What is an ohm and what is Ohms Law?

An Ohm is the SI unit for electrical resistance and is symbolised by the Greek letter Ω. It is named after the German physicist Georg Simon Ohm.

The Ohm is related to the current and voltage in a system: a current of 1 amp through 1 ohm of electrical resistance produces a voltage of 1 volt across it. 

The formula for this is I=V/R where:

I = the current through the conductor

V = the voltage measured across the conductor

R = the resistance of the conductor

What is resistance?

Resistance measures how strongly a material opposes the flow of electric current.

A "perfect" conductor would let electrons move through it completely freely. In practice, every conductor used in cables (including copper and aluminium) has some resistance, so there is always some opposition to the flow of electrons. This opposition comes at a cost. As current flows, some of the electrical energy is converted into heat instead of reaching the equipment it's meant to power. These are known as power losses, and they rise with the square of the current (P=I²R), so doubling the current produces four times the heat. That's why resistance is so important when selecting and sizing cables. 

The resistance of a conductor depends on four main factos: the material it's made from, its length, its cross-sectional area and its temperature. 

What is Ohm's Law

To understand current in practical terms, Ohms law describes the 3-way relationship between Voltage (potential difference/PD), Current and Resistance.

The current flowing in a circuit is equal to the voltage across it divided by its resistance:

The same formula can be rearranged to find any of the three values. The Ohm's last triangle above is an easy way to remember this. 

To further understand, it is easy to imagine an electrical circuit in a similar way to a water pipe, transporting water from A to B.

Resistance, as the name implies is the difficulty encountered by the flow of current; if you can imagine water trying to pass through grit/sand, the 2 options would be to a) increase the size of the tube to reduce the resistance or (b) to put more pressure behind the water to force it through the obstacle (resistance)

Current equates to the intensity of electric charge passing through the conductor, so it therefore would mean that the larger the conductor (or bore of a hose) the larger the possible current.

Voltage or PD is the pressure that it takes to move the electricity (or water) - so  we can consider voltage as electrical pressure or more correctly Electromotive force.

Find out more

Now that you know how voltage, current and resistance relate to each other, find out how conductor material affects resistance in our FAQ What is electrical conductivity and conductor resistance?, or use our cable calculator to work out voltage drop and cable sizing for your circuit.

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