Resistors: Fixed, Variable, LDR, Thermistor and Diodes — AQA GCSE Physics
Different circuit components have different I–V characteristics, which can be investigated experimentally.
I–V characteristics
An I–V graph shows how the current through a component changes with the potential difference across it.
- Fixed resistor (ohmic, constant temperature): current is directly proportional to p.d. — a straight line through the origin (constant resistance).
- Filament lamp: as current increases, the filament gets hotter, so its resistance increases. The graph is an S-shaped curve (it gets less steep).
- Diode: allows current to flow in one direction only. It has very high resistance in the reverse direction, so the graph shows current only on one side.
Special resistors
- Thermistor — its resistance decreases as temperature increases. Used in temperature sensors and thermostats.
- Light-dependent resistor (LDR) — its resistance decreases as light intensity increases. Used in light sensors and automatic lighting.
Required practical
Investigate the I–V characteristics of a resistor, a filament lamp and a diode, using a variable resistor to change the current and recording matching V and I values.
Exam tips
- Fixed resistor → straight line (constant resistance); filament lamp → curve (resistance rises as it heats); diode → current one way only.
- Thermistor: resistance down as temperature up.
- LDR: resistance down as light up.
- Know the uses: thermistor (temperature), LDR (light).