Electric Current (Edexcel International A Level Physics)

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Electric Current

Electric Current

  • Electric current is defined as:

The rate of flow of charge

  • It is measured in units of amperes (A) or amps
  • The charge, current and time are related by the equation:

Charge equation, downloadable AS & A Level Physics revision notes

  • There are several examples of electric currents, including in household wiring and electrical appliances

Electric Charge

  • Electric charge is a property of some particles
    • For example, protons have a positive charge and electrons have a negative charge
  • Charge has the unit Coulombs, C
  • In electric circuits, electrons are usually the charge carriers
    • They have a charge of 1.6 × 10−19 C
  • Charge, Q, can be calculated using the equation

Q = ne

  • Where:
    • Q = charge (C)
    • n = number of electrons
    • e = electron charge (C)

  • If 1 electron has a charge of 1.6 × 10−19 C
    • Then, 1 C of charge contains 6.25 × 1018 electrons

  • Charge is sometimes written as ΔQ which means 'change in charge'
    • Similarly, time is written as Δt means 'change in time'

  • When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current
  • Therefore, rearranging for current, I gives the equation:

Current Equation

Flow of charge, downloadable AS & A Level Physics revision notes

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

Direction of Flow of Charge

  • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
    • This is the opposite to the direction of electron flow, as the conventional current was described before electric current was really understood

Electric current flow, downloadable AS & A Level Physics revision notes

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

Measuring Current

  • Current is measured using an ammeter
  • Ammeters should always be connected in series with the part of the circuit the current is to be measured through
    • This is because the current is the same in all components connected in series

Ammeter in series, downloadable AS & A Level Physics revision notes

An ammeter can be used to measure the current around a circuit and always connected in series

Worked example

The current in a filament lamp is  8 mA.

Which answer below explains how this current can be obtained?

A.    1 J of energy is used by 1 C of charge

B.    A charge of 4 C passes in 500 s

C.    A charge of 8 C passes in 100 s

D.    A charge of 1 C passes in 8 s


Step 1: Write out the equation relating current, charge and time

Q = It

Step 2: Rule out any obviously incorrect options

    • Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

    • Consider option B:

I = 4 / 500 = 8 × 10–3 = 8 mA

    • Consider option C:

I = 8 / 100 = 80 × 10–3 = 80 mA

    • Consider option D:

I = 1 / 8 = 125 × 10–3 = 125 mA

    • Therefore, the correct answer is B

Exam Tip

Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers.

An example of this is electrolysis where ions are used as charge carriers in the ionic solution and the ions travel through a liquid as an electric current

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