OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(a) Kirchhoff’s second law; the conservation of energy
Full specification wording
Series and parallel circuits: (a) Kirchhoff’s second law; the conservation of energy
Kirchhoff’s second law states that in any closed loop of a circuit, the sum of the electromotive forces equals the sum of the potential differences.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Kirchhoff’s second law states that in any closed loop of a circuit, the sum of the electromotive forces equals the sum of the potential differences. It is a statement of conservation of energy: charge returning to its starting point must have the same total energy, so energy gained from sources equals energy lost in components. In a series circuit with a 6.0 V battery and two resistors of 2.0 Ω and 4.0 Ω carrying 1.0 A, the p.d.s are 2.0 V and 4.0 V, which sum to 6.0 V. In multiple-choice questions, identify the loop, assign signs consistently and check that the algebraic sum of e.m.f.s and p.d.s around the loop is zero.
What earns the marks
- States Kirchhoff’s second law as the sum of e.m.f.s equals the sum of p.d.s around a closed loop.
- Links the law to conservation of energy for charge moving around a complete circuit.
- Applies the law to series circuits where p.d.s sum to the supply e.m.f.
- Uses the law to find an unknown p.d. or e.m.f. in a loop.
- Recognises that the algebraic sum of potential changes around a closed loop is zero.
Common mistakes and how to put them right
Confusing Kirchhoff’s second law with the first law.
Put it right: The second law concerns energy around a loop, while the first law concerns charge at a junction.
Ignoring the direction of the loop when assigning signs.
Put it right: Choose a direction and treat e.m.f.s and p.d.s consistently.
Assuming the law applies only to series circuits.
Put it right: It applies to any closed loop, including those in parallel networks.
Exam tips
- Mark the chosen loop direction on a diagram before writing the equation.
- List e.m.f.s and p.d.s with their signs, then sum them to zero.
- Check that the total p.d. across series components equals the supply e.m.f. as a quick verification.
Try the lesson
The start of the Series and parallel circuits: (a) Kirchhoff’s second law; the conservation of energy lesson, made from checked teaching notes for this specification point.
Listen
Kirchhoff’s second law states that in any closed loop of a circuit, the sum of the electromotive forces equals the sum of the potential differences. It is a statement of conservation of energy: charge returning to its starting point must have the same total energy, so energy gained from sources equals energy lost in components. In a series circuit with a 6.0 V battery and two resistors of 2.0 Ω and 4.0 Ω carrying 1.0 A, the p.d.s are 2.0 V and 4.0 V, which sum to 6.0 V. In multiple-choice questions, identify the loop, assign signs consistently and check that the algebraic sum of e.m.f.s and p.d.s around the loop is zero. A strong answer earns marks for these points. States Kirchhoff’s second law as the sum of e.m.f.s equals the sum of p.d.s around a closed loop. Links the law to conservation of energy for charge moving around a complete circuit. Applies the law to series circuits where p.d.s sum to the supply e.m.f. Uses the law to find an unknown p.d. or e.m.f. in a loop. A common slip: Confusing Kirchhoff’s second law with the first law. Instead, the second law concerns energy around a loop, while the first law concerns charge at a junction. Exam tip: Mark the chosen loop direction on a diagram before writing the equation.
See it
Key words
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Kirchhoff’s second law
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Conservation of energy in electrical circuits
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Series circuit potential differences
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Sign conventions in loop analysis
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See every definition
- Kirchhoff’s second law
- In any closed loop of a circuit, the sum of the electromotive forces equals the sum of the potential differences.
- Conservation of energy in electrical circuits
- Charge returning to its starting point must have the same total energy, so energy gained from sources equals energy lost in components.
- Series circuit potential differences
- In a series circuit the potential differences across the components sum to the supply electromotive force.
- Sign conventions in loop analysis
- A loop direction is chosen and electromotive forces and potential differences are treated consistently, so their algebraic sum around the loop is zero.
Label the diagram
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1Box 1
2Box 2
3Box 3
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