AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Circuits: Conservation of charge and conservation of energy in dc circuits.
In any dc circuit, charge is neither created nor destroyed, so the current entering a junction equals the total current leaving it.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In any dc circuit, charge is neither created nor destroyed, so the current entering a junction equals the total current leaving it. Energy is also conserved: each charge carrier that completes the circuit transfers energy from the source to the components, and the sum of potential differences around any closed loop equals the source emf. For example, in a series circuit the current is the same at every point because charge is conserved, while the p.d.s across components add to the supply p.d. because energy is conserved. In parallel branches, the branch currents sum to the total current, and each branch has the same p.d. Applying these principles lets you analyse unknown currents and p.d.s without memorising every circuit configuration.
What earns the marks
- State that electric charge is conserved at a junction: the sum of currents entering equals the sum of currents leaving.
- Apply conservation of charge to series and parallel circuits to deduce unknown currents.
- State that energy is conserved in a closed loop: the sum of potential differences around the loop equals the source emf.
- Apply conservation of energy to series circuits to deduce unknown potential differences.
- Use the relationship between energy transferred, charge and potential difference, W = QV, to support energy conservation arguments.
- Recognise that in a parallel circuit the p.d. across each branch is the same because each branch connects the same two nodes.
Common mistakes and how to put them right
Thinking current is used up as it passes through components.
Put it right: Current is the rate of flow of charge; charge is conserved, so the same current enters and leaves a series component.
Adding branch currents incorrectly in a parallel circuit, for example treating total current as the difference between branch currents.
Put it right: The total current is the sum of the branch currents.
Assuming energy is used up in the sense that charge loses energy permanently.
Put it right: Energy is transferred from the source to components; the total energy transferred per unit charge around a closed loop is conserved.
Exam tips
- When asked to find an unknown current, identify a junction and write an equation based on charge conservation before substituting numbers.
- When asked to find an unknown p.d., trace a closed loop and write an equation based on energy conservation, taking care with the direction of travel.
- Show the conservation principle explicitly in your working; a correct numerical answer without the principle may not gain full credit.
Try the lesson
The start of the Circuits: Conservation of charge and conservation of energy in dc circuits. lesson, made from checked teaching notes for this specification point.
Have a go first
In a series dc circuit, a current of 2.0 A flows through the first resistor. What is the current in the second resistor, which is in series with the first?
Listen
In any dc circuit, charge is neither created nor destroyed, so the current entering a junction equals the total current leaving it. Energy is also conserved: each charge carrier that completes the circuit transfers energy from the source to the components, and the sum of potential differences around any closed loop equals the source emf. For example, in a series circuit the current is the same at every point because charge is conserved, while the p.d.s across components add to the supply p.d. because energy is conserved. In parallel branches, the branch currents sum to the total current, and each branch has the same p.d. Applying these principles lets you analyse unknown currents and p.d.s without memorising every circuit configuration. A strong answer earns marks for these points. State that electric charge is conserved at a junction: the sum of currents entering equals the sum of currents leaving. Apply conservation of charge to series and parallel circuits to deduce unknown currents. State that energy is conserved in a closed loop: the sum of potential differences around the loop equals the source emf. Apply conservation of energy to series circuits to deduce unknown potential differences. A common slip: Thinking current is used up as it passes through components. Instead, current is the rate of flow of charge; charge is conserved, so the same current enters and leaves a series component. Exam tip: When asked to find an unknown current, identify a junction and write an equation based on charge conservation before substituting numbers.
Key words
Say what each word means before you turn the card.
Conservation of charge
Try answering aloud or in your head before turning the card.
Conservation of energy
Try answering aloud or in your head before turning the card.
See every definition
- Conservation of charge
- In any dc circuit, charge is neither created nor destroyed, so the current entering a junction equals the total current leaving it.
- Conservation of energy
- Each charge carrier that completes the circuit transfers energy from the source to the components, and the sum of potential differences around any closed loop equals the source emf.
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