← Back to Electricity contents

ELECTRICITY · CHAPTER 9 OF 13

Measuring potential difference

A voltmeter compares the energy transferred per coulomb between two points, so it must be connected across the component being measured.

By the end, you’ll be able to…

  • explain how a voltmeter is connected in a circuit
  • read potential difference from analogue and digital voltmeters
  • record potential difference using the correct unit

Potential difference is measured with a voltmeter

Potential difference is measured in volts, with the unit symbol V. A voltmeter measures the energy transferred per coulomb of charge between two points.

Analogue voltmeter

This is an image of an analogue voltmeter.

Unit: volt (V)

A device used to measure potential difference. Its circuit symbol is a circle containing the letter V.

V

How is a voltmeter connected?

Incorrect: connected in series

A voltmeter has a very high resistance. Putting it in the main path would greatly reduce the current and change the circuit.

Connected in parallel

The two voltmeter leads connect to opposite sides of the component, allowing it to compare those two points.

INTERACTIVE INVESTIGATION

Where should the voltmeter go?

Select one of the three positions, then press Test position. A voltmeter measures the potential difference between two points, so it needs one connection on each side of the bulb.

Select position A, B or C, then press Test position.
VOLTMETER PLACEMENT
V POSITION A V POSITION B V POSITION C
Choose a position first. Select A, B or C, then test it.
Not quite. Position A places the voltmeter in series. A voltmeter must connect to both sides of the bulb.
Not quite. Position B places the voltmeter in series. It would not be comparing the two points on opposite sides of the bulb.
Correct. Position C connects the voltmeter in parallel across the bulb, with one connection on each side.

☆ A voltmeter connected across the cell measures the cell’s potential difference. Connected across the bulb, it measures the bulb’s potential difference.

How do you read an analogue voltmeter?

First, find the difference between two labelled values. Count the number of equal intervals between them, then divide to find the value of one interval. Finally, use the needle position to determine the reading.

*Count the spaces between the scale marks—not the marks themselves.

INTERACTIVE SIMULATION

Practise reading an analogue voltmeter

This scale runs from 0 V to 6 V. Choose a mystery reading, then work out the value shown.

Select a mystery reading, read the voltmeter scale, then choose your answer.
0 3 6V
How to read this scale The gap from 0 V to 3 V is divided into six equal intervals. 3 V ÷ 6 = 0.5 V per interval

Choose a mystery reading:

What potential difference does the needle show?

Not quite. Each small interval represents 0.5 V.
Correct. The needle shows 2.0 V.
Not quite. Each small interval represents 0.5 V.
Correct. The needle shows 3.5 V.
Not quite. Each small interval represents 0.5 V.
Correct. The needle shows 4.5 V.
Not quite. Each small interval represents 0.5 V.
Correct. The needle shows 1.0 V.
Not quite. Each small interval represents 0.5 V.
Correct. The needle shows 5.5 V.

The reading here is 4.20 V. Never just write “4.20”—a measurement needs its unit!

Digital Voltmeter

Digital Voltmeters

A digital display gives the reading directly

A digital voltmeter removes the need to interpret a scale. The potential difference is shown directly on the display, but you must still record the correct unit and pay attention to the number of decimal places shown.

INTERACTIVE QUICK CHECK

Check your understanding

Answer all three questions about measuring potential difference.

Select one answer for each question. You will get feedback straight away.
1 How should a voltmeter be connected across a bulb?
2 A scale runs from 0 V to 6 V and contains 12 equal intervals. What is each interval worth?
3 Why does a voltmeter have a very high resistance?
INTERACTIVE CHALLENGE

A student says, “A voltmeter measures the current that passes through it, so it should be connected in series like an ammeter.” Explain why this is incorrect.

Think about what a voltmeter measures and how it is connected, then reveal the answer.

A voltmeter measures the potential difference between two points, so it is connected in parallel across a component. It has a very high resistance, so very little current flows through it and it does not significantly change the circuit.

CHAPTER 9

Chapter summary

  • Potential difference is measured between two points using a voltmeter and is recorded in volts (V).
  • A voltmeter is connected in parallel across the component being measured.
  • A voltmeter has a very high resistance so that it draws very little current and does not significantly change the circuit.
  • To read an analogue voltmeter, calculate the value of one scale interval before reading the needle.

← Previous chapter
08 · Potential difference

← Back to Electricity contents

Next chapter →
10 · Resistance