Electrical charge and current

There are two types of current - direct and alternating. In a direct current, the flow of electrons is consistently in one direction around the circuit, in an alternating current, 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. 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 large groups of electrons. One coulomb of charge is equivalent to 6,250,000,000,000,000,000 electrons.

Current

In a closed circuit that includes a source of potential difference (voltage) there will be a current flowing around the circuit.

curriculum-key-fact
Electrical current in a metal is a flow of electrons

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

charge = current × time

Q = I \times t

This is when:

  • charge (Q) is measured in coulombs (C)
  • current (I) is measured in amperes (A)
  • time (t) is measured in seconds (s)

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

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'Amps' is an abbreviation for 'amperes', the unit for current (also abbreviated as 'A') but the symbol for 'current' is 'I'.

Example

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

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

Q = I \times t

Q = 1.5 \times 60

Q = 90 \ C

Question

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

Q = I \times t

Q = 13 \times 10

Q = 130 \ C

Question

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

Q = I \times t

I = \frac{Q}{T}

I = \frac{10}{2}

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.