Section 1 of 6Components you must know
Filament lamp
- Filament lamp Resistance rises as the filament gets hotter, so the current-voltage graph curves.
Diode
- Diode Has very high resistance in one direction and very low resistance in the other, so current flows one way only.
Thermistor (NTC)
- Thermistor (NTC) Resistance falls as the temperature rises.
LDR
- LDR Resistance falls as light intensity rises.
Section 2 of 6Circuit for a lamp or diode
1
Connect the component in series with a low-voltage power supply, an ammeter and a variable resistor.
2
Connect a voltmeter in parallel across the component only.
3
Use the variable resistor to set a chosen current (the independent variable).
4
Read the potential difference on the voltmeter (the dependent variable).
5
Calculate the resistance each time using R = V ÷ I, where R is in ohms (Ω), V in volts (V) and I in amperes (A).
6
Repeat across a range of currents, and for a diode also reverse the connections.
Section 3 of 6Circuit for an LDR or thermistor
Same circuit, changing the environment
LDR method
- LDR method Start in a dark room with the lamp off and record the ammeter and voltmeter readings.
- Raise the lamp brightness in steps using a dimmer and record both readings each time.
Thermistor method
- Thermistor method Start with the heater off and record both readings.
- Raise the heater temperature in steps and record both readings each time.
Good practice
- Good practice Keep the lamp or heater close to, but not touching, the component.
- Wait a few seconds after each change so the component can respond before reading.
Processing
- Processing Work out R = V ÷ I for each setting and plot resistance against light level or temperature.
Section 4 of 6Circuit diagram checklist
| Feature | Correct arrangement |
|---|---|
| Ammeter | In series with the component |
| Voltmeter | In parallel across the component |
| Symbols | Correct circuit symbol for each component |
| Varying current | Include a variable resistor |
| Supply | Low voltage, below about 15 V, so a large current does not change the component's resistance by heating |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Resistance | A measure of how much a component opposes current; R = V ÷ I, in ohms. |
| Thermistor | A component whose resistance changes with temperature. |
| LDR | Light-dependent resistor: resistance decreases as light intensity increases. |
| Diode | A component that allows current in one direction only. |
| Variable resistor | A resistor whose resistance can be changed to alter the current. |
Section 6 of 6Exam technique
1
Connecting the voltmeter in series - it must be in parallel across the component.
2
Saying a thermistor's resistance increases with temperature - for the type on the specification it decreases.
3
Reading the meters immediately after changing the conditions, before the component has responded.
4
Using a high voltage supply, which heats the component and alters its resistance.