IB Physics HL B.5.15 Variable Resistance Question Bank

Practise IB Physics HL B.5.15 by analysing thermistor control circuits, potential dividers and light-dependent resistor responses quantitatively.

Exam points

Analyse a thermistor divider to predict switching voltage as temperature and resistance change.
Use divider ratios and slider position to calculate current or potential at a circuit node.
Explain how an LDR or thermistor changes a control signal and the connected output.

[Maximum number: 3]

This question is about an operational amplifier (op-amp).

An op-amp is used as a comparator in a circuit, as shown.

Figure for Question B.5.15—Variable resistance question 1 — IB Physics HL

The circuit is used to monitor the temperature in a greenhouse. The resistance of a thermistor decreases as its temperature increases.

(a)

Explain how the potential at the inverting input changes as the temperature decreases.

[ 2 ]

(b)

Describe the purpose of component X in the circuit.
When the potential of the non-inverting input is set at 0 V , the heater switches on at a temperature of around $10^{\circ} \mathrm{C}$ . The ideal temperature to grow plants in the greenhouse is $18^{\circ} \mathrm{C}$ or greater. At $18^{\circ} \mathrm{C}$ the resistance of the thermistor is $193 \Omega$ .

[ 1 ]

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B.5.15—Variable resistance question 1

Question (a)

Question (b)