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2.2.1Forces and Motion

Current in Series and Parallel

5 min read 9 parts Lesson 1 of 26 Unit quiz

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Section 1 of 9Series circuits

In a series circuit the components are connected one after another in a single loop, so there is only one path for the charge. The current is the same at every point in the loop:

Itotal = I1 = I2 = I3

Charge is not used up as it passes through a component, and it cannot pile up anywhere, so the same amount of charge passes each point every second. An ammeter placed before or after a lamp reads the same value. If one component in a series circuit breaks, the whole circuit stops, as in old-style strings of lights.

Imagine the circuit as a loop of water pipes with a pump.

1
In a single loop, the same volume of water passes every point each second.
2
If the pipe splits into two branches, the flow divides, and the two flows add up again afterwards.
3
Electric current behaves in the same way.
4
This analogy is not perfect, but it helps to remember the rules: series means one flow, parallel means flow splits and recombines.

Section 2 of 9Parallel circuits

In a parallel circuit the components are connected on separate branches between the same two points. The current from the cell splits at a junction, flows through the branches, and then recombines. The total current equals the sum of the branch currents:

Itotal = I1 + I2 + I3

This is because charge is conserved at a junction: the charge arriving each second equals the charge leaving each second. The branch with the lower resistance carries the larger current.

At a junction the rule is called Kirchhoff's first law at advanced level: the sum of currents entering equals the sum of currents leaving. In a diagram, label every wire with its current and check each junction. If a current arrow is unknown, assume a direction; a negative answer then simply means the current actually flows the other way.

Section 3 of 9Identical and different branches

If two branches have identical components, the current splits equally.

Identical and different branches
If two lamps of different resistance are in parallel across the same cell, the lamp of lower resistance takes the larger current,
  • because the p.d. across each branch is the same and I = V/R.
A branch can be switched off without affecting the others,
  • which is why houses use parallel circuits for lights and sockets: each appliance gets the full mains voltage and works independently.

The relative sizes of branch currents follow from V = IR, because each branch has the same p.d.

Identical and different branches
A 6.0 Ω lamp and a 12 Ω lamp across 12 V carry 2.0 A and 1.0 A respectively, a total of 3.0 A.
The total current is larger than for either lamp alone, and the combined resistance is smaller than either: 12/3.0 = 4.0 Ω.

Section 4 of 9Comparing series and parallel

SeriesParallel
Paths for currentOneMore than one
CurrentSame everywhereSplits at junctions
If one component failsAll stopOthers keep working
Total current from the cellSmallLarger as branches are added

Section 5 of 9Adding components

When more lamps are added in series, the total resistance increases and the current falls, so the lamps become dimmer.

Adding components
When more lamps are added in parallel, each lamp still has the full p.d. and the same current as before, and the cell must supply more total current.
A cell supplying too many parallel branches can run flat quickly, or overheat the wires, which is why fuses are fitted.

This is also why plugging too many appliances into one extension lead is dangerous: the total current is the sum of the currents of each appliance, and it may exceed the rating of the cable and the fuse, so the cable overheats.

Section 6 of 9Using ammeters

To measure the current in a branch, place the ammeter in series with that branch.

Using ammeters
To measure the total, put it in the main wire next to the cell.
In circuit questions, work from known values: find the unknown branch current by subtracting known branch currents from the total,
  • or add the branch currents to find the total.
The next lesson looks at voltage in the same two types of circuit.

Section 7 of 9Worked examples

  1. Three lamps in series carry a current of 0.40 A at the first lamp. What is the current through the third lamp?

  2. A parallel circuit has branches with currents 0.30 A, 0.50 A and 0.20 A. Find the total current from the cell.

  3. The total current in a parallel circuit is 2.4 A. One branch carries 0.90 A. The two other branches are identical. Find the current in each.

Section 8 of 9Key terms

TermMeaning
SeriesComponents connected in one loop.
ParallelComponents on separate branches between the same two points.
JunctionPoint where wires meet and current splits or joins.
BranchOne path in a parallel circuit.
Conservation of chargeCharge entering a point equals charge leaving.

Section 9 of 9Exam technique

1
Thinking current is used up as it passes through a component in series.
2
Believing the current splits equally in every parallel circuit, regardless of resistance.
3
Placing an ammeter across a component instead of in line.
4
Adding currents in series circuits.
5
Forgetting that the total current rises when more parallel branches are added.
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