Required Practical: I-V Characteristics

This lesson covers the Required Practical on investigating the I–V characteristics of a filament lamp, a diode and a resistor, as specified by AQA GCSE Physics (4.2.1). This is Required Practical Activity 16 and is frequently examined — you must be able to describe the method, draw the circuit, sketch the results and explain the shapes of the graphs.

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Aim

To investigate the relationship between potential difference and current for a filament lamp, a diode and a resistor at constant temperature, and to produce I–V characteristic graphs for each component.

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Equipment List

Equipment	Purpose
Filament lamp	Component under investigation
Diode (silicon)	Component under investigation
Resistor (fixed value, e.g., 10 ohms)	Component under investigation
Variable resistor (rheostat)	To vary the p.d. across the component
Ammeter	To measure the current through the component
Voltmeter	To measure the p.d. across the component
Power supply (d.c., low voltage)	To provide the potential difference
Switch	To control the circuit
Connecting leads	To connect the circuit

Exam Tip: The variable resistor is essential in this practical — it allows you to smoothly vary the p.d. across the component being tested. Without it, you would only be able to test a single p.d. value from the power supply. Some setups use a variable power supply instead.

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Variables

Variable Type	Variable	Details
Independent	Potential difference (V) across the component	Varied using the variable resistor (or variable power supply)
Dependent	Current (I) through the component	Measured using the ammeter
Control variables	Component being tested	Same component for each set of readings
	Temperature of surroundings	Conduct experiment at room temperature

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Circuit Diagram

The circuit for investigating I–V characteristics uses the following arrangement:

graph LR
    A[DC Power Supply] --> B[Switch]
    B --> C[Variable Resistor]
    C --> D[Ammeter]
    D --> E["Component (lamp/diode/resistor)"]
    E --> A
    F[Voltmeter] -.-> |parallel across component| E

The variable resistor is connected in series to allow the p.d. across the component to be changed.

The ammeter is connected in series to measure the current through the component.

The voltmeter is connected in parallel across the component to measure the p.d. across it.

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Method

1. Set up the circuit as shown in the circuit diagram, with the resistor as the first component to test.

2. Adjust the variable resistor to its maximum resistance setting.

3. Close the switch and record the voltmeter reading (V) and the ammeter reading (I).

4. Gradually decrease the resistance of the variable resistor, recording V and I at regular intervals. Aim for at least six to eight pairs of readings across a suitable range.

5. Reverse the connections to the power supply (swap the positive and negative terminals) to obtain readings with negative p.d. and negative current. This allows you to complete the I–V graph for both directions.

6. For the resistor, record readings for both positive and negative p.d. values.

7. Switch off between readings if testing the filament lamp, to allow the lamp to cool between readings and reduce the effect of temperature changes on results.

8. Repeat the procedure for the filament lamp and then for the diode.

9. For the diode, note that no current flows in the reverse direction (negative p.d.) and current only begins to flow in the forward direction once the threshold voltage (approximately 0.6–0.7 V) is reached.

10. Plot I–V graphs for each component with p.d. (V) on the x-axis and current (I) on the y-axis.

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Safety Precautions

Hazard	Risk	Precaution
Filament lamp gets hot	Burns from touching the hot lamp	Do not touch the lamp during or immediately after the experiment
Overheating components	Damage to components or burns	Use low voltages; switch off between readings
Electrical connections	Electric shock if connections are faulty	Check all connections; keep hands dry; use low voltages

Exam Tip: When describing safety in any electricity practical, always mention using low voltages and ensuring secure connections. For the filament lamp specifically, mention that it gets hot and should not be touched.

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Expected Results and Graph Shapes

1. Resistor (at constant temperature)

p.d. (V)	Current (A)
-6.0	-0.30
-4.0	-0.20
-2.0	-0.10
0	0
2.0	0.10
4.0	0.20
6.0	0.30

Graph shape: A straight line through the origin. The line extends into both the positive and negative quadrants, showing the same behaviour in both directions.

What this tells us: The resistor has a constant resistance and obeys Ohm's Law. Current is directly proportional to p.d.

2. Filament Lamp

p.d. (V)	Current (A)
-6.0	-0.45
-4.0	-0.35
-2.0	-0.20
0	0
2.0	0.20
4.0	0.35
6.0	0.45

Graph shape: An S-shaped curve through the origin. The curve is steeper near the origin and flattens as p.d. increases. The shape is symmetrical in both positive and negative directions.

What this tells us: As the p.d. and current increase, the filament heats up. The increased temperature causes the metal ions to vibrate more, leading to more collisions with electrons. This increases the resistance of the filament, causing the curve to flatten.

3. Diode

p.d. (V)	Current (A)
-3.0	0
-2.0	0
-1.0	0
0	0
0.4	0
0.6	0.01
0.7	0.05
0.8	0.20
1.0	0.60