Edexcel A-Level Physics Variable Potential Dividers

Practise Edexcel A-Level Physics variable-resistance potential dividers using thermistors, changing supply voltage and switching thresholds.

Exam points

calculate thermistor resistance and temperature from potential-divider voltmeter readings
assess supply-voltage changes by linking current, thermistor temperature and voltage ratio
deduce switching behaviour from thermistor voltage at a specified temperature threshold

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Resistors are included in circuits that are used as potential dividers.

The circuit shown below includes a light dependent resistor (LDR). The battery has no internal resistance.

Figure for Question 2.4.76 - Variable-resistance potential dividers question 1 — Edexcel A-Level Physics AS

In normal daylight, this particular type of LDR has a resistance of $670\,\Omega$ .

Calculate the power dissipated in the fixed resistor in normal daylight.

resistance of fixed resistor $=8.00\,\mathrm{k}\Omega$

ci

Use of V=I R to determine circuit current

Use of $P=I^{2} R$ or $P=V^{2} / R$ or P=V I

$P=1.5 \times 10^{-2} \mathrm{~W}$

Or

Ratio of resistances used to calculate p.d. across R

Use of $P=V^{2} / R$ $P=1.5 \times 10^{-2} \mathrm{~W}$

Example of calculation

$\mathrm{I}=12.0 \mathrm{~V} /(8000+670 \Omega)=1.38 \times 10^{-3} \mathrm{~A}$ $\mathrm{P}=\left(1.38 \times 10^{-3} \mathrm{~A}\right)^{2} \times 8000=0.015 \mathrm{~W}$

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2.4.76 - Variable-resistance potential dividers question 1