IB Physics SL B.5.15 Variable Resistance

Practise interpreting potentiometers, thermistors, light-dependent resistors and rheostats, using their changing resistance to predict current, voltage or measurement sensitivity.

Exam points

use a variable resistor to collect V-I points and find resistance or internal resistance from a graph gradient
predict current or divider output when thermistor or LDR resistance changes with temperature or light
read a resistance-response graph to identify the range with greatest sensitivity

[Maximum number: 3]

This question is in two parts. Part 1 is about a thermistor circuit. Part 2 is about vibrations and waves.
Part 1 Thermistor circuit
The circuit shows a negative temperature coefficient (NTC) thermistor X and a $100 \mathrm{k} \Omega$ fixed resistor R connected across a battery.

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

The battery has an electromotive force (emf) of 12.0 V and negligible internal resistance.

(a)

The graph below shows the variation with temperature T of the resistance $R_{\mathrm{x}}$ of the thermistor.

[ 3 ]

(i)

State the range of temperatures for which the change in the resistance of the thermistor is most sensitive to changes in temperature.

[ 1 ]

50 to (up to) $-30^{\circ} \mathrm{C} /$ at low temperatures;

(ii)

State and explain the effect of a decrease in temperature on the ratio

\frac{\text { voltage across } \mathrm{X}}{\text { voltage across } \mathrm{R}}

Part 2 Vibrations and waves

The cone and dust cap D of a loudspeaker L vibrates with a frequency of 1.25 kHz with simple harmonic motion (SHM).

Figure for Question (ii) — IB Physics SL

[ 2 ]

as the temperature decreases $R_{X}$ increases; same current through R and X so the ratio increases or $V_{X}$ increases and $V_{R}$ decreases so the ratio increases;

Part 2 Vibrations and waves

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B.5.15—Variable resistance question 1

Question (a)

Question (i)

Question (ii)