Current, I, in amperes, A (amp is acceptable for ampere)

The idea

Electric current, I, is the rate of flow of electric charge, measured in amperes, A, where one ampere is one coulomb of charge passing a point each second (1 A = 1 C s⁻¹). The term 'amp' is completely acceptable for ampere in both speech and written exam answers. In the motor effect, current in a conductor placed in a magnetic field produces a force, and Fleming's left-hand rule relates the mutually perpendicular directions of current, field and force.

For example: A current of 2.0 A in a 0.50 m wire perpendicular to a 0.30 T field gives F = B I L = 0.30 T × 2.0 A × 0.50 m = 0.30 N. Current is measured with an ammeter connected in series, and conventional current flows from positive to negative outside the source.

What earns the marks

• State that current I is measured in amperes, A, and that 'amp' is an acceptable written and spoken form of ampere.
• Define current as the rate of flow of charge, I = Q ÷ t, with charge in coulombs and time in seconds.
• Use I in the motor-effect equation F = B I L, identifying current as the flow through the conductor perpendicular to the magnetic field.
• Describe how to measure current with an ammeter connected in series, and explain that current is the same everywhere in a single closed loop.
• Apply Fleming's left-hand rule to predict the direction of force from the directions of current and magnetic field.
• Recognise that increasing current increases the force on the conductor in a magnetic field, for a fixed field and length.

Common mistakes and how to put them right

• Connecting an ammeter in parallel with a component.

  Put it right: An ammeter must be connected in series so the current to be measured passes through it.

• Confusing current with potential difference or charge.

  Put it right: Current is the rate of charge flow in amps, potential difference is energy per unit charge in volts, and charge is in coulombs.

• Using the symbol A for both the ampere and area.

  Put it right: In equations, use I for current and A for area, and include units to avoid ambiguity.

Exam tips

• Write the unit A or amp after every current value and check that the ammeter is described as connected in series.
• When using F = B I L, confirm that the current direction is perpendicular to the magnetic field before substituting values.
• Use the left-hand rule with the first finger for field, second finger for current and thumb for force, and state the direction clearly.