Section 1 of 7Aim and equipment
Aim
- Aim To find how the current through a component depends on the p.d. across it, for a fixed resistor, a filament lamp and a diode.
Equipment
- Equipment Variable power supply or a cell with a variable resistor, ammeter, voltmeter, the test component, switch and leads.
Circuit layout
- Circuit layout Component and ammeter in series; voltmeter in parallel across the component only.
- A variable resistor lets the p.d. be changed in small steps.
Section 2 of 7Method
1
Build the circuit with the fixed resistor as the test component and check the meters are connected correctly.
2
Set the p.d. to a low value, close the switch, and read the current and the p.d.
3
Increase the p.d. in equal steps, recording the ammeter and voltmeter reading each time, and open the switch between readings to avoid heating.
4
Reverse the connections to the cell (or swap the leads) and repeat to obtain negative values.
5
Replace the resistor with a filament lamp, then a diode, and repeat. For the diode, place a protective resistor in series so the current cannot become too large.
Section 3 of 7Variables and control
| Type | Variable |
|---|---|
| Independent | Potential difference across the component |
| Dependent | Current through the component |
| Control | Temperature of the component (keep the circuit switched off between readings), the same component and leads, same meters |
Section 4 of 7Expected results
1Fixed resistor
- Fixed resistor A straight line through the origin: current is directly proportional to p.d.
2Filament lamp
- Filament lamp A curve that flattens as p.d. rises, because the hot filament has greater resistance.
3Diode
- Diode Almost no current in reverse and below the forward threshold, then a sharp rise in current.
4Conclusion
- Conclusion Only the fixed resistor is ohmic; the others are non-ohmic.
Section 5 of 7Safety and improving accuracy
Evaluating the practical
Safety
- Safety A lamp or resistor can become hot, so switch off between readings and do not touch the component.
- Never exceed the voltage rating of the lamp or the current rating of the diode.
Accuracy
- Accuracy Use more readings, especially where the graph changes shape, and use digital meters with a good resolution.
Reliability
- Reliability Repeat the sets of readings, check the zero error of each meter, and compare the shape with expected results.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| I–V characteristic | The graph of current against potential difference for a component. |
| Variable resistor | A resistor whose value can be adjusted to alter the current and p.d. |
| Ammeter | A meter connected in series that measures current in amperes. |
| Voltmeter | A meter connected in parallel that measures potential difference in volts. |
| Protective resistor | A fixed resistor in series that limits the current to a safe value. |
Section 7 of 7Exam technique
1
Connecting the voltmeter across the whole circuit rather than just across the component being tested.
2
Leaving the circuit switched on between readings so the filament heats and the resistor warms, changing resistance.
3
Omitting a protective resistor when testing a diode, which risks damaging it.
4
Drawing a straight line of best fit through the filament lamp or diode results instead of a smooth curve.