Investigating I-V Characteristics (WJEC GCSE Physics: Combined Science)

Aim of the Experiment

• The aim of the experiment is to construct appropriate circuits to investigate the I–V characteristics of a variety of circuit components
• These include a fixed resistor at a constant temperature, a lamp and diode

Variables:

• Independent variable = Voltage, V
• Dependent variable = Current, I
• Control variables:
  • Voltage of the power supply
  • Use of the same equipment eg. wires, diodes

Equipment List

List of Equipment Used to Investigate the I-V Characteristics of a Range of Circuit Components

Method

Circuit Diagram of Set-up to Investigate I-V Characteristics of Components

Circuit diagram of the apparatus set up. The fixed resistor will be replaced by a filament lamp and diode

1. Set up the circuit as shown with the fixed resistor
2. Vary the current across the component by changing the resistance of the variable resistor, this will change the voltage across the component
3. For each voltage, record the value of the current from the ammeter 3 times and calculate the average current
4. Repeat steps 2 and 3 for 8-10 different voltage values increasing by around 0.5 V each time
5. Make sure to switch off the circuit in between readings to prevent heating of the component and wires
6. Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)
7. Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each

Example Results Table

A good results table includes space for all measurements and associated calculations (like averages) presented in a clear and logical way

Analysis of Results

• Plot a graph of average current against voltage (an I–V  graph) for each component
• If the I–V graph is a straight line, it is a linear conductor. This is expected from the fixed resistor

• If the I–V graph is a curve, it is a non-linear conductor. This is expected from the filament lamp and diode
• Compare the results from the graphs obtained to the known I–V graphs for the resistor, filament lamp and diode. These should look like:

I-V  Graph Shapes for a Resistor, Diode & Filament Lamp

The expected I-V graphs for the resistor, diode and filament lamp

Evaluating the Experiment

Systematic Errors:

• The voltmeter and ammeters should start from zero, to avoid zero error in the readings

Random Errors:

• In practice, the voltmeter and ammeter will still have some resistance, therefore the voltages and currents displayed may be slightly inaccurate
• The temperature of the equipment could affect its resistance. This must be controlled carefully
• Taking multiple readings of the current for each component will provide a more accurate result and reduce uncertainties

Safety Considerations

• When there is a high current and a thin wire, the wire will become very hot
  • Make sure you never touch the wire directly when the circuit is switched on

• Switch off the power supply right away if burning is smelled
• Make sure there are no liquids close to the experiment, as this could damage the electrical equipment
• Electrical components will get hot, especially at higher voltages
  • Be careful when handling them - especially the filament lamp

• Disconnect the power supply in between readings to avoid the components heating up too much