Potential difference

Potential difference in an electrical circuit indicates that there is a difference in electrical potential across a circuit component (such as a battery).

Potential difference is measured in volts (V) and the term voltage is also used instead of potential difference.

Potential difference is measured using a voltmeter.

Cells, batteries and e.m.f

There is a special term for the voltage across a cell. It is known as an electro motive force (e.m.f) of the cell and it is also measured in volts.

Any component that pushes a current around it has an e.m.f.

Examples of components having an e.m.f are:

  • Cells
  • Batteries
  • Power supplies
  • Dynamos

Common misconceptions: Though e.m.f stands for electro motive force, it is a voltage and not a force! Be careful!

Voltage and energy

We have observed that an e.m.f of a supply tells us about how much energy it transfers to charges flowing around the circuit.

The greater the current flowing around the circuit, the faster that energy is transferred. Thus, in a circuit, the rate at which energy is transferred (the power P) depends on the e.m.f (V) and the current (I) that it pushes around the circuit.

These relationships help construct an important equation:

Exam tip!

You may find it easier to remember the above equation as:

Calculating energy

As energy transformed = power x time, we can modify the equation P = IV to give an equation for energy transformed:

Voltages around a series circuit

In a series circuit, the voltage at the terminals of all batteries equals to the sum of voltages across the devices in the external circuit from one battery terminal to the other.

In short, if a circuit has a battery of 4.5V and has three lamps, each of the lamps will have a voltage of 1.5 volts.

This is demonstrated by the equation below:

and so on.

Voltages around a series circuit

In a parallel circuit, the voltages across all devices are equal.

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