Testing Series & Parallel Circuits
- The GCSE examination will require an understanding of the variation of resistance in the following devices:
- Filament lamps
- Diodes
- Thermistors
- LDRs
- Luckily, the design and use of the circuits to investigate how resistance varies is broadly similar for each component
- It is important to recognise that:
- Voltmeters are always connected in parallel with the component they are measuring the potential difference of
- Ammeters are always connect in series with the rest of the components in the circuit
Resistance in Filament Lamps & Diodes
- In order to investigate the variation of resistance in a filament lamp or diode, the following circuits should be set up:
- The current is the independent variable
- The variable resistor is used to change the current flowing through the filament lamp / diode
- The voltage is the dependent variable
- The voltmeter is used to measure the voltage across the filament lamp / diode
- Calculate the resistance of the filament lamp / diode for each chosen value of current and measured value of voltage using the equation:
These circuits enable the variation of resistance in a filament lamp or a diode to be investigated
Resistance in LDRs & Thermistors
- In order to investigate the variation of resistance in a LDR or a thermistor, the following circuits should be set up:
These circuits enable the variation of resistance in a LDR or a thermistor to be investigated
- For the LDR circuit:
- Begin with the lamp turned off in a dark room
- Record the reading on the voltmeter and ammeter
- Slowly increase the light intensity of the lamp using the dimmer switch
- Record the reading on the voltmeter and ammeter for each increase in light intensity
- For the thermistor circuit:
- Begin with the heater turned off
- Record the reading on the voltmeter and ammeter
- Slowly increase the temperature of the heater using the dimmer switch
- Record the reading on the voltmeter and ammeter for each increase in temperature of the heater
- In both situations, make sure the lamp and heater are close, but not touching, the LDR and thermistor respectively
- Wait a few seconds before taking the voltmeter and ammeter readings to allow the LDR and thermistor to react to the change in the environment
- Calculate the resistance of the LDR or thermistor for each change in light intensity or temperature using the equation:
Examiner Tip
It's really common for examiners to test your understanding of setting up and using a circuit to measure current, voltage and resistance of a component. Generally, make sure the following are included in your circuit diagrams:
- The ammeter is connected in series to the component
- The voltmeter is connected in parallel to the component
- The component with the appropriate circuit symbol
- If you need to vary the current, include a variable resistor
- The component is connected to a power supply with a low voltage (below 15 V) otherwise too high a current in the circuit will start to affect the resistance of the component
Circuit diagram for measuring the variation of resistance of a component, using current and voltage
