Electrical charge and current: A current has the same value at any point in a single closed loop.

The idea

In a single closed loop, charge cannot accumulate or escape, so the rate of flow of charge — the current — is identical at every point. Imagine a simple series circuit with a cell, a lamp and a resistor. The cell drives charge around the loop; the same number of coulombs passes through the cell, the lamp and the resistor each second. If the current were larger in one part, charge would build up there, which does not happen in a steady circuit. This conservation of charge explains why ammeters placed anywhere in the loop give the same reading. It also means that adding components in series increases resistance and reduces the current everywhere, but the current remains equal at all points.

What earns the marks

• State that current is the rate of flow of charge, measured in amperes (A).
• Explain that in a single closed loop there is only one path, so charge cannot leave or accumulate.
• Apply conservation of charge to conclude that the current is the same at every point in the loop.
• Predict that ammeters connected at different positions in the same series loop give identical readings.
• Use the equation I = Q ÷ t to support the idea that equal charge transfer per second means equal current.

Common mistakes and how to put them right

• Thinking current is used up as it passes through components.

  Put it right by stating that charge is conserved and energy, not current, is transferred to components.

• Believing the current before a component is larger than after it.

  Put it right by explaining that the same charge passes through every point each second.

• Confusing current with potential difference or energy.

  Put it right by defining current as charge flow per second and pd as energy transfer per unit charge.

Exam tips

• When asked to compare ammeter readings in a series loop, state that they are equal and justify with conservation of charge.
• Use the phrase 'rate of flow of charge' when defining current to secure the definition mark.
• If a circuit diagram shows one loop, check whether the question asks about current, pd or resistance before writing your answer.