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Course: OCR A Level Physics A H556
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OCR A Level Physics A H556

1. Development of Practical Skills in Physics

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2. Foundations in Physics

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3. Forces and Motion

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4. Electrons, Waves and Photons

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OCR H556 chapter questions and answers

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Text lesson

4.3.6 Series and Parallel Circuits

4.3.6 Series and Parallel Circuits: Compare how current and potential difference behave in series and parallel circuits and predict ammeter and voltmeter readings.

4.3.6 Series and Parallel Circuits

Series and parallel connections have opposite sharing rules for current and p.d.

Current
  • There is only one path, so the current is identical at every point in the circuit.
P.d.
  • The supply p.d. is shared between components in proportion to their resistances.
Failure
  • If one component fails, the circuit is broken and every component stops working.
Total resistance
  • Resistances add, so each extra component lowers the current.
Parallel Circuits
P.d.
  • Each branch is connected directly across the source, so each has the same p.d. as the supply.
Current
  • The supply current splits at junctions and the branch currents add up to the total.
Failure
  • If one branch breaks, the others keep working normally, which suits household wiring.
Total resistance
  • Each extra branch gives charge another route, so the total resistance falls and the supply current rises.
Quantity Series Parallel
Current Same everywhere Sum of branch currents
P.d. Sum of component p.d.s Same across each branch
Total R R₁ + R₂ + … 1/R₁ + 1/R₂ + …
1

An ammeter measures the current through a component and must be connected in series with it.
2

A voltmeter measures the p.d. across a component and must be connected in parallel with it.
3

An ideal ammeter has zero resistance so it does not change the current it measures.
4

An ideal voltmeter has infinite resistance so it draws no current from the circuit.
5

Predict readings with the sharing rules, then check that p.d.s and currents obey the sum rules.

Question: A 10 Ω resistor and a 15 Ω resistor are connected in parallel across a 6.0 V supply of negligible internal resistance. Find the current in each branch, the total current and the combined resistance.

  1. Branch 1: I = V/R = 6.0 / 10 = 0.60 A.
  2. Branch 2: I = 6.0 / 15 = 0.40 A.
  3. Total current = 0.60 + 0.40 = 1.0 A.
  4. Combined resistance = V / I = 6.0 / 1.0 = 6.0 Ω, which is below both branch values.

Answer: 0.60 A, 0.40 A, total 1.0 A, combined resistance 6.0 Ω.

4.3.6 – Key Points for the Exam

  • Series circuit: A circuit with a single path for current.
  • Parallel circuit: A circuit with branches that each connect across the same two points.
  • Ammeter: A meter placed in series to measure current.
  • Voltmeter: A meter placed in parallel to measure p.d.

Common Mistakes

  • Connecting a voltmeter in series, which blocks the current and gives a nonsense reading.
  • Saying the current is used up as it passes round a series circuit; it is the energy that is transferred.
  • Believing the p.d. is shared between parallel branches when each has the full supply p.d.
  • Thinking more parallel branches increase total resistance.
YOUR STUDY PATH

4.3.6 Series and Parallel Circuits

OCR A Level Physics A H556 / 4. Electrons, Waves and Photons

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