent resistors (LDRs) depends on the light intensity.


There is a resistance [resistance: The degree to which a component impedes the passage of current. Shown by the letter R. The unit of resistance is the ohm. ] to the flow of an electric current [current: Moving electric charges, for example, electrons moving through a metal wire. ] through most conductorsconductor: An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. A thermal conductor allows thermal energy to be transferred through it easily..

The resistance in a wire increases as:

  • The length of the wire increases
  • The thickness of the wire decreases

Resistance - Higher tier

An electric current flows when electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ] move through a conductor, such as a metal wire. The moving electrons can collide with the ions [ion: Positively- or negatively-charged particles - eg positively charged hydrogen, sodium and potassium atoms. Ion charge helps determine a substance's acidity or alkalinity ] in the metal. This makes it more difficult for the current to flow, and causes resistance.

The resistance of a long wire is greater than the resistance of a short wire because electrons collide with ions more often.

The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current.

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