Potential Difference

Potential difference measures the work done per unit charge

The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points.

V = W/Q


W = VQ

A volt is a 'joule per coulomb' OR a coulomb is a 'joule per volt'


What matters most to an electron?

If you were an electron you would have 'charge' as well as 'mass'.

The force exerted on your mass by gravity would be negligible compared to the electrostatic force that you would experience because you were charged.

You would therefore not be very bothered by the 'ups and downs' in the physical dimension around you - hills and dales, slopes and steps - because the force of gravity does not have a very big effect on you.... but the proximity of any charged object would have a big effect on you - negative charge would repel you strongly and positive charge would attract you.

Click here for the calculation

Don't worry if it looks hard, it is hard maths (not needed until A level)... just look at the answer!

Scientific Model

A scientific model is like a scientific parable.... whereas a parable is an earthly story with a heavenly (spiritual) meaning, a scientific model is an earthly description in everyday terms with a deeper scientific meaning. Just as the analogy of a parable can't be taken too literally, so the model can't.... but its basic features help us to understand how something behaves.

A Scientific Model for Electric Potential

The fact that you are charged makes you susceptible to changes in something we call electric potential gradient (a type of electrical slope).

The bigger the change in gradient (the steeper the 'electric slope') the bigger push you experience from the electric force.

You therefore have to think of potential difference as an 'electric slope' that charged particles slide down (or up... if they are of opposite charge!).

- the bigger the potential difference the 'steeper this slope' and therefore the bigger the force or push they receive.

- the bigger the charge on the particle the more charge moves each time therefore the bigger the current.

- the bigger the potential gradient for an electron in a wire the faster it moves, so the bigger the current flow (as current is rate of flow of electrons in a wire)

The voltmeter

This is an instrument for measuring potential difference. It measures the difference in electrical height between to levels of electric potential. Each of these are in volts, so it measures potential difference in volts.

Try these questions:

Q1. A component has a potential drop of 4.0 V across it. This makes a current of 0.25A flow through it. What current will flow if the drop is increased to 8.0 volts?

Q2. A component has a potential drop of 3.0 V across it. This makes a current of 0.15 A flow through it. What potential drop must be across the component if the current flowing through it drops to 0.05 A?

Click here for the answers