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Understanding Electrical Terms

As PAT testing is based around electrical safety, it is important to have a good understanding of the various fundamental aspects of electricity and what they actually mean.


“Voltage is directly related to the force electrons free in a wire. This force propels them through the wire. You can think of voltage (more correctly expressed as a difference in electrical potential) as an analogy to water pressure in a pipe. The pressure is what causes water (current) to flow out of the tap, for example.”

If you have no voltage then no current will flow.

Voltage is basically the driving force behind electricity. It is defined as the ‘electrical pressure’ that causes current to flow around a circuit, and is an essential part of absolutely any circuitry.

When you press a switch on a socket and there’s something plugged into it, the voltage supplied to that socket drives the current towards the appliance, thus powering it.

Voltage is represented by a V symbol, so if you see that anywhere you’ll know what it means.

UK mains voltage is always kept at 230V, while our American cousins step theirs down to 110V.


“Current is a measure of the number of electrons that pass through some given point in a wire per unit time. The larger the current, the more electrons flow through the wire per unit time.”

Although the most common name for the unit of current is ‘amp’, the full technical term is in fact ampere (A).  The general symbol for current is an I, which can seem a bit confusing at first (it comes from the French intensité de courant, which means current intensity).

Current is the flow of electricity across a circuit, driven on by the power of the voltage behind it.  The electrical flow is really tiny electrons flowing across a circuit.

The amount of current required by an appliance varies depending on the appliance type.


“Resistance is caused by a material’s interaction with electrons. Some material inhibit electrons from moving quickly through them, in the same way that a sponge shoving into your water faucet would inhibit the flow of water.”

Resistance is measured in Ohms (represented by the symbol Ω) and the actual symbol for resistance itself is R.

Resistances basically apply to all electrical current, as resistance opposes the flow of current as it is being driven along by a voltage.

Basically the higher the resistance is, the more difficult it is for current to flow.

Ohm’s Law

Ohm’s law is a series of equations that can be used to work out either of the three examples above.

Voltage (V) = Current (I) X Resistance (R)

Resistance (R) = Voltage (V)/Current (I)

Current (I) = Voltage (V)/ Resistance (R)


Power refers to how much energy flows through a circuit every second, and is denoted by the symbol P. The measuring unit of Power is the Watt, which is represented by a W.

The higher the amount of voltage and current flowing around a circuit, the higher the amount of power will be produced.  This power can then be converted into useful energy, such as lighting up our rooms.

Although it doesn’t come under Ohm’s law, power still has direct relation to voltage, current and resistance and can be worked out using the following formulas.

Watt (W) = Current (I) X Voltage (V)

Watt (W) = Voltage (V) x Voltage (V)/Resistance (R)

Watt (W) = Current (I) x Current (I) X Resistance (R)

Alternating Current and Direct Current

Known more commonly as AC and DC current, there’s a stark different between the two.

Direct current is usually used in an area where there’s not much need for the electricity to travel far, for example in a car battery.

When used at a distance, most of the energy generated by DC current is lost as heat. That’s why we use alternating current with a voltage of 230V.

Written by Barry Atkins