Testing Series & Parallel Circuits (OCR GCSE Physics A (Gateway))

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: