KS3 · Physics

Voltage and the voltmeter

Put a voltmeter in the wrong place and the lamp goes out. Add one cell too many and it blows. By the end, you will predict both.

Add cells. Watch what the push does.

+terminal0 VBattery1 cell1.5 V lamplitV1.5 VVoltmeterreading1Cells in seriesdrag up:add a cell

Cells in series: 1. Voltmeter reading: 1.5 V

Drag the handle up to add cells, all lined up the same way. Before each step, predict the voltmeter reading, then check yourself.

There is one lamp in this loop, so the voltage across the battery and the voltage across the lamp are the same. One voltmeter reads both.

Voltage · Current · Energy

So what actually is voltage?

The lamp is lit. A friend asks what the battery is really doing. Three people give three different answers.

Which answer is closest to what you think right now?
How sure are you?

The rope loop and the real circuit

The tape loopvsThe real circuit

Picture a loop of tape. Some hands pull it round and other hands grip it. Match each part of the picture to its real circuit partner.

Focus

Who does the pulling?

The tape loop

The pulling hands

The real circuit

The cell or battery

The insight

The pulling hands are the push. In the circuit, that push is the battery’s voltage.

What moves round?

The tape loop

The moving tape

The real circuit

The current

What gets in the way?

The tape loop

The still hands gripping the tape

The real circuit

The components, such as lamps

Pull harder

The tape loop

The tape moves faster

The real circuit

A higher voltage gives a higher current

Add more grippers

The tape loop

The tape moves more slowly

The real circuit

More lamps give a lower current, and dimmer lamps

Predict, then check

A voltmeter’s circuit symbol is a circle with a capital V inside it. Here, one has been wired into the loop in series with a lamp, so any current has to go through the meter.

What does the lamp do?

Cell sums

Finish the working

Four 1.5 V cells are joined in a row. Three of them point the same way, but one cell is turned the wrong way round, so it pushes against one of the others. What does a voltmeter read when it is connected across the whole battery?

  1. Start with the cells. Pair the backward cell with one of the forward cells. Those two push against each other, so they are an opposed pair.
  2. missing step
Which line is step 2?

Typical voltages

Each tick is ten times the one before. In science, “battery” means more than one cell joined in series, so a single AA cell is a cell. Tap a chip to read about it.

1 V10 V100 V1000 VVoltage (V), log scale

AA cell

1.5 V

A single AA cell is rated 1.5 V.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Current is the flow. Voltage is the push that makes it flow.

What you need to know

  • Voltage and potential difference mean the same thing. It is measured in volts, V.
  • A voltmeter has its own circuit symbol and is connected across a cell, battery or component.
  • How cell voltages add up, cancel, and get shared out in a series circuit.
  • What a bigger voltage does to the current and to the brightness of a lamp, and why a lamp can blow.
  • Typical voltages, from a single AA cell to UK mains.

The big picture

Voltage, also called potential difference, is the push from a cell or battery that moves charge round a circuit. It is measured in volts, measured with a voltmeter connected across a component, added up cell by cell, and shared out across components in series. A bigger voltage means a bigger current and a brighter lamp, but too much blows it.

Key points

1Voltage (potential difference) is the size of the push from a cell or battery, and the size of the push across a component. Its unit is the volt, V.
2Current flows round the circuit; voltage does not. A voltmeter goes across a component, never in line with it.
3Cells lined up the same way add their voltages. Cells turned the other way push against each other and cancel.
4In a series circuit the voltages across the components add up to the battery’s voltage. Identical lamps share it equally.
5A bigger voltage gives a bigger current and a brighter lamp, but a voltage that is too high for a lamp can blow it. A cell’s voltage depends on its chemistry, not its size.

Worked example

Problem

A toy has a battery made of six 1.5 V cells in series, all lined up the same way. The battery is connected in series to three identical lamps. What does a voltmeter read (a) across the battery, (b) across one lamp, (c) across two of the lamps?

⚠ Watch out

Wiring the voltmeter in line with the lamp, as if voltage flowed round the loop like current. In line, the meter lets almost no current through, so the lamp goes out. A voltmeter goes across the component.

🧠

Memory hook

Pull, flow, grip. The hands PULL (voltage), the tape FLOWS (current), the other hands GRIP (the components). And a voltmeter goes ACROSS, like a bridge over a river, never sailing along the river like a boat.

✓

Check yourself

No peeking: where does a voltmeter go, what actually flows round the circuit, and why does a 1.5 V lamp blow on a 6 V battery?

Flashcards

(13)
What is another name for voltage, and what is it measured in?
Potential difference (p.d.). It is measured in volts, written V.
Finish the sentence: voltage is the ___ from a cell or battery that moves charge round a circuit.
Push. A component has a voltage across it too: the size of the push moving current through it.
Does voltage flow round a circuit?
No. Current is what flows (a flow of charge). Voltage is the push across a cell, battery or component.
What does the voltmeter symbol look like, and where does the meter go?
A circle with a capital V inside, with wires coming out. The meter goes across the component, close to it on either side.
Why must a voltmeter not go in series with a lamp?
It lets almost no current through, so it blocks the circuit and the lamp will not light.
You slide a voltmeter’s probes along the wires on either side of a component. What happens to the reading?
Nothing. The leads do not change the voltage.
Several cells in a row all point the same way. How do you find the battery’s voltage?
Add the cell voltages. The pushes go the same way, so they add.
What happens when two equal cells in a row point opposite ways?
They push against each other and cancel out, giving 0 V.
In a series circuit, what do the voltages across the components add up to?
The voltage of the cell or battery.
Add more cells (same way) to a circuit with the same lamp. What happens to the current and the brightness?
A bigger push, so more current and a brighter lamp.
Add more lamps to a circuit with the same cell. What happens?
The current is harder to push, and the cell cannot push harder, so the lamps get dimmer.
What can happen if the voltage is too high for a lamp?
Too much current flows and the lamp could blow.
A small lithium cell can be 3 V while a bigger alkaline cell is 1.5 V. What decides a cell’s voltage?
The chemical reactions inside it, not its size.

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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