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4.3.5Electricity and Magnetism

Potential Dividers — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 41 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.3.5 Potential Dividers — IGCSE PHYSICS notes: Explain how two resistors in series divide a supply p.d., calculate the output p.d. and describe sensor circuits using LDRs and thermistors.Definition — a circuit that splits the p.d.

  • Construction
  • Larger resistor
  • V1 / V2 = R1 / R2
  • Vout = Vin × R2 / (R1 + R2)
  • Output range
  • Potentiometer

Part 1 of 7The idea of a potential divider (Extended)

Definition
  • Definition a circuit that splits the p.d. from a power source so that only a fraction goes to a component.
Construction
  • Construction two or more resistors in series across a supply.
Sharing
  • Sharing the same current passes through both, so the p.d. across each is proportional to its resistance.
Larger resistor
  • Larger resistor it takes the larger share of the p.d.

Part 2 of 7Potential divider equations

V1 / V2
R1 / R2
  • the p.d.s are in the same ratio as the resistances.
Vout
Vin × R2 / (R1 + R2)
  • the output across R2.
Supply
  • Supply the e.m.f. equals V1 + V2.
Output range
  • Output range the output lies between 0 and Vin.
  • Equal resistors give half the input.
  • A much larger R2 gives an output close to Vin.
  • A much smaller R2 gives an output close to zero.

Part 3 of 7Variable resistor as a divider

Potentiometer
  • Potentiometer a variable resistor across the supply with a sliding contact.
Output
  • Output moving the slider changes the ratio of the two parts, so the output varies smoothly from 0 V to the supply p.d.
Uses
  • Uses volume controls, dimmer controls and obtaining a range of p.d.s from zero for an I–V experiment.
One resistor variable
  • One resistor variable the p.d. across the other, and across any component in parallel with it, can be altered.

Part 4 of 7Sensor circuits

LDR
  • LDR bright light lowers its resistance, so the p.d. across it falls and the p.d. across the fixed resistor rises.
  • In the dark the LDR takes a large share of the p.d.
  • Swapping the positions of the LDR and the resistor reverses the effect.
Thermistor
  • Thermistor a rise in temperature lowers its resistance, so the p.d. across it falls and that across the fixed resistor rises.
Switching
  • Switching the output can operate a relay or transistor switch to turn on a lamp, heater or alarm.
Adjustment
  • Adjustment a variable resistor in place of the fixed one lets the switching point be adjusted.
  • To switch on a component with light or temperature, make the resistor it is in parallel with become larger compared with the other.

Part 5 of 7Loading effect

Load
  • Load a device connected across the output draws current and, being in parallel with R2, lowers the output p.d.
Requirement
  • Requirement the load resistance should be much larger than R2.
Energy
  • Energy a divider wastes some energy continuously, so it is used for small sensor currents, not large loads.

Part 6 of 7Using the formula correctly

1
Decide which resistor the output is taken across.
2
Substitute into Vout = Vin × R / (R1 + R2) or use the ratio.
3
Check the two p.d.s add up to the supply.
4
Check the larger resistor has the larger p.d. and the output lies between zero and the supply value.

Part 7 of 7More from the specification

More from the specification
Voltage divider ratio
  • Voltage divider ratio adjusting resistance ratios in series circuits provides an adjustable means to control output voltage levels.
Potential divider output
  • Potential divider output splits total supply potential difference in direct proportion to the resistance values of series components.
Mains electrical safety
  • Mains electrical safety protective fuses and earth wiring protect appliances from dangerous current surges and insulation breakdown.
Current in divider branches
  • Current in divider branches total current drawn from the voltage source divides proportionally across parallel branch loads.
Potential Divider Circuits and Voltage Sharing
  • A potential divider circuit splits the input potential difference across two series resistors in proportion to their resistance values: Vout = Vin × [R₂ / (R₁ + R₂)],
  • allowing control of voltage supplied to an electrical component or sensor.
Current Rules at Junctions and Combined Resistance
  • At any circuit junction, total current entering equals total current leaving; in a parallel circuit branch, the combined resistance Rtotal satisfies 1/Rtotal = 1/R₁ + 1/R₂,
    • (conservation of charge)
  • reducing total circuit resistance below that of any individual branch.
Electrical Safety Devices
  • Fuses, Earth Wire,
  • and Insulation — Electrical safety protects users and appliances from electrical faults;
    • fuses contain a thin wire that melts and breaks the circuit if current exceeds safe limits,
  • while the earth wire and double insulation prevent the metal casing of an appliance from becoming live.
Branch
  • Path connecting two junctions.
Junction
  • Place where a pipe splits or joins; flow rates add.
Insulation
  • Material that reduces thermal energy transfer.
Potential difference
  • Work done by a unit charge passing through a component.
Combined resistance
  • Single resistance that could replace several resistors.
Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Circuit diagram
  • Drawing of a circuit using standard symbols.
Circuit symbol
  • Standard picture for a component.

Key terms

TermMeaning
Potential dividerSeries resistors that share the supply p.d. in proportion to resistance.
Output p.d.P.d. taken across one resistor of the divider.
PotentiometerVariable resistor used as a potential divider.
LDRResistor whose resistance falls as light increases.
LoadingLowering of the output p.d. by a connected load.

Avoid these mistakes

1
Putting the output across the wrong resistor and reversing the answer.
2
Forgetting that a load across the output changes the p.d.
3
Saying the p.d. is shared equally — it is shared in proportion to resistance.
4
Using an LDR the wrong way round, so the lamp comes on in the day instead of at night.

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