Calculating Resistance

A steady direct current flows through a conductor.

The resistance R is the ratio of the potential difference V to the current I.

R = V ÷ I

S.I. Units
symbol	quantity	unit	unit symbol
V	potential difference (voltage)	volt	V
I	current	amp or ampere	A
R	resistance	ohm	Ω

Ω is the Greek letter omega.

Equations

R = V ÷ I ; resistance equals voltage divided by current.

V = I × R ; voltage equals current multiplied by resistance.

I = V ÷ R ; current equals voltage divided by resistance.

Example 1

When a six volt filament bulb is operating at its correct voltage (6 V) the current through it is 0.3 A. Calculate the resistance of the bulb.

I = 0.3 A
V = 6 V

R = V/I = 6/0.3 = 20 Ω

Example 2

A one metre length of Constantan wire has a current of 70 mA when the potential difference across it is 0.147 V. Calculate the resistance of the wire.

I = 70 mA = 0.070 A
V = 0.147 V

R = V/I = 0.147/0.070 = 2.10 Ω