Electrical charge and current: For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

The idea

Electrical charge flows only when there is a complete closed conducting path and a source of potential difference, such as a cell or power supply. The source does work on the charge, giving energy to the charges so they move around the circuit; without it, charge does not flow even if the wires form a closed loop. A useful model is a battery pushing charge around a track, like a pump pushing water around a closed pipe system. If the switch is open, the path is not closed and current stops. If the source is removed, there is no potential difference to drive the charge, so again no current flows. Current is the rate of flow of charge, and it is measured in amperes with an ammeter connected in series.

What earns the marks

• States that a closed circuit is needed so charge can complete a continuous path.
• States that a source of potential difference, such as a cell or battery, is needed to make charge flow.
• Explains that the source does work on the charge, transferring energy to it and causing it to move.
• Uses the equation Q = It, where Q is charge in coulombs, I is current in amperes and t is time in seconds.
• Describes current as the rate of flow of electrical charge.
• Applies the idea that removing the source or opening the switch stops the current.

Common mistakes and how to put them right

• Thinking that a closed circuit alone is enough for charge to flow.

  Put it right: A closed circuit must also contain a source of potential difference.

• Confusing current with potential difference.

  Put it right: Potential difference is the energy transferred per unit charge and drives the current, while current is the rate of flow of charge.

• Using Q = It with time in minutes.

  Put it right: Convert time to seconds before substituting into the equation.

Exam tips

• In explanations, name both requirements: a closed circuit and a source of potential difference.
• When calculating charge, write the equation, substitute values with units, and give the answer in coulombs.
• Use the water-pump analogy only as a model, then link it back to the physics of charge, energy and potential difference.