Investigating Ohm’s Law

Aim

To investigate the relationship between current and potential difference for a resistor and a light bulb, and to determine which component obeys Ohm’s Law.

Learning Outcome

• Measure current through a component.
• Measure potential difference across a component.
• Record current and voltage readings accurately.
• Draw a graph of potential difference against current.
• Identify an ohmic conductor and a non-ohmic conductor.
• Determine resistance from the gradient of a graph.

Time Allocation

Approximately 60 to 90 minutes.

How Does It Work?

When a potential difference is applied across a conductor, electric current flows through the conductor.

Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

A conductor that obeys this relationship is called an ohmic conductor. A conductor that does not obey this relationship is called a non-ohmic conductor.

Interactive Ohm’s Law Visual

Scientific Background

Current is the rate of flow of electric charge through a conductor.

Potential difference is the electrical push supplied to charges in a circuit.

Resistance is the opposition to the flow of current.

For an ohmic conductor, V ÷ I remains constant. This constant is the resistance.

Hypothesis

If a component obeys Ohm’s Law, then the potential difference across it will be directly proportional to the current through it, producing a straight-line graph through the origin.

Variables

Independent Variable

Current through the component, adjusted using the rheostat.

Dependent Variable

Potential difference across the component.

Control Variables

• Same power source.
• Same measuring instruments.
• Same circuit layout.
• Same procedure for each reading.
• Constant temperature as far as possible for the resistor.

Apparatus

• Battery holder
• 4 × 1.5 V cells or low-voltage power supply
• Fixed resistor or resistance wire
• Light bulb and holder
• Rheostat or variable resistor
• Ammeter
• Voltmeter
• Switch
• Connecting leads with crocodile clips
• Circuit board, optional
• Graph paper

Safety Precautions

Experiment Part A: Resistor Investigation

Aim

To investigate whether a resistor obeys Ohm’s Law.

Method

1. Connect the resistor, ammeter, rheostat, switch and battery in series.
2. Connect the voltmeter in parallel across the resistor.
3. Set the rheostat to maximum resistance before closing the switch.
4. Close the switch briefly.
5. Record the current reading from the ammeter and the potential difference reading from the voltmeter.
6. Open the switch.
7. Adjust the rheostat to decrease the resistance slightly.
8. Repeat the investigation until at least five sets of readings have been recorded.
9. Open the switch between readings to reduce heating.

Results Table: Resistor

Reading	Current, I (A)	Potential Difference, V (V)	Resistance, R = V/I (Ω)
1
2
3
4
5

Experiment Part B: Light Bulb Investigation

Aim

To investigate whether a light bulb obeys Ohm’s Law.

Method

1. Replace the resistor with the light bulb.
2. Keep the ammeter, rheostat, switch and battery connected in series.
3. Connect the voltmeter in parallel across the light bulb.
4. Set the rheostat to maximum resistance.
5. Close the switch briefly.
6. Record the current reading and potential difference reading.
7. Open the switch.
8. Adjust the rheostat to change the current.
9. Repeat until at least five sets of readings have been recorded.
10. Open the switch between readings.

Results Table: Light Bulb

Reading	Current, I (A)	Potential Difference, V (V)	Resistance, R = V/I (Ω)
1
2
3
4
5

Graph

Draw a graph of potential difference, V, on the vertical axis against current, I, on the horizontal axis.

Plot the resistor readings and the light bulb readings on the graph to compare the two relationships.

Graph Interpretation

• The resistor should produce a straight-line graph through or close to the origin.
• The straight-line graph shows that potential difference is directly proportional to current.
• The gradient of the resistor graph represents resistance.
• The light bulb should produce a curved graph because its resistance changes as the filament heats up.

What Learners Should Observe

Resistor

• A straight-line graph.
• Approximately constant resistance values.
• Direct proportionality between current and voltage.

Light Bulb

• A curved graph.
• Changing resistance values.
• The bulb becoming brighter as current increases.

Expected Results

• Resistor: obeys Ohm’s Law, straight-line graph, constant resistance.
• Light bulb: does not obey Ohm’s Law, curved graph, changing resistance.

Conclusion

The resistor obeys Ohm’s Law because the potential difference across it is directly proportional to the current through it when temperature remains approximately constant.

The light bulb does not obey Ohm’s Law because its resistance changes as the filament heats up.

Important Notes

Questions for Learners

1. State Ohm’s Law in words.
2. Which component obeyed Ohm’s Law?
3. What shape was the graph for the resistor?
4. What shape was the graph for the light bulb?
5. Why does the light bulb not obey Ohm’s Law?
6. What does the gradient of the graph represent?
7. Why should the switch be opened between readings?
8. Why is the resistor considered an ohmic conductor?
9. Why does resistance increase in the light bulb filament?
10. What would happen if the resistor became very hot?

Common Mistakes

• Connecting the ammeter incorrectly.
• Connecting the voltmeter incorrectly.
• Starting with the rheostat at low resistance.
• Leaving the switch closed too long.
• Heating the resistor excessively.
• Mixing up graph axes.
• Using unstable readings.

Teacher Notes

• Complete the resistor investigation first, then repeat using the light bulb.
• Emphasise correct graph plotting and interpretation.
• Reinforce the difference between ohmic and non-ohmic conductors.
• Ensure learners understand that temperature affects resistance.
• Do not use time intervals as the main variable for the bulb. Vary current or potential difference using the rheostat.

Teacher Tip

Use low-voltage equipment and short measuring intervals to reduce heating effects.

Extension Activity

• Calculate resistance from graph gradients.
• Compare different resistors.
• Investigate the effect of temperature on resistance.

Real-World Application

Ohm’s Law is important in electrical engineering, circuit design, electronics, appliance manufacturing and electrical troubleshooting.

Resistors are used to control current, protect components and regulate circuits.