Current, resistance and potential difference: current, I, in amperes, A (amp is acceptable for ampere)

The idea

Electric current is the rate of flow of electrical charge, measured in amperes (A), where one ampere equals one coulomb per second (1 A = 1 C/s). The symbol I represents current, so write I = 0.50 A. In a series circuit the current is the same at every point because charge is conserved and there is only one path. In parallel branches, the current splits so that the total current from the supply equals the sum of the branch currents. Current is measured with an ammeter connected in series with the component. For a fixed resistor at constant temperature, current is directly proportional to potential difference, so the current–potential difference graph is a straight line through the origin.

What earns the marks

• State that current is the rate of flow of charge, with the ampere defined as one coulomb per second (1 A = 1 C/s).
• Use the symbol I for current and the unit A for amperes, writing values such as I = 0.50 A.
• Describe how an ammeter is connected in series with the component whose current is being measured.
• Explain that current is the same at all points in a series circuit and that branch currents sum to the supply current in a parallel circuit.
• Apply the relationship I = Q/t to calculate current from charge and time, or rearrange to find charge or time.
• Interpret a current–potential difference graph for a fixed resistor as a straight line through the origin, showing direct proportionality at constant temperature.

Common mistakes and how to put them right

• Connecting the ammeter in parallel with a component.

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

• Confusing current with potential difference and giving the unit as V.

  Put it right: Current is measured in amperes (A), while potential difference is measured in volts (V).

• Writing 'amps' as the unit symbol in calculations.

  Put it right: The unit symbol is A; 'amp' is acceptable as a word for ampere, but calculations should use A.

Exam tips

• Check that every current value in an answer carries the unit A, and convert milliamperes to amperes by dividing by 1000 before substituting into equations.
• When describing a circuit, state that the ammeter is in series and that the reading is the current through that component.
• Use the equation I = Q/t and show the rearrangement clearly if you are asked to find charge or time rather than current.