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# Definition of resistance

Resistance can be thought of as a quantity that prevents or slows down the flow of charge.

The higher the resistance of a wire, the more energy is lost as a charge flows through it.

Objects lodged in water pipes prevent the flow of water. Similarly, metal atoms in wires prevent the flow of electrons.

The resistance ($R$) of a conductor is the ratio of the potential difference ($\Tblue{V}$) across the conductor to the current ($\Tred{I}$) flowing through it.

$$\text{resistance} = R =\frac{\Tblue{\text{potential difference}}}{\Tred{\text{current}}} = \frac{\Tblue{V}}{\Tred{I}}$$

The SI unit for resistance is the ohm $(\Omega)$. One ohm is equivalent to one volt per ampere (i.e. $1\text{ }\Omega=1\text{ V } / \text{ A}$).

Metals like copper or gold have low resistance while non-metals like plastic and rubber have high resistance.

A common resistor. The coloured bands on the resistor indicate its resistance (given by a colour code chart).