Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.2.2Electricity

Voltage in Series and Parallel — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 14

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 7Series circuits

In a series circuit the e.m.f. of the supply is shared among the components. The sum of the p.d. across all the components equals the p.d. of the supply:

Vsupply = V1 + V2 + V3

This follows from conservation of energy

  • each coulomb leaving the cell carries a certain amount of energy, and this is transferred in the components it passes through.
  • If the components are identical, they share the voltage equally;
    • if not, the component with the larger resistance has the larger p.d. across it, because V = IR with the same current in each.

Notice how the series rules connect to the equation V = IR.

Series circuits
With a single current I passing through two resistors, the p.d. across the larger resistor is greater.
If one resistor has three times the resistance of the other, it takes three quarters of the supply p.d. and the other takes one quarter.
This is the basis of the potential divider and of scale reading in many sensors.

Part 2 of 7Parallel circuits

In a parallel circuit, each branch is connected directly across the same two points, so each branch has the same p.d. as the supply:

V1 = V2 = Vsupply

For this reason, two lamps in parallel across a 6.0 V cell each have 6.0 V across them, however different they are. The lamps in a house are all in parallel so each receives the full 230 V of the mains supply.

Because every branch in parallel sees the full supply voltage, the same lamp shines with the same brightness in a parallel circuit no matter how many other lamps are added, as long as the supply can provide enough current.

Parallel circuits
In a series circuit, adding lamps splits the voltage, and each one gets dimmer.
This is why Christmas lights are often wired in parallel or in groups.

Part 3 of 7Cells and batteries

When cells are connected in series, positive end to negative end, their e.m.f. values add: two 1.5 V cells in series give 3.0 V.

Cells and batteries
If one cell is reversed, its e.m.f. subtracts: 1.5 − 1.5 = 0 V.
Cells connected in parallel give the same voltage as one cell, but can supply current for longer.
Batteries in torches normally use cells in series.

Part 4 of 7Measuring voltage

Voltmeters are connected in parallel across the component.

Measuring voltage
To check the rule for series circuits, connect one voltmeter across each of two lamps and one across both; the readings add.
To check the parallel rule, connect a voltmeter across each branch and across the cell; all readings are the same.
If the voltages do not match, check the meter range and zero error, and consider the small p.d. lost in the connecting wires and the cell.

When you draw circuits for these questions, label the p.d. across each component with an arrow or a V label. Then check that around any closed loop, the voltage gains from cells equal the voltage drops across components. This rule is the energy conservation law for circuits, and a quick check on your answers.

Part 5 of 7Potential divider idea

Two resistors in series across a supply divide the supply voltage in proportion to their resistances.

Potential divider idea
If R1 = 10 Ω and R2 = 20 Ω across 6.0 V, the current is 6.0/30 = 0.20 A and the p.d. values are 2.0 V and 4.0 V.
This idea is used in sensor circuits with an LDR or thermistor, where changing resistance changes the output voltage.

Remember that a switch that is open has the full supply p.d. across it, because no current flows and so there is no p.d. across the other components, whereas a closed switch has zero p.d. across it as its resistance is negligible. This is a useful fault-finding clue: a voltmeter across a broken wire reads the full supply voltage.

Part 6 of 7Summary table

SeriesParallel
VoltageShared; adds to supply voltageSame across every branch
CurrentSameSplits

Combine this with V = IR to solve circuits with several components.

Part 7 of 7Worked examples

  1. Two lamps are in series with a 9.0 V battery. The first has 3.5 V across it. Find the p.d. across the second lamp.

  2. Three resistors in parallel are connected to a 12 V supply. What is the p.d. across each?

  3. A 4.0 Ω and a 8.0 Ω resistor are in series with a 6.0 V cell. Find the p.d. across the 8.0 Ω resistor.

Key terms

TermMeaning
Potential dividerSeries resistors that split a supply voltage.
E.m.f.Energy per coulomb given by a source.
Series cellsCells in a chain whose e.m.f. add.
Branch p.d.Voltage across one branch of a parallel circuit.

Avoid these mistakes

1
Saying that the voltage is the same in series (it is the current that is the same).
2
Adding p.d. in parallel branches.
3
Placing a voltmeter in series.
4
Connecting cells in series the wrong way round and expecting their e.m.f. to add.
5
Thinking identical lamps in series each get the full supply voltage.

Last updated: