I-V characteristics
GCSE Physics (8463) · Required practical 4 — method, variables, the marks examiners report students losing.
Investigate the current-potential difference (I-V) characteristics of circuit elements: a fixed resistor at constant temperature, a filament lamp and a diode.
Apparatus
- Component under test: a fixed resistor, a filament lamp and a diode (with a protective resistor)
- Ammeter connected in series with the component
- Voltmeter connected in parallel across the component
- Variable resistor (rheostat) or variable power supply to change the p.d.
- Cell or power supply, switch and connecting leads
Method
- 1Connect the component in series with the ammeter and the variable resistor; connect the voltmeter across the component.
- 2Use the variable resistor to set the potential difference to a small value and record V and the current I.
- 3Increase the p.d. in steps, recording I at each value.
- 4Reverse the connections to the component to get negative p.d. and current values.
- 5Plot current (y-axis) against potential difference (x-axis) for each component; the diode needs a protective resistor in series to limit the current.
Variables
Independent
Potential difference across the component
Dependent
Current through the component
Control
- The component being tested
- Temperature (for the fixed resistor)
- The rest of the circuit
Results & processing
- Fixed resistor (constant temperature): a straight line through the origin — current is proportional to p.d. (ohmic).
- Filament lamp: an S-shaped curve that gets less steep as p.d. rises, because the filament heats up and its resistance increases.
- Diode: current flows in one direction only; its resistance is very high in the reverse direction.
Where students lose marks
Placing the meters wrongly.
Fix: Ammeter in series with the component, voltmeter in parallel across it — swapping them gives meaningless readings.
Letting the fixed resistor heat up.
Fix: Take readings quickly (or keep the current low) so the resistor stays at constant temperature, or its line will curve too.
Testing the diode without a protective resistor.
Fix: A diode has very low forward resistance, so a series protective resistor is needed to stop a large current damaging it.
Improve the method
- Reverse the connections to record negative values and get the full characteristic.
- Take readings quickly so a fixed resistor stays at constant temperature.
- Repeat readings and take a mean current at each p.d.
Try it — exam-style
Describe the shape of the I-V graph for a filament lamp and explain why it has this shape.
A fixed resistor obeys Ohm's law. At 6.0 V the current through it is 0.50 A. Calculate the current when the p.d. is 3.0 V at the same temperature.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on i-v characteristics?
The three characteristic graphs and their explanations come up every year — I make sure you can draw and justify all three, and your first lesson is free.