I–V characteristics required practical

Aim: To use a circuit to plot current against potential difference for a fixed resistor, a filament lamp and a diode, and explain the shape of each graph.

Variables

• Independent: Potential difference across the component
• Dependent: Current through the component
• Control: The component being tested; Temperature of the fixed resistor (keep currents low); The circuit set-up and meters used

Equipment

• Variable power supply or battery with a variable resistor
• Ammeter
• Voltmeter
• Fixed resistor
• Filament lamp
• Diode with a protective resistor in series
• Switch
• Connecting leads

Method

1. Set up a circuit with the component in series with an ammeter and a variable resistor, and a voltmeter connected in parallel across the component.
2. Adjust the variable resistor (or power supply) so the potential difference is small and record the current and potential difference.
3. Increase the potential difference in equal steps, recording the current each time, and switch off between readings.
4. Swap the connections to the component so the current flows the other way.
5. Repeat the readings, recording current and potential difference as negative values.
6. Repeat the whole process for the filament lamp and then for the diode.
7. For the diode, use a protective resistor and small steps near 0.6 V, where the current starts to rise quickly.
8. Plot current (y-axis) against potential difference (x-axis) for each component, including negative values.

Safety

• A filament lamp gets hot: do not touch it while it is on or just afterwards.
• Keep potential differences low and use a protective resistor with the diode so it does not overheat.
• Switch off between readings to stop components heating up.

Results and calculations

Record potential difference and current in both directions in a table and plot an I–V graph for each component. A fixed resistor gives a straight line through the origin (ohmic, so R = V ÷ I is constant); a filament lamp gives an S-shaped curve because resistance rises with temperature; a diode lets current flow only above about 0.6 V in one direction.

Common mistakes and improvements

• The fixed resistor warms up and its resistance changes: keep the current small and switch off between readings.
• Forgetting to reverse the component, so only half of each graph is drawn: take readings in both directions.
• Taking too few readings near where the diode switches on: use smaller steps in that region to show the sharp rise.
• Meters with the wrong range give poor resolution: choose a range that suits the readings, or use digital meters, and check for zero errors.

Exam tips

• Learn to sketch the three graphs and describe each in words, including what happens in the negative direction.
• Explain the lamp graph: as current increases the filament gets hotter, ions vibrate more and resistance increases, so the line curves.
• Say that the diode has a very high resistance in the reverse direction, so almost no current flows.
• Remember you can find resistance at any point using R = V ÷ I, not from the gradient of the line.
• Circuit diagram questions often test the positions of the ammeter, voltmeter and variable resistor.