Electrical charge and current: Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:

The idea

Electric current is the movement of electrical charge through a conductor, like electrons flowing through a copper wire. The size of the current indicates how quickly charge passes a point: a larger current means more charge flows each second. Current is measured in amperes (A) using an ammeter in series. Charge flow is measured in coulombs (C) and time in seconds (s). The relationship is charge flow = current × time, or Q = I × t. For example, a steady direct current of 2 A flowing for 5 s transfers 10 C of charge. Rearranging gives current = charge flow ÷ time. This equation lets you calculate any one quantity when the other two are known, and it applies to steady direct currents.

What earns the marks

• State that electric current is a flow of electrical charge, commonly electrons in a metal conductor.
• Explain that current is the rate of flow of charge, so current equals charge flow divided by time.
• Recall and apply the equation charge flow = current × time, with charge in coulombs, current in amperes and time in seconds.
• Rearrange the equation to find current or time when the other two quantities are given.
• Use correct units and unit symbols: coulombs (C), amperes (A) and seconds (s).
• Substitute numerical values correctly into the equation and evaluate the answer with an appropriate unit.

Common mistakes and how to put them right

• Writing the equation as charge flow = current ÷ time.

  Put it right: Charge flow is current multiplied by time, because more current or more time transfers more charge.

• Using minutes instead of seconds without converting.

  Put it right: Convert time to seconds before substituting, for example 2 minutes = 120 s.

• Confusing current with charge.

  Put it right: Current is the rate of charge flow measured in amperes, while charge flow is measured in coulombs.

Exam tips

• Write the equation, then substitute values with units before calculating to reduce errors.
• Check that time is in seconds and convert minutes or hours if necessary.