GCSE · Physics · Edexcel · Spec 1PH0

Current conserved at junction

Here is a challenge: try to break one rule of circuits. Change a branch however you like and see if the ammeters can ever disagree.

Physics · Parallel circuits

Try to break the junction rule

Parallel circuit with three ammeters

Parallel circuit with three ammeters. Resistance of branch 2: 5 Ω. When you change the resistance of one branch, what happens to the current in each branch, and to the current going into the junction?

When you change the resistance of one branch, what happens to the current in each branch, and to the current going into the junction?

PredictYou are about to swap branch 2 from a high resistance (30 Ω) to a low one (5 Ω). What will the ammeters show?

Resistance of branch 2

One battery feeds two branches. Branch 1 stays at 10 Ω. Pick a resistance for branch 2 and run it: the dial shows A1, the current going into the junction, and the readings for A2 (branch 1) and A3 (branch 2) appear below, ready to record in the table. A1 reads the current going into the junction. A2 reads branch 1 (always 10 Ω). A3 reads branch 2, the one you change. The values are illustrative.

Watch out: Equal shares only appeared when the two resistances matched. Current does not split equally by default.

Physics · Why does it work?

Where does the current go?

A wire carries current into a junction, where it splits into two branches.

Which of these is closest to what you think happens?

Each branch uses up some of the current, so less comes out than went in.The current always splits equally, so each branch gets half.Charge is not used up or created, so all the charge arriving each second leaves along a branch.The branch with more resistance carries the larger share, because it has more to push through.

How sure are you?

I'm sureI think soI'm guessing

Each branch uses up some of the current, so less comes out than went in.

This is a common misconception

Easy to believe, because lamps and resistors do things to the circuit. But current is a flow of charge, and charge is not used up in a circuit. So nothing at the junction or in the branches can eat the current. Look back at the ammeters: the branch readings never added up to less than A1.

The current always splits equally, so each branch gets half.

This is a common misconception

It only looks equal when the two branches happen to match. In the ammeter runs, a 5 Ω branch took 1.20 A while the 10 Ω branch beside it carried 0.60 A. How the current divides depends on the resistance of each branch, and a lower-resistance branch carries a larger share.

Charge is not used up or created, so all the charge arriving each second leaves along a branch.

This matches the evidence

Yes. Current is a flow of charge, and every coulomb of charge that arrives at the junction each second must leave along one of the branches. That is why the current going in always equals the current going out, whatever way it divides.

The branch with more resistance carries the larger share, because it has more to push through.

This is a common misconception

This one is the wrong way round. A branch with less resistance carries the larger share. In the ammeter runs, the 30 Ω branch carried only 0.20 A while the 10 Ω branch carried 0.60 A. Either way, the shares still add up to the current entering the junction.

Current is not used up. What goes into a junction comes out, and resistance decides who gets the bigger share.

Where does the junction rule apply?

Series circuitvsParallel circuit

Same battery, same kind of ammeter. Different layout.

Focus

Junctions

Series circuit

None. The charge has one path to follow.

Parallel circuit

Yes. The path divides, then the branches join again.

The insight

The junction rule only has work to do where the path divides or joins.

What the current does

Series circuit

The same at every point in the circuit.

Parallel circuit

Divides between the branches, then the branch currents add up again.

Ammeter readings at different points

Series circuit

Every ammeter reads the same, wherever you put it.

Parallel circuit

It depends where the ammeter is: in one branch, it reads only that branch.

Is current used up along the way?

Series circuit

No. The reading is the same before and after a lamp.

Parallel circuit

No. The current leaving the branches equals the current that entered.

Physics · Your turn

Find the missing branch current

A parallel circuit has three branches. The supply provides a total current of 1.00 A. Branch 1 carries 0.45 A and branch 2 carries 0.30 A. What current flows in branch 3?

The branch currents add up to the total current through the supply, so I1 + I2 + I3 = 1.00 A.
missing step

Which line is step 2?

I3 = total − (I1 + I2)I3 = I1 + I2I3 = total ÷ 3

Physics · Check the working

Two branches rejoin

Two branches of a parallel circuit rejoin at a junction. One carries 0.15 A and the other carries 0.35 A. What current flows in the single wire leaving the junction?

A student's answer — which line goes wrong?

0.15 A and 0.35 A arrive at the junction.
The current leaving is the difference between the two, because the bigger one wins.
0.35 A − 0.15 A = 0.20 A
So 0.20 A flows in the wire leaving the junction.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

A junction is a point in a circuit where the path divides into two or more branches, or where branches join again.
Have a goPicture one battery and two lamps, joined end to end in a single loop of wire. How many junctions does that circuit have?
None.
The wire never divides into branches or joins branches back together, so there is no point that counts as a junction.
Current is conserved at a junction: the total current flowing in equals the total current flowing out.
For example, 0.5 A flowing in can split into 0.2 A and 0.3 A, since 0.2 A + 0.3 A = 0.5 A.
Current is a flow of charge, and charge is not used up or created in a circuit.
Have a goSam says: "1.0 A goes into a junction, and the wires leaving carry 0.4 A and 0.4 A. The missing 0.2 A just got used up in the junction." Is Sam right?
No. The wires leaving should carry 1.0 A in total.
Charge is not used up in a circuit, so the current out must equal the current in. Sam's readings can't both be right.
So every coulomb of charge arriving at a junction each second leaves along one of the branches.
In a parallel circuit, the branch currents add up to the total current through the supply.
To find an unknown branch current, subtract the known branch currents from the total.
A lower-resistance branch carries a larger share, but the shares always add up to the current entering the junction.
Have a goBranch A has a resistance of 5 Ω and branch B has 20 Ω, side by side. Which carries the larger current?
Branch A, the 5 Ω one.
The branch with less resistance carries the larger share, though both shares still add up to the current entering the junction.
A series circuit has no junctions, so the current is the same at every point.

The big picture

At a junction, the total current flowing in always equals the total current flowing out, because charge is not used up or created. In a parallel circuit the branch currents add up to the current from the supply, and the branch with less resistance takes the larger share. A series circuit has no junctions, so its current is the same everywhere.

Key points

1Current in equals current out at every junction.

2Charge is not used up or created, so current is not used up either.

3In a parallel circuit the branch currents add up to the supply current, so subtract to find a missing one.

4A lower-resistance branch carries a larger share, but the shares always add up to the current entering.

5A series circuit has no junctions, so the current is the same everywhere.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

A student wires a battery to two lamps side by side. An ammeter on the lead from the battery reads 0.50 A. Ammeters on the two lamp wires read 0.20 A and 0.30 A. Are these readings consistent with the junction rule, and which lamp has the lower resistance?

⚠ Watch out

Thinking that lamps and resistors use up current, or that current always splits equally. Charge is not used up, and each branch's share depends on its resistance.

🧠

Memory hook

Think of cars arriving at a fork in the road. However they split between the two roads, every car that arrives has to leave by one of them.

✓

Check yourself

Cover the page. In your own words: why can't a junction swallow any of the current, and what decides which branch gets the bigger share?

Flashcards

(9)

What is a junction in a circuit?

A point where the path divides into two or more branches, or where branches join again.

What is the rule for current at a junction?

Current is conserved: the total current flowing in equals the total current flowing out.

Why is current conserved at a junction?

Current is a flow of charge and charge is not used up or created, so every coulomb arriving each second leaves along a branch.

In a parallel circuit, how do the branch currents compare with the supply current?

They add up to the total current through the supply.

How do you find an unknown branch current?

Subtract the known branch currents from the total.

Which parallel branch carries the larger share of the current?

The one with less resistance. The shares still add up to the current entering the junction.

True or false: the current always splits equally between branches.

False. How it divides depends on the resistance of each branch.

Why is the current the same at every point in a series circuit?

A series circuit has no junctions, so the current has nowhere to divide.

True or false: a lamp in a series circuit uses up some of the current.

False. Charge is not used up, so the current is the same before and after the lamp.

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.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Try to break the junction rule

Where does the current go?

Find the missing branch current

Two branches rejoin

More Edexcel GCSE Physics topics