Electromotive force and internal resistance Flashcards – AQA A-level Physics (7408)

What is electromotive force (emf), and what is its unit?

The energy transferred by a supply to the circuit per unit charge: $\varepsilon = E/Q$. It is measured in volts and is not a force.

What happens to energy within a real supply when it delivers current?

Energy is dissipated within the supply because of its internal resistance.

What is the internal potential difference of a supply with internal resistance $r$ delivering current $I$?

$Ir$

What is the terminal potential difference of a supply?

The energy transferred per unit charge to the external circuit.

How are emf, terminal potential difference, current and internal resistance related for a supply delivering current?

$\varepsilon = V + Ir$

How does a supply’s terminal potential difference compare with its emf when it delivers no current and when it delivers current?

With no current, it equals the emf; when delivering current, it is lower than the emf.

What is the terminal potential difference across an external circuit of equivalent resistance $R$ carrying current $I$?

$V = IR$

How are emf, current, external resistance and internal resistance related for a supply delivering current?

$\varepsilon = I(R+r)$