Electrical current

There are two types of current: direct and alternating. In a direct current (dc), the flow of electrons is consistently in one direction around the circuit. In an alternating current (ac), the direction of electron flow continually reverses.

Charge

Electrons are negatively charged particles and they transfer energy through wires as electricity.

Charge is a property of a body which experiences a force in an electric field. The flow of charge is measured in coulombs (C).

Since electrons are so small and one electron will not have much of an effect anywhere, it is more useful to refer to packages of electrons. One coulomb of charge is a package equivalent to 6,250,000,000,000,000,000 electrons.

Current

curriculum-key-fact
Electrical current is a flow of electrons.

When current flows, electrical work is done and energy transferred. The amount of charge flowing past a point in the circuit can be calculated using the equation:

charge flow = current × time

\text Q = \text I \times \text t

This is when:

  • charge flow ( \text{Q}) is measured in coulombs (C)
  • current ( \text{I}) is measured in amperes or amps for short (A)
  • time ( \text{t}) is measured in seconds (s)

One amp is the current that flows when one coulomb of charge passes a point in a circuit in one second.

Example

A current of 1.5 amps (A) flows through a simple electrical circuit.

How many coulombs of charge flow past a point in 60 seconds?

\text Q = \text I \times \text t

\text{Q} = 1.5 × 60

\text{Q} = 90 C

Question

How much charge has moved if a current of 13 A flows for 10 s?

\text Q = \text I \times \text t

\text{Q} = 13 × 10

\text{Q} = 130 C

Question

How much current flows when 10 C passes down a wire in 2 s?

\text Q = \text I \times \text t

\text{I} = \frac{\text{Q}}{\text{t}}

\text{I} = \frac{10}{2}

\text{I} = 5 A

Measuring current

Current is measured using an ammeter. To measure the current through a component, the ammeter must be placed in series with that component.

Circuit with a cell, switch, lamp and ammeter connected in series.

Charge will only flow in a circuit if the circuit is closed, or complete, and if there is a potential difference applied to the circuit either by using a power supply or a battery.