KS3 · Physics

Voltage in parallel circuits

One battery, two branches: one full of resistance, one easy. Surely they split the battery's voltage? They don't. Each branch gets the whole lot, and here's why.

Parallel circuit · three voltmeters

Add cells. Read every branch.

View: 1 cell. Showing 2 layers: Circuit, One cell

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switch the cells, then tap a voltmeter

One cell. Read all three voltmeters: the battery, branch A and branch B all show 1.5 V. The high-resistance branch does not get less.

The readings are example values. The pattern between them is the physics.

Why the rule works

Reason it through

Why does every branch get the battery's full voltage, instead of a share of it?

Link 1 of 4

First link · your turn

Start with the battery. What does the voltage across it actually tell you?

Locked — reveal the link above first

Measuring voltage

Where is this voltmeter connected?

What will this voltmeter do?

Still to sort

Measures a voltage (0)

Across a battery or a component, one wire each side.

Where the line is: The wires must have a component (or the battery) between them, not just wire.

Reads zero (0)

Both wires on the same stretch of connecting wire.

Where the line is: Correctly across something, but that something is only wire.

Wrongly connected (0)

In series: the current would have to go through it.

Where the line is: Put into the loop instead of across a part of it.

5 of 5 still to sort.

Each card describes a voltmeter in a parallel circuit. Sort it, then read why.

The one place voltage IS shared

Two lamps in one branch

A parallel circuit has two branches. Branch A has two identical lamps one after the other (in series). Branch B has just one lamp.

Which is closest to what you think about the two lamps in branch A?

Each lamp in branch A gets the battery's full voltage, like every branch does.Branch A gets the battery's full voltage, and its two lamps share it, half each.The battery's voltage is shared equally between all three lamps in the circuit.Branch A gets less voltage than branch B, because it has more lamps in it.

How sure are you?

I'm sureI think soI'm guessing

Each lamp in branch A gets the battery's full voltage, like every branch does.

This is a common misconception

The full-voltage rule is about branches, not about every component. Branch A as a whole gets the battery's voltage, but its two lamps are in series inside that branch, so they share it: half each.

Branch A gets the battery's full voltage, and its two lamps share it, half each.

This matches the evidence

Exactly. Each branch still gets the battery's whole voltage. Inside branch A, the two identical lamps are in series, so they share that branch's voltage and each has half of it.

The battery's voltage is shared equally between all three lamps in the circuit.

This is a common misconception

That treats the branches as if they share too. They don't: branch B's lamp has the battery's full voltage on its own. Only the two lamps in series within branch A share, and they share branch A's voltage.

Branch A gets less voltage than branch B, because it has more lamps in it.

This is a common misconception

More lamps in a branch doesn't lower that branch's voltage. Branch A is still a direct loop to the battery, so across the whole branch the voltmeter reads the same as the battery. What changes is that its two lamps split it.

Between branches: no sharing. Between components in series within one branch: sharing.

Spot the slip

Find the line where this goes wrong

A battery reading 9 V is connected to three branches in parallel, with one lamp in each branch. What is the voltage across each lamp? What would it be if a fourth branch were added?

A student's answer (example values) — which line goes wrong?

The voltmeter across the battery reads 9 V.
There are three branches, with one lamp in each.
The 9 V is shared out between the three branches: 9 ÷ 3 = 3 V per branch.
So each lamp has 3 V across it.
With a fourth branch, the 9 V would be split four ways: 9 ÷ 4 = 2.25 V per lamp.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why every branch gets the battery's full push, and how to measure it.

What you need to know

Voltage, also called potential difference, is measured in volts (V). It tells you the size of the push on the current.
A voltmeter is connected across a battery or component, one wire each side, with its positive side nearest the battery's positive side.
In a parallel circuit, the voltage across each branch is the same as the voltage across the battery, whatever the branch's resistance.
Components in series inside one branch share that branch's voltage.

The big picture

In a parallel circuit, every branch has the same voltage as the cell or battery. Voltage is not shared out between branches, whatever their resistance; only components in series inside one branch share. You measure voltage with a voltmeter connected across a component or the battery, one wire each side.

Key points

1Voltage and potential difference mean the same thing. Both are measured in volts (V).

2The voltage across a cell or battery measures how strongly it can push current round a circuit; the voltage across a component measures the size of the push moving current through it.

3A voltmeter (symbol: a circle with a V inside) goes across the thing it measures. Connected in series, it stops the current in that loop.

4A parallel circuit has more than one complete loop, and each branch is a direct loop to the battery. So each branch gets the battery's full voltage: it is not shared out between branches.

5Across a plain connecting wire a voltmeter reads zero, and sliding its connections along the wires does not change its reading.

6More cells in series in the battery give a bigger voltage across every branch (roughly double with two cells, roughly three times with three), still the same for each branch.

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Worked example

Problem

A parallel circuit has a battery reading 4.5 V. Branch 1 has one lamp. Branch 2 has two identical lamps in series. What does a voltmeter read across (a) the lamp in branch 1, (b) the whole of branch 2, (c) one lamp in branch 2?

⚠ Watch out

Dividing the battery's voltage between the branches. In a parallel circuit each branch gets the battery's full voltage; only components in series inside one branch share it.

🧠

Memory hook

Branches don't share, rows do. Every branch is plugged straight into the battery, so it feels the full push. Only components in a row inside one branch split it.

✓

Check yourself

Cover the page. In two sentences, explain to a friend why a voltmeter across any branch of a parallel circuit shows the same reading as a voltmeter across the battery.

Flashcards

(13)

What is the more formal name for voltage?

Potential difference. The two words mean the same thing, and both are measured in volts (V).

What does the voltage across a cell or battery tell you?

How strongly it can push current around the circuit.

What does the voltage across a component measure?

The size of the push moving current through that component.

How is a voltmeter connected to measure the voltage across a component?

Across it (in parallel with it), with one wire on each side.

What is the circuit symbol for a voltmeter?

A circle with a capital V inside.

Which way round should a voltmeter be connected?

With its positive side on the side closest to the positive side of the cell or battery.

What happens if a voltmeter is connected in series with a lamp?

It stops the current flowing in that loop, so the lamp does not light.

What makes a circuit a parallel circuit?

It has more than one complete loop from one end of the battery to the other. Each branch is a direct loop to the battery.

In a parallel circuit, how does a high-resistance branch's voltage compare with the battery's?

It is the same as the battery's, and so is every other branch's. Voltage is not shared out between branches.

One branch has two identical lamps in series. What voltage does each lamp have?

Half of that branch's voltage. Components in series within a branch share its voltage.

What does a voltmeter read if both its wires are on a plain connecting wire?

Zero. Connecting wires have (effectively) no resistance.

Does sliding a voltmeter's connections along the wires change its reading?

No. The wires add no voltage, so the reading stays the same.

What happens to the branch voltages when more cells are added in series to the battery?

Every branch voltage gets bigger (roughly double with two cells) and they all stay the same as each other.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Where is this voltmeter connected?

Two lamps in one branch

Find the line where this goes wrong

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