Ohmic conductors
- For an ohmic conductor, such as a fixed resistor at constant temperature, the current is directly proportional to the potential difference.
- Its I–V graph is a straight line through the origin, so its resistance stays the same.
Components whose resistance changes
- Filament lamp: as the current increases, the filament gets hotter and its resistance increases, so the I–V graph curves.
- Diode: current flows in one direction only. It has a very high resistance in the reverse direction.
- Thermistor: its resistance decreases as the temperature increases. Used in thermostats.
- Light-dependent resistor (LDR): its resistance decreases as light intensity increases. Used to switch on lights when it gets dark.
Required practical: I–V characteristics
- Set up a circuit with a power supply, the component, an ammeter in series, a voltmeter in parallel and a variable resistor.
- Change the variable resistor to change the current, and record the current and pd each time.
- Reverse the connections to the component to get negative values, then plot current against pd.
Required practical: length of wire
- Connect a length of resistance wire along a metre ruler, with an ammeter and voltmeter.
- Measure the current and pd for different lengths, and calculate R = V ÷ I.
- Resistance is directly proportional to length. Keep the current low so the wire doesn't heat up.
Key terms
- Ohmic conductor
- A conductor in which current is directly proportional to pd at constant temperature.
- I–V characteristic
- A graph of current against potential difference for a component.
- Filament lamp
- A lamp whose resistance increases as it gets hotter.
- Diode
- A component that lets current flow in one direction only.
- Thermistor
- A resistor whose resistance decreases as temperature increases.
- Light-dependent resistor
- A resistor whose resistance decreases as light intensity increases.
- Directly proportional
- When one quantity doubles, the other doubles; the graph is a straight line through the origin.