1.1.5Electricity, Energy and Waves
Investigating I-V Characteristics — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 6 of 36
Part 1 of 5Aim and variables
Aim
- Aim To construct circuits that investigate the I-V characteristics of a fixed resistor at constant temperature, a filament lamp and a diode.
Variables
- Variables Independent variable: the voltage across the component.
- Dependent variablethe current through the component.
Control variables
- Control variables The same power supply and the same wires and equipment throughout,
- and the temperature of the resistor kept constant by using low voltages.
Part 2 of 5Equipment
| Equipment | Purpose |
|---|---|
| Ammeter | Measures the current, connected in series |
| Voltmeter | Measures the voltage, connected in parallel across the component |
| Variable resistor | Changes the current and so the voltage across the component |
| Fixed resistor (100 to 500 Ω) | First component tested |
| Filament lamp and diode | Other components tested in turn |
| Low-voltage power supply | Drives current round the circuit |
| Wires | Complete the circuit |
Part 3 of 5Method
1
Build the circuit with the fixed resistor, the ammeter in series and the voltmeter in parallel across the resistor.
2
Adjust the variable resistor to set a voltage, and read the current from the ammeter.
3
Take three current readings at each voltage and calculate an average.
4
Repeat for eight to ten voltages, increasing by about 0.5 V each time.
5
Switch off between readings so that the component and wires do not heat up.
6
Reverse the connections to the power supply and repeat to obtain readings for negative voltage and current.
7
Replace the resistor with the filament lamp and then the diode, repeating the whole procedure.
Part 4 of 5Processing and analysing results
1Tabulate
- Tabulate Record voltage, the three currents and the mean current in a clear table with units in the headings.
2Plot
- Plot Draw a graph of mean current against voltage for each component.
3Describe the shape
- Describe the shape A straight line through the origin means a linear conductor;
- a curve means a non-linear conductor.
4Compare
- Compare Expect a straight line for the resistor and curves for the lamp and diode, and compare with the known I-V graphs.
Part 5 of 5Safety and evaluation
Reliable results
Safety
- Safety A thin wire carrying a high current gets very hot, so never touch it while the circuit is on.
- Components, especially the filament lamp, become hot, so handle them carefully.
- Switch off straight away if you smell burning and keep liquids away from the equipment.
Accuracy
- Accuracy Repeating the readings and averaging reduces the effect of random errors.
- Using a low voltage stops the heating effect changing the resistance of the resistor.
Identifying an unknown component
- Identifying an unknown component Zero current for negative voltages and a rapid rise for positive voltages indicates a diode or LED.
Key terms
| Term | Meaning |
|---|---|
| Independent variable | The variable that is deliberately changed, here the voltage. |
| Dependent variable | The variable that is measured, here the current. |
| Control variable | A variable kept the same so that the test is fair. |
| Mean | The average of repeated readings, found by adding them and dividing by the number of readings. |
| Non-linear conductor | A component with a curved I-V graph whose resistance changes. |
Avoid these mistakes
1
Placing the ammeter in parallel or the voltmeter in series, which gives wrong readings.
2
Leaving the circuit on between readings so that the components heat and change resistance.
3
Plotting voltage on the vertical axis when the question asks for an I-V graph, with current on the vertical axis.
4
Taking only one reading at each voltage, which makes results less reliable.
5
Forgetting to reverse the supply to obtain negative values, which are needed to show diode behaviour.
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