KS3 · Physics

Parallel circuits

Two circuit diagrams can look nothing alike — and still be exactly the same circuit. By the end of this lesson, you'll spot that in seconds.

Physics · Electric circuits

Series or parallel? Count the loops.

Same battery, same two lamps. Flip the wiring, then read the current and follow the dashed line round.

Two lamps · 3 V supply

Dashed line = current flow

Voltage

3 V

Total R

—

Current

0.2 A round the loop

Brightness

Both on

Series. One loop. Put your finger on one end of the battery and trace the wire: you go through L1, then L2, and straight back to the other end. There's only one route and no junctions, so the charge has nowhere to split — one loop, one current, 0.2 A all the way round.

Watch out: Don't compare the two totals yet — why the parallel circuit takes more current from the battery is a story for a later lesson. Today, watch where the current splits and where it joins.

What do you really think?

Where does the current go?

Picture another parallel circuit: a battery and two lamps, each lamp on its own loop. The ammeter on the wire leaving the battery reads 0.6 A.

What happens to the current once it reaches the lamps? Pick the idea closest to what you think right now.

The first lamp uses up some of the current, so less comes back to the battery than went out.All the current takes one route — through whichever lamp it reaches first — and the other lamp gets none.The current splits at the junction, but each part goes back to the battery separately and never joins up again.The current splits at the first junction, some goes through each lamp, and the branch currents add back up to 0.6 A where the branches meet.

How sure are you?

I'm sureI think soI'm guessing

The first lamp uses up some of the current, so less comes back to the battery than went out.

This is a common misconception

This is the most common idea about circuits, and it's worth letting go of. Current isn't fuel that a lamp burns — it's a flow of charge. The charge that goes into a lamp comes out of the other side and keeps flowing round its loop. In a parallel circuit the branch currents add back up where the branches meet, so the current returning to the battery is the same 0.6 A that left it.

All the current takes one route — through whichever lamp it reaches first — and the other lamp gets none.

This is a common misconception

Look at the circuit's shape: each lamp sits on its own complete loop from one end of the battery to the other. At the junction the current splits between the loops, so some flows through each lamp. Neither branch is left out — that's exactly why both lamps are on.

The current splits at the junction, but each part goes back to the battery separately and never joins up again.

This is a common misconception

Half right — it does split. But follow the wires further: the two branches meet at a second junction before the wire goes back to the battery. There the branch currents join and add back up, so one 0.6 A current flows back into the battery.

The current splits at the first junction, some goes through each lamp, and the branch currents add back up to 0.6 A where the branches meet.

This matches the evidence

Spot on. Current is a flow of charge: it splits at one junction and adds back up at the other. That lets you work out a missing reading — if an ammeter showed 0.2 A in one branch, the other branch must carry 0.4 A, because together they add up to 0.6 A.

Current isn't used up. It splits between the branches at one junction, and the branch currents add back up to the full current at the other.

Predict, then check

Put your finger on the battery and trace each path before you choose.

In the model above, the parallel circuit has L1 on the top wire and L2 on a wire drawn underneath it. Sam redraws it with the battery along the bottom and L1 and L2 standing side by side as two upright wires. In Sam's drawing, the wire from the battery reaches a junction, splits into one path through L1 and one through L2, and the two paths meet at a second junction before going back to the battery. Is Sam's drawing the same circuit?

Build it, then draw it — one loop at a time

The order is the point: finish one loop before you start the next.

Collect the componentsRead the circuit diagram and gather every component it shows before you connect anything.
Build one complete loopStart from any point on the diagram. Connect the components one by one, always working in the same direction, until you have one complete loop from one end of the battery to the other.
Find the junctionNow find the junction on the diagram — the point where the wire splits. That's where the next loop starts.
Complete each further loop in turnFrom the junction, connect the components of the next loop one by one until it is complete. Then move on to the next loop, until every loop on the diagram is built.
Draw it the same wayDrawing a parallel circuit diagram follows the same pattern: follow one loop round and finish it, then complete each further loop in turn, using the correct symbol for each component.

Drawing circuit diagrams

Would this pass as a proper circuit diagram?

Sort each feature of a hand-drawn circuit diagram: does it follow the conventions or break them?

Still to sort

Follows the conventions (0)

Ruled, straight, joined up, correct symbols.

Breaks the conventions (0)

Wobbly or slanted lines, gaps, or wires through symbols.

Where the line is: A wire that is complete but slanted or freehand still breaks the conventions — the lines themselves must be straight, horizontal or vertical, and ruled.

7 of 7 still to sort.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

More than one loop — and the current splits at one junction and adds back up at the other.

What you need to know

Electric current is a flow of charge, measured in amperes (A).
A series circuit has one complete loop from one end of the battery to the other, so it has no junctions.
A parallel circuit has more than one complete loop from one end of the battery to the other.
In a parallel circuit the current splits between the branches at one junction, and the branch currents add back up where the branches meet.
Parallel circuits are built and drawn one loop at a time, using correct symbols and ruled, straight wires with no gaps.

The big picture

Electric current is a flow of charge, measured in amperes (A). A series circuit has one complete loop from one end of the battery to the other, so it has no junctions. A parallel circuit has more than one complete loop: at one junction the current splits between the branches, and at another junction the branch currents add back up. Two diagrams show the same circuit when their junctions are in the same places and each loop has the same components. Parallel circuits are built and drawn one loop at a time, with ruled, straight, joined-up wires.

Key points

1Count the loops: one loop means series; more than one loop means parallel.

2A junction is where the wire splits into branches, or where branches meet again.

3The current before the first junction equals the branch currents added together — lamps don't use current up.

4Layout on the page doesn't decide the circuit; the positions of the junctions and the components in each loop do.

5Wires in a diagram: pencil and ruler, straight horizontal and vertical lines, no tiny gaps, never through a symbol.

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Worked example

Problem

A parallel circuit has three branches, each with one lamp. The ammeter on the wire leaving the battery reads 0.9 A. Ammeters in two of the branches read 0.3 A and 0.4 A. What current flows through the third branch?

⚠ Watch out

Thinking the first lamp 'uses up' some of the current, so less flows back to the battery. Current is a flow of charge — it splits between the branches, and the branch currents add back up to exactly what went in.

🧠

Memory hook

Loops tell you the type; junctions tell you the story — split at one, add up at the other.

✓

Check yourself

A parallel circuit has two branches. One branch carries 0.25 A and the other carries 0.35 A. What current flows in the wire just before the first junction — and how do you know?

Flashcards

(11)

What is electric current?

A flow of charge.

What unit is electric current measured in?

Amperes (A).

How do you tell a series circuit from a parallel circuit?

Count the complete loops from one end of the battery to the other: one loop (no junctions) is series; more than one loop is parallel.

In a parallel circuit, what happens to the current at the junction where the wire splits?

It splits between the loops (branches).

In a parallel circuit, what happens where the branches meet again?

The currents from the branches join and add back up.

Two branches carry 0.1 A and 0.5 A. What current flows after they meet?

0.6 A — the branch currents add together.

How can you tell two different-looking diagrams show the same parallel circuit?

The junctions are in the same places and each loop contains the same components.

When building a parallel circuit from its diagram, how do you build the first loop?

Start anywhere and connect the components one by one, in one direction, until the loop is complete.

Once the first loop is built, what comes next?

Find the junction, then complete each further loop in turn.

How are wires drawn in a circuit diagram?

As straight horizontal and vertical lines, with a ruler, in pencil — never through a component symbol.

Why must there be no tiny gaps at corners and junctions in a circuit diagram?

A gap would break the circuit.

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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Series or parallel? Count the loops.

Where does the current go?

Would this pass as a proper circuit diagram?

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