Electric current

Electric current describes how quickly electric charge flows past a point. The more charge that passes the point in a given time, the greater the current.

The average current is the charge that passes the point divided by the time taken:

Here, $$I$$ is the average current, $$\Delta Q$$ is the charge passing the point, and $$\Delta t$$ is the time interval. The formula gives the average over that interval, even if the current changes during it.

Current is measured in amperes (A). One ampere means that one coulomb of charge passes a point per second: $$1,\mathrm{A} = 1,\mathrm{C/s}$$.

Potential difference

When charge moves between two points, energy can be transferred. The potential difference between the points is the energy transferred, or work done, per unit of charge moving between them.

Here, $$V$$ is the potential difference, $$W$$ is the work done or energy transferred, and $$Q$$ is the charge. Work done is measured in joules ($$\mathrm{J}$$) and charge in coulombs ($$\mathrm{C}$$).

Potential difference is measured in volts ($$\mathrm{V}$$). One volt is one joule per coulomb: a potential difference of $$4,\mathrm{V}$$ means that $$4,\mathrm{J}$$ of energy is transferred for each coulomb of charge moving between the two points.

Electrical resistance

A component’s electrical resistance tells us how much potential difference is associated with a given current through it. For the same current, a component with greater resistance has a greater potential difference across it.

Resistance is the ratio of the potential difference across the component to the current through it:

Here, $$R$$ is resistance, $$V$$ is potential difference and $$I$$ is current.

Resistance is measured in ohms, written $$\Omega$$. One ohm is one volt per ampere: $$1,\Omega = 1,\text{V/A}$$.

For example, if the potential difference across a component is $$6,\text{V}$$ and the current through it is $$2,\text{A}$$, its resistance is $$6,\text{V} \div 2,\text{A} = 3,\Omega$$.