Electricity

Review of Year 12Year 13 Material

Internal Resistance

Electrostatic Forces and Coulomb’s Law Like charges are

repelled from one another, and unlike charges are attracted.

The forces involved in this repulsion/ attraction are given by Coulomb’s law:

Electric Current The electric force can be used to push

charge (measured in coulombs) around a circuit.

A large positive charge is maintained at one terminal of the battery, and a large negative charge at the other terminal – when they are linked by a circuit, charge flows towards the negative terminal.

This results in electric current.

Energy and Potential Difference For current to be maintained, the

difference in charge between the terminals needs to be maintained.

This means energy must be continuously added to the system.

The energy source (battery, power pack, wall-socket) provides a potential difference – this is the energy that it gives to one coulomb of charge.

Internal Resistance (def’n) Internal resistance

refers to the resistance of a voltage source when a current is flowing.

This results in a lowered terminal voltage.

Internal Resistance: A battery in a circuit can be

regarded as a source of EMF (electromotive force), ε, and a resistor in series.

If we measure the voltage, V, across the terminals of a battery in a circuit, we are therefore measuring:

V = ε – Ir

V is the terminal voltage,ε is the EMFI is the current flowingr is the internal resistance

ε

r

Practical Activity: Internal Resistance