A level revision

Resistance and conductance

As electrons move through a wire they collide with other electrons and particles causing them to slow down as they lose energy which is transferred as heat. This is known as resistance, it is equal to the potential difference divided by the current:

Resistance is measured in Ohms () with 1 being equal to a potential difference of 1V causing a current of 1A to flow.

Conductance

Conductance is a measure of how much an electrical component carries an electric current. Conductance is measured in siemens (S) and is represented by the letter G in equations. Conductance is the reciprocal of resistance so:

As resistance is equal to voltage divided by current and conductance is the reciprocal conductance can also be calculated using:

Series and parallel circuits

The total resistance of resistors connected in series is equal to the sum of all the resistances. In other words:

The total conductance is just the reciprocal:

In a parallel circuit the total resistance can be calculated like so:

And again conductance is simply the reciprocal: