3 Electric potential difference (voltage)

Water analogy.

• Two connected containers X and Y, with a closed valve between them.
• The water level in X is higher than in Y, so the water pressure at the valve is higher on X's side.
• When the valve is opened, water flows from X to Y until the levels become equal, then the flow stops.

• In the same way, charges flow through a conductor from a point of higher electric potential to a point of lower electric potential.
• If the two points have the same potential, there is no current between them.
• A potential difference (cause) produces a current (effect).

The electric potential difference between two points (voltage) is the work done to move one coulomb of electric charge between the two points.

V = W / Q

W in joules (J), Q in coulombs (C), V in volts (V). So 1 volt = 1 joule per coulomb.

"The potential difference across a conductor is 3 volts" means that 3 joules of work are done (3 J of electrical energy transformed) to move each coulomb of charge through it.

A voltmeter measures potential difference. It is connected in parallel across the two points.

Worked example 3

A lamp uses 120 J of electrical energy to carry 20 C through it. What is the potential difference across the lamp? V = W/Q = 120 / 20 = 6 V.