Variable Resistance Components (OCR A Level Physics)

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Variable Resistance Components

  • Variable and sensory resistors are used in potential dividers to vary the output voltage
    • This could cause an external component to switch on or off e.g. a heater switching off automatically when its surroundings are at room temperature

  • Sensory resistors used are light dependent resistors (LDRs) and thermistors

Sensory potential divider circuits, downloadable AS & A Level Physics revision notes

LDR and thermistor in a potential divider circuit with a fixed resistor R

  • The voltmeter in both circuits is measuring Vout
  • Recall that the resistance of an LDR varies with light intensity
    • The higher the light intensity, the lower the resistance and vice versa

  • An LDR circuit is often used for street and security lights
  • The resistance of a thermistor varies with temperature
    • The hotter the thermistor, the lower the resistance and vice versa

  • A thermistor circuit is used in fire alarms, ovens and digital thermometers

 

  • From Ohm’s law V = IR, the potential difference Vout from a sensory resistor in a potential divider circuit is proportional to its resistance
    • If an LDR or thermistor's resistance decreases, the potential difference through it also decreases
    • If an LDR or thermistor's resistance increases, the potential difference through it also increases

  • Since the total p.d of the components must be equal to Vin, if the p.d of the sensory resistor decreases then the p.d of the other resistor in the circuit must increase and vice versa

Worked example

A potential divider consists of a fixed resistor R and a thermistor.WE - Thermistor potential divider question image, downloadable AS & A Level Physics revision notesWhat happens to the p.d through resistor R and the thermistor when the temperature of the thermistor decreases?WE - Thermistor potential divider question table image, downloadable AS & A Level Physics revision notes

ANSWER:  DWorked example - Thermistor potential divider (2), downloadable AS & A Level Physics revision notes

  • Due to Ohm’s Law (V = IR), both the resistor and thermistor are connected in series and have the same current I
  • If resistance R increases, the potential difference across the thermistor also increases
  • In series, the potential difference is shared equally amongst the components. Their sum equals the e.m.f of the supply (Kirchhoff’s second law)
  • If the potential difference across the thermistor increases, the potential difference across the resistance R must decreases, to keep the same overall total e.m.f
  • This is row D

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.