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4.3.3Electricity and Magnetism

Potential Difference in Circuits — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 21 of 31

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Hub / Topical Revision
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Part 1 of 7E.m.f. of cells in series

  • Rule
    • cells connected in series have a combined e.m.f. equal to the sum of their individual e.m.f.s.

Part 2 of 7P.d. in series circuits (Extended)

E
V1 + V2 + V3
  • the sum of the p.d.s across the components equals the e.m.f. of the supply.
Sharing
  • Sharing the components share the e.m.f. of the supply.
Larger resistance
  • Larger resistance a component with more resistance takes a larger share of the p.d.
Identical lamps
  • Identical lamps they share the p.d. equally, so two lamps on a 6.0 V battery each have 3.0 V and are dim.
Energy view
  • Energy view each coulomb gains E joules in the cell and loses it in portions in the components.

Part 3 of 7P.d. in parallel circuits (Extended)

P.d. in parallel circuits (Extended)
Rule
  • Rule the p.d. across each branch equals the e.m.f. of the supply.
  • A 12 V supply gives 12 V across every branch, whatever its resistance.
Branch with several components
  • Branch with several components the p.d. is split between the components on that branch.
Branch current
  • Branch current a branch with smaller resistance takes a larger current.
Houses
  • Houses appliances need the full mains p.d., so they are wired in parallel and can be switched independently.

Part 4 of 7Voltmeter placement

Position
  • Position a voltmeter is always connected in parallel with the component or group whose p.d. is needed.
Series example
  • Series example across the first resistor it reads V1, across the second V2, and across both V1 + V2.
Across a switch
  • Across a switch a reading of the full supply value means the switch is open and zero means it is closed.

Part 5 of 7Terminal p.d. and internal resistance

Internal resistance
  • Internal resistance a real cell has some, so p.d. is lost inside it when a current flows.
Terminal p.d.
  • Terminal p.d. it is slightly less than the e.m.f. when current flows.
Calculations
  • Calculations for a fresh cell and a small current the terminal p.d. is taken as equal to the e.m.f.
Ageing
  • Ageing internal resistance increases, the terminal p.d. drops under load and lamps are dimmer.
Heavy appliance
  • Heavy appliance switching it on lowers the total resistance and the cell's terminal p.d. dips slightly, so room lights can dim.

Part 6 of 7Solving circuits with both arrangements

Solving circuits with both arrangements
Find the equivalent resistance of the parallel part and the p.d. across it using V = I R.
Subtract that p.d. from the e.m.f. to get the p.d. across the series part.
Work out the currents from the p.d.s and resistances.
  • Lamp replaced by one of larger resistance in series
    • the current falls and the p.d. across that lamp rises, which is the basis of the potential divider.

Part 7 of 7More from the specification

More from the specification
Circuit electromotive force and power
  • Circuit electromotive force and power multiplying total source electromotive force by circuit current determines the total electrical power supplied.
Electromotive Force vs Potential Difference
  • Electromotive force (EMF) is the energy provided per unit charge by an electrical source,
    • (such as a battery or generator)
  • whereas potential difference (p.d.) is the electrical energy dissipated per coulomb by circuit components.
Current Conservation at Circuit Junctions
  • According to Kirchhoff's first law, total electric current flowing into any circuit junction must equal the total current flowing out, reflecting fundamental conservation of charge.
Electrical Safety with Fuses and Circuit Breakers
  • A fuse contains a thin wire that melts and breaks the circuit when an excessive current flows, protecting electrical appliances from overheating and preventing electrical fires.
Earthing and Double Insulation
  • The earth wire provides a low-resistance path to the ground for fault currents if a live wire contacts a metal casing, blowing the fuse;
    • appliances with double insulation require no earth wire as internal live parts are fully shielded.
Junction
  • Place where a pipe splits or joins; flow rates add.
Insulation
  • Material that reduces thermal energy transfer.
Circuit diagram
  • Drawing of a circuit using standard symbols.
Circuit symbol
  • Standard picture for a component.

Key terms

TermMeaning
E.m.f.Energy supplied per coulomb by the source.
Series p.d. ruleThe p.d.s across series components add up to the e.m.f.
Parallel p.d. ruleEach parallel branch has the full supply p.d.
Internal resistanceResistance inside a cell that causes lost p.d.
Terminal p.d.P.d. across the terminals of a cell while it supplies current.

Avoid these mistakes

1
Saying the p.d. is the same across series components — the current is the same, the p.d.s add.
2
Giving the p.d. across parallel branches as a share of the supply — each branch gets the full p.d.
3
Connecting a voltmeter in series — it must be in parallel.
4
Ignoring that a battery has an internal resistance.

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