KS3 · Physics

Comparing series and parallel circuits

Two bulbs, one cell. Wire them one way and they glow dimly; rewire the same parts and both shine brighter — but the battery dies sooner. What changed?

Physics · Electric circuits

Same two bulbs, same cell — flip the wiring

Current is the flow of charge around a circuit, measured in amps (A). Voltage is the push the cell gives the charges, measured in volts (V) — you'll also hear it called potential difference. Switch between series and parallel and watch what that push does to each bulb.

Two lamps · 3.0 V supply

Dashed line = current flow

Voltage

3.0 V

Total R

Higher

Current

Smaller

Brightness

Dimmer

Series. One single loop. The two bulbs share the cell's 3.0 V, so each one gets just 1.5 V. A smaller push on each bulb, a smaller current round the loop — and both glow dimly.

Watch out: The Voltage reading is the cell's own push, and it's 3.0 V in both circuits. What changes is how much of that push each bulb gets: half each in series, all of it in parallel.

Predict, then check

Trace each circuit with your finger. Can you still get from one side of the cell to the other without going through the broken bulb?

One of the two bulbs blows. What happens to the other bulb in the series circuit, and in the parallel circuit?

What do you think?

Add a branch — where does the extra current come from?

A parallel circuit has one bulb in one branch. You add a second, identical bulb in a brand-new branch.

What happens to the current? Pick the idea closest to what you think right now.

The cell sends out a fixed amount of current. Now it has to split between two branches, so each branch gets less.The total current from the cell stays the same — the new branch is just another way round for the same current.Each branch draws its own current, like a separate series circuit. The first branch keeps its current, and the total through the cell goes up.

How sure are you?

I'm sureI think soI'm guessing

The cell sends out a fixed amount of current. Now it has to split between two branches, so each branch gets less.

This is a common misconception

This is the picture most people start with, so it's worth letting go of carefully. The cell does not produce a fixed-size current that gets shared out. Instead, picture each branch as its own separate series circuit, plugged into its own identical battery. The first branch still gets the full cell voltage, so it still draws exactly the same current as before. The new branch doesn't take anything away from it.

The total current from the cell stays the same — the new branch is just another way round for the same current.

This is a common misconception

The new branch isn't a detour for current that was already flowing — it's a whole extra circuit. It gets the full cell voltage and draws its own current. Where the branches meet, the currents add together, so the total current through the cell goes up. Two branches carrying 0.2 A each means 0.4 A through the cell.

Each branch draws its own current, like a separate series circuit. The first branch keeps its current, and the total through the cell goes up.

This matches the evidence

Exactly right. Each branch is connected directly to the cell, so each gets the cell's full voltage and pushes its own current. Adding a branch doesn't change the current in the branches already there, because the voltage across them hasn't changed. The currents add up where the branches meet: 0.2 A + 0.2 A gives 0.4 A through the cell.

A battery doesn't hand out a fixed current to be shared. Each branch takes its own current from the full cell voltage — and the branch currents add up at the cell.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

	Current through the cell	Voltage across each bulb	Bulbs already there
Add a bulb in seriesone more bulb in the single loop
Add a cell in seriesone more cell in the battery
Add a bulb to one branchparallel: that branch now has two
Add a bulb in a new branchparallel: one more branch

Each cell hides a short answer and the reason behind it. Predict before you tap.

The price of brightness

Reason it through

Why does a battery run out faster when it lights bulbs in parallel than when it lights the same bulbs in series?

Link 1 of 4

First link · your turn

Start with the circuit itself. Each time you add another branch, what happens to the total resistance?

Locked — reveal the link above first

Check the answer

One line goes wrong — can you find it?

Two identical bulbs are connected to a 3.0 V cell, first in series and then in parallel. In the parallel circuit, each branch carries a current of 0.2 A. Compare the two circuits.

A student's answer — which line goes wrong?

In series, the two bulbs share the cell's voltage, so each bulb gets 1.5 V.
Each series bulb only gets half the push, so both glow dimly.
In parallel, the two branches share the cell's voltage too, so each bulb gets 1.5 V.
So the parallel bulbs glow just as dimly as the series bulbs.
The total current through the cell is 0.2 A + 0.2 A = 0.4 A.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Same bulbs, same cell, different wiring — and a completely different circuit.

What you need to know

A series circuit is a single loop; a parallel circuit has junctions that lead to separate loops, called branches.
Current is the rate of flow of charge, measured in amps. Voltage (potential difference) is the push the cell gives the charges, measured in volts.
In series, the current is the same everywhere in the loop, and the components share the cell's voltage.
In parallel, each branch is connected directly to the cell and behaves like its own series circuit: every branch gets the full cell voltage.
The branch currents add up where the branches meet, so the current through the cell is their total.
With the same battery, two bulbs in parallel are brighter than two in series and make the battery run out faster.

The big picture

A series circuit is one single loop: the current is the same everywhere and the components share the cell's voltage, so two identical bulbs on a 3.0 V cell get 1.5 V each and glow dimly. A parallel circuit splits into branches, and each branch behaves like its own series circuit connected straight to the cell: it gets the full cell voltage, and the branch currents add up at the cell. That is why, with the same battery, parallel bulbs are brighter, keep working if one blows, and drain the battery faster.

Key points

1Series = one loop. If one bulb blows, the gap stops the current everywhere and every bulb goes out.

2Parallel = separate branches. If one bulb blows, the others stay lit because each has its own connections to the cell.

3Adding a bulb in series raises the resistance, so the current falls and each bulb gets a smaller share of the voltage.

4Adding a cell in series raises the voltage, so the current rises — and it is still the same all the way round the loop.

5A branch with more bulbs has a higher resistance and a lower current, and its bulbs share that branch's voltage.

6Adding a component to one branch doesn't change the current in the other branches; adding a new branch lowers the total resistance and increases the current through the cell.

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Worked example

Problem

A 3.0 V cell has two branches connected to it. Branch A contains one bulb. Branch B contains two bulbs identical to the one in branch A. Find the voltage across each bulb, and say which bulbs glow brighter.

⚠ Watch out

Thinking the cell pushes out a fixed amount of current that splits between the branches. It doesn't: each branch draws its own current from the full cell voltage, so adding a branch leaves the others unchanged and makes the total current through the cell bigger.

🧠

Memory hook

One loop shares; every branch gets the lot. In series the bulbs share the cell's push. In parallel, each branch is its own little circuit plugged straight into the cell — so each gets the whole push.

✓

Check yourself

Two identical bulbs are wired in parallel to a cell. One blows. Does the other stay lit — and does the current through the cell go up, go down or stay the same?

Flashcards

(13)

What is a series circuit?

A circuit with a single loop — the current has only one path to follow.

What is a parallel circuit?

A circuit with junctions that lead to separate loops, called branches.

What is current, and what is it measured in?

The rate of flow of charge in a circuit, measured in amps (A).

What is voltage, and what is it measured in?

The push a cell gives to charges to move current through the components, measured in volts (V). It is also called potential difference.

How does the current compare at different points in a series loop?

It is the same everywhere in the loop.

What happens to the current when you add another component to a series loop?

The resistance increases, so the current decreases — while the voltage from the cell stays the same.

What happens when you add another cell in series to the battery?

The voltage increases, so the current increases — and it is the same everywhere along the loop.

How do identical bulbs in series share the cell's voltage?

Equally. Each gets a lower voltage than the cell gives, which is why they are dimmer.

What is the voltage across each branch of a parallel circuit?

The same as the voltage across the cell — however many components are in that branch.

How do you find the current through the cell in a parallel circuit?

Add up the currents in all the branches — currents add where branches meet.

What is the best way to picture a parallel circuit?

As separate series circuits, each connected to its own identical battery. The battery does not send out a fixed current that splits.

You add a component to one branch of a parallel circuit. What happens to the current in the other branches?

Nothing — the voltage across those branches stays the same, so their current stays the same.

What happens to the total resistance when you add more branches to a parallel circuit?

It decreases, so more current flows through the cell and its energy store drains more quickly.

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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Same two bulbs, same cell — flip the wiring

Add a branch — where does the extra current come from?

One line goes wrong — can you find it?

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