Potential Difference
Potential Difference (p.d.) – the electrical energy transferred per unit charge when electrical energy is converted into any other type of energy.
Potential difference is often just referred to as voltage.
When charge carriers, such as electrons, pass through a component (i.e. a bulb) energy is transferred to the component (work is done). Potential difference is a measure of how much energy (per unit charge) is transferred to the component.
Therefore, potential difference can be measure between two points either side of a component, as the transfer of energy to the component causes a difference in energy between the points.
Potential difference (V) can be calculated using the following equation:
Where W is the energy measured in joules (J) and Q is the charge measured in coulombs (C). The SI unit for potential difference is the volt:
Volt – one volt is one joule per coulomb.
Electromotive Force
Electromotive Force (e.m.f.) – the energy transferred per unit charge when any type of energy is converted into electrical energy.
It is important to note that electromotive force is not a ‘force’.
As you can see from the definition e.m.f. it is very similar to potential difference. The difference between the two is the energy transfer.
For e.m.f. we are looking at the amount of energy converted into electrical energy per unit charge. For example if we have a circuit in which a cell (or battery) is the source of energy the e.m.f. would be the amount of chemical energy from the cell converted into electrical energy per unit charge.
Whereas, potential difference is the amount of electrical energy converted into another type of energy per unit charge. For example, if we had a circuit with a bulb it would be the amount of electrical energy converted into light energy per unit charge.
We can calculate e.m.f. (ε) using the following equation:
In symbol form we have:
The unit of energy is the joule (J) and the unit of charge is the coulomb (C). As with potential difference the SI unit of e.m.f. is the volt.
Worked Examples
Example 1
250, 000J of electrical energy are required in a circuit. Calculate the e.m.f. of the supply if the charge supplied is 1087C.
Example 2
The voltage between two points is 3.5V. Calculate the charge required to transfer 45J of energy between these two points.
Example 3
The potential difference across a mini motor is 6V. Calculate the current in the circuit if 50J of electrical energy is converted into thermal and mechanical energy in the motor in 40s.
This question requires knowledge of charge and current.
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