Resistors and I–V characteristics
Reading I–V graphs
An is a graph of (up the side) against (along the bottom). Negative values show what happens when you connect the component the other way round.
The steeper the line, the more current flows for each volt, so the lower the resistance.
(a resistor at constant temperature): a straight line through the origin. The current is to the pd, so the resistance stays constant.
Key terms
Filament lamps and diodes
the line goes through the origin but curves, getting flatter as the pd rises, in both directions. As the current heats the filament, its ions vibrate more, so electrons collide with them more often. The as the temperature rises.
a diode lets current flow in one direction only (forward). In the reverse direction it has a very high resistance, so the graph is flat along zero. In the forward direction, the current stays near zero for a small pd, then rises steeply.
An behaves like a diode and gives out light when current flows forward.
Key terms
LDRs and thermistors
its resistance decreases as light intensity increases. In the dark its resistance is high. LDRs are used in night lights and outdoor lights that switch on automatically when it gets dark.
its resistance . Thermistors are used in thermostats, which switch heating on when a room gets cold and off when it warms up, and in fire alarms.
Remember
For both: more light or more heat means less resistance.
Key terms
Required practical 4: I–V characteristics
The circuit, testing a filament lamp:
- Connect the component in series with a , an and a power supply. Connect a across the component.
- Adjust the variable resistor to change the pd. Record the current and pd each time.
- Swap the connections to the component to get negative values, and repeat.
- Do this for a resistor, a filament lamp and a diode (with a protecting resistor in series).
- Plot current against pd for each.
- the pd across the component.
- the current.
- the temperature of the resistor.
Hazard: the lamp gets hot, so don't touch it.
Improving accuracy: switch off between readings so the resistor stays cool, and repeat each reading.