4.5.2 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

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

• The voltmeter in both circuits is measuring V_out
• 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 V_out 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 V_in, 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

ANSWER:  D

• 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