Required Practical: Investigating I–V Characteristics (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Required Practical 4: Investigating I–V Characteristics

Aim of the Experiment

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

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

Variables

• Independent variable = Potential difference, V

• Dependent variable = Current, I

• Control variables:

  • Potential difference of the power supply

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

Equipment List

• Ammeter

• Voltmeter

• Variable resistor

• Fixed resistor, for example 10 Ω

• Filament lamp

• Diode

• Voltage Supply

• Wires

Method

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. Connect the ammeter in series with the component and the voltmeter in parallel across the component

2. Use the variable resistor to change the potential difference across the component in equal steps (for example 0.5 V) and record 8–10 values. Check each value on the voltmeter

3. For each voltage, record the value of the current from the ammeter 3 times and calculate the average current

4. Keep the current low and switch the circuit off between readings, so the fixed resistor stays at a constant temperature

5. Reverse the connections to the power supply (not the meters) and repeat the readings to obtain the negative values. Take the same range and interval of potential differences as before

6. Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each

• An example of a suitable table might look like this:

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 through the origin, the component is an ohmic conductor. This is expected from the fixed resistor at constant temperature

  • This means the current is directly proportional to the potential difference, so the resistance stays constant as the current changes

• If the I–V graph is a curve, it is a non-ohmic 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:

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

Evaluating the Experiment

Improving the accuracy of the result

• Check each meter before connecting it. If it shows a reading when it is not connected to a circuit, it has a zero error. Subtract this reading from every measurement

• 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 never to 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 equipment, as this could damage the electrical equipment

• The components will get hot, especially at higher voltages

  • Be careful when handling them – especially the filament lamp

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