Measuring Current and Potential Difference

Aim

To measure current and potential difference in a simple electric circuit using an ammeter and a voltmeter.

Learning Outcome

• Identify an ammeter and a voltmeter.
• Connect an ammeter correctly in a circuit.
• Connect a voltmeter correctly in a circuit.
• Measure current in a simple circuit.
• Measure potential difference across a component.
• Record electrical readings correctly.
• Explain the difference between current and potential difference.

Time Allocation

Approximately 45 to 60 minutes.

How Does It Work?

An electric circuit allows electric charge to flow when the circuit forms a complete closed path.

Current is the flow of electric charge through a conductor.

Potential difference is the electrical push supplied by the battery that causes charge to move through the circuit.

An ammeter is used to measure current.

A voltmeter is used to measure potential difference.

Key Rules

Scientific Background

Current is measured in amperes (A).

Potential difference is measured in volts (V).

An ammeter must always be connected in series with the component being measured.

A voltmeter must always be connected in parallel across the component being measured.

If measuring instruments are connected incorrectly, the readings may be wrong or the instruments may be damaged.

Hypothesis

If an ammeter and voltmeter are connected correctly in a simple circuit, then current and potential difference can be measured accurately.

Variables

Independent Variable

Position of the measuring instrument in the circuit.

Dependent Variables

• Current readings.
• Potential difference readings.

Control Variables

• Same battery or cells.
• Same light bulb.
• Same connecting wires.
• Same circuit setup.

Apparatus

• Torch cells or battery pack
• Light bulb
• Bulb holder
• Ammeter
• Voltmeter
• Switch
• Connecting leads
• Circuit board, optional

Safety Precautions

Experiment Part A: Measuring Current

Aim

To measure current in a simple electric circuit using an ammeter.

Method

1. Connect the battery, switch, light bulb and ammeter in series.
2. Ensure the ammeter is connected correctly.
3. Close the switch.
4. Observe the light bulb.
5. Record the ammeter reading.
6. Open the switch.
7. Move the ammeter to another position in the same series circuit.
8. Close the switch again.
9. Record the new ammeter reading.
10. Repeat for another position in the circuit.

Results Table: Current

Ammeter Position	Current Reading (A)
Position 1
Position 2
Position 3

What Learners Should Observe

• The bulb lights when the circuit is complete.
• The ammeter gives a reading when current flows.
• The current reading is approximately the same at different positions in the same series circuit.

Conclusion

Current can be measured using an ammeter connected in series in the circuit.

Experiment Part B: Measuring Potential Difference

Aim

To measure potential difference across a component using a voltmeter.

Method

1. Build a simple circuit using a battery, switch and light bulb.
2. Connect the voltmeter in parallel across the light bulb.
3. Close the switch.
4. Record the voltmeter reading.
5. Open the switch.
6. Connect the voltmeter across the battery.
7. Close the switch again.
8. Record the voltmeter reading across the battery.
9. Open the switch after taking readings.

Results Table: Potential Difference

Component Measured	Potential Difference (V)
Light bulb
Battery

What Learners Should Observe

• The voltmeter only gives a reading when connected across a component.
• Potential difference can be measured across different parts of the circuit.
• The battery supplies the electrical push in the circuit.

Conclusion

Potential difference can be measured using a voltmeter connected in parallel across a component.

Important Note

Expected Results

• Measurable current readings.
• Measurable potential difference readings.
• Similar current readings at different points in the same series circuit.
• Voltage readings across the battery and light bulb.

Questions for Learners

1. What does an ammeter measure?
2. What does a voltmeter measure?
3. What is the unit of current?
4. What is the unit of potential difference?
5. Why must an ammeter be connected in series?
6. Why must a voltmeter be connected in parallel?
7. What happens when the switch is open?
8. Why does the bulb light when the circuit is complete?
9. What provides the electrical push in the circuit?
10. What could happen if the meters are connected incorrectly?

Common Mistakes

• Connecting the ammeter in parallel.
• Connecting the voltmeter in series.
• Leaving the switch closed while changing connections.
• Creating a short circuit.
• Reading the wrong scale on the meter.
• Recording incorrect units.
• Loose wire connections.

Teacher Notes

• This practical should focus on correct use of measuring instruments.
• Avoid detailed circuit analysis during this experiment.
• Ensure learners understand the difference between current and potential difference.
• Demonstrate correct meter placement before learners begin.
• Use simple low-voltage circuits suitable for beginners.

Teacher Tip

Allow learners to predict what will happen before taking measurements. This improves understanding and engagement.

Extension Activity

• Measure current using different numbers of cells.
• Measure potential difference across additional components.
• Compare brighter and dimmer bulbs.

Real-World Application

Electricians and engineers use ammeters and voltmeters to test and troubleshoot electrical circuits.

Electrical measurements are important in homes, vehicles, appliances, electronic devices and industrial systems.