Investigating V-I Characteristics (Oxford AQA IGCSE Physics)

Investigating V-I Characteristics

Aim of the experiment

• The aim of the experiment is to construct appropriate circuits to investigate the V–I characteristics of a variety of circuit components

• These include a fixed resistor at a constant temperature, a lamp and a diode

Variables:

• Independent variable = Voltage, V

• Dependent variable = Current, I

• Control variables:

  • Voltage of the power supply

  • Temperature of the components

  • Use of the same equipment e.g. wires, diodes

Equipment list

List of Equipment used to Investigate the V-I Characteristics of Components

Method

Circuit diagram of set-up to investigate V-I characteristics of components

1. Set up the circuit as shown with the fixed resistor

2. Change the resistance of the variable resistor to change the voltage across the component, this will in turn vary the current through 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 on the voltmeter, 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

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.

I-V  graph shapes for a 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

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