Electrical resistance is the opposition by a material to the flow of electrical current when a potential difference is applied. A resistor is an electronic component designed to have a particular resistance value.

The SI unit of resistance is the ohm. The unit is named after German mathematician / physicist Georg Ohm (1789 – 1854), but the unit symbol is the upper case Greek letter omega (Ω), rather than 'O', due to the potential for confusion between letters and numbers.

Mathematically, a resistance of one ohm will be found when a potential difference of one volt causes a current of one ampere to flow. Ohm's law generalises this relationship as:

Equation: R = V / I

Worked Example 1

Calculate the equivalent resistance if an applied voltage of 12 V causes a current of 50 mA to flow.
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R = V ÷ I

  = 12 ÷ 50 × 10-3

  = 240 Ω

Worked Example 2

Find the potential difference across a 330 Ω resistor, caused by a current of 30 mA.
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Transposing the above expression to make V the subject gives:

V = I × R

  = 30 × 10-3 × 330

  = 9.9 V

The reciprocal of resistance is conductance.

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