GCSE · Physics · Edexcel · Spec 1PH0

Ammeter in series

Read an ammeter on one side of a lamp, then on the other side. Bigger, smaller or the same? Guess first, then go and move the meter yourself.

Physics · Example circuit: one cell, one lamp

Move the ammeter round the loop

View: Left of the lamp. Showing 2 layers: Circuit, Left of the lamp

View

try every position ↓

Ammeter on the wire to the left of the lamp: it reads 0.30 A. It's in series, in the one loop.

Example reading of 0.30 A. Pick a position, then read the meter.

Why the position doesn't matter

Reason it through

Why does an ammeter anywhere in the loop give the same reading?

Link 1 of 4

First link · your turn

A series circuit is a single loop. How many routes can the charge take round it?

Locked — reveal the link above first

Physics · Circuits

Does the lamp change the reading?

A cell, a lamp and an ammeter are in one loop. The ammeter is next to the cell and reads 0.30 A. You move it to the wire on the far side of the lamp.

What do you think it reads now?

Less than before. The lamp uses up some of the current, so there's less left after it.It reads the most next to the cell, because that's where the current starts, and it gets smaller as it goes round.The same, 0.30 A. In one loop the same charge passes every point each second.It depends where it sits. To get the lamp's current, the meter has to be right up against the lamp.

How sure are you?

I'm sureI think soI'm guessing

Less than before. The lamp uses up some of the current, so there's less left after it.

This is a common misconception

It sounds sensible: the lamp is doing something, so surely something gets lost. But current is the charge passing a point each second, and the charge that passes the lamp carries on round the one loop. The same charge passes every point each second, so the reading is the same either side of the lamp.

It reads the most next to the cell, because that's where the current starts, and it gets smaller as it goes round.

This is a common misconception

Easy trap, because the cell feels like the start. But a loop has no start and finish. The same charge passes every point in it each second, so next to the cell the meter reads exactly what it reads anywhere else.

The same, 0.30 A. In one loop the same charge passes every point each second.

This matches the evidence

Yes. Same charge per second at every point means the same current. So the current through the lamp equals the reading of an ammeter anywhere in that series loop.

It depends where it sits. To get the lamp's current, the meter has to be right up against the lamp.

This is a common misconception

Touching the lamp isn't what matters. What matters is being in series in the same single loop. Anywhere in that loop gives the same reading, so you can put the meter wherever is easiest.

Same loop, same charge passing each second, same current: the ammeter reads the same anywhere in series.

Physics · Circuits

Does this ammeter measure the lamp's current?

Sort each ammeter position: does it measure the current through the lamp?

Still to sort

Measures the lamp's current (0)

In series with the lamp, in its single loop.

Where the line is: The charge flowing through the lamp also flows through the ammeter.

Doesn't measure the lamp's current (0)

Not in series with the lamp in its single loop.

Where the line is: Being close to the lamp isn't enough. It has to be in series in the same loop.

5 of 5 still to sort.

One cell and one lamp. Judge each ammeter position, then read why.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

An ammeter is the meter that measures electric current.
Current is the amount of charge passing a point in the circuit each second, measured in amperes (amps, A).
Have a goQuick prediction. If twice as much charge passes a point every second, what happens to the reading on an ammeter there?
It doubles.
Current is the charge passing a point each second, so twice the charge per second means twice the current.
To measure the current through a component, connect the ammeter in series with it, in the same single loop.
That way, the charge flowing through the component also flows through the ammeter.
Have a goSam wants the current through his lamp. He puts the ammeter on the table next to it and glares at the dial. Why won't that tell him the lamp's current?
The ammeter isn't connected in series with the lamp, in the same single loop.
Only then does the charge flowing through the lamp also flow through the ammeter.
A series circuit is a circuit with a single loop.
In a series circuit the current is the same everywhere: in the cell, in the wires and in each component.
Have a goIn a series circuit the current in one wire is 0.40 A. What is the current in the lamp, and in the cell?
0.40 A in both.
In a series circuit the current is the same everywhere: in the cell, in the wires and in each component.
So an ammeter at any position in series in the loop gives the same reading.
Moving the ammeter to another position in series doesn't affect the current. You're only changing where you read it.
So the current through a component equals the reading of an ammeter anywhere in the same series loop.

The big picture

An ammeter measures current: the amount of charge passing a point in the circuit each second, in amperes (A). To measure the current through a component, connect the ammeter in series with it, in the same single loop. In a series circuit the current is the same everywhere, so an ammeter at any position in series gives the same reading.

Key points

1An ammeter measures current: charge passing a point each second, in amperes (A).

2Connect it in series with the component, in the same single loop.

3Series circuit = single loop = the same current everywhere: cell, wires and every component.

4So an ammeter anywhere in series in the loop gives the same reading, which is the current through the component.

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Worked example

Problem

A cell and one lamp are joined in a single loop. You want the current through the lamp. An ammeter on the wire between the cell and the lamp reads 0.45 A. Your friend says you should move it to the other side of the lamp 'to check'. What is the current through the lamp, and is the check needed?

⚠ Watch out

Mixing up 'in series' with 'next to'. The ammeter doesn't have to touch the component. It has to be in series with it, in the same single loop, and anywhere in that loop will do.

🧠

Memory hook

Picture a ring road with no exits: cars passing any spot each minute is the same number wherever you stand. Cars aren't charge, but the counting idea carries over. One loop, so the meter doesn't care where it sits.

✓

Check yourself

Cover the page. Explain to an imaginary friend why ammeters on opposite sides of a lamp read the same. Use 'single loop' and 'charge passing each second'.

Flashcards

(9)

What does an ammeter measure?

Electric current.

What is current?

The amount of charge passing a point in the circuit each second.

What symbol and unit does current have?

The symbol is I, and it is measured in amperes (amps, A).

How do you connect an ammeter to measure the current through a component?

In series with the component, in the same single loop.

Why must the ammeter be in the same single loop as the component?

So the charge flowing through the component also flows through the ammeter.

What is a series circuit?

A circuit with a single loop.

In a series circuit, where is the current the same?

Everywhere: in the cell, in the wires and in each component.

You move an ammeter to another position in series in the loop. What happens?

Its reading stays the same, and the current isn't affected.

How can you find the current through a component in a series circuit?

Read an ammeter placed anywhere in series in the same loop.

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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Does the lamp change the reading?

Does this ammeter measure the lamp's current?

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