Investigating Resistance in Thermistors and LDRs

Section 1 of 7The circuit

Components needed

• Components needed A low-voltage power supply, the thermistor or LDR, an ammeter, a voltmeter and connecting leads.
• A heat source or a lamp that can be varied, for example a beaker of water at different temperatures or a dimmable lamp.

Meter positions

• Meter positions The ammeter goes in series with the component.
• The voltmeter goes in parallel directly across the component.

Keep the voltage low

• Keep the voltage low Use a supply of a few volts;
  • a large current would heat the component and change its resistance, spoiling the result.

Section 2 of 7Method for the thermistor

1

Set up the circuit and place the thermistor in a boiling tube sitting in a beaker of water, with a thermometer beside it.

2

Start with cold water, let the thermistor settle for about a minute and record the temperature, the current and the p.d.

3

Warm the water in steps, for example every 10 °C, stirring so the temperature is even.

4

At each step wait for the reading to stabilise, then record the thermometer, ammeter and voltmeter values.

5

Calculate the resistance at each temperature using R = V ÷ I.

Section 3 of 7Method for the LDR

1

Set up the same circuit with an LDR and place a lamp a measured distance from it, in a dim room.

2

Record the current and p.d. with the lamp off to find the resistance in the dark.

3

Switch on and move the lamp to several different distances, or change the dimmer setting, recording the current and p.d. each time.

4

Calculate R = V ÷ I for each light level and plot resistance against light intensity or distance.

Section 4 of 7Variables

Investigation	Independent	Dependent	Control variables
Thermistor	Temperature of the water	Resistance (from V and I)	Supply p.d., same thermistor, time allowed to settle
LDR	Light intensity (lamp distance or brightness)	Resistance (from V and I)	Supply p.d., same LDR, background light, lamp position

Section 5 of 7Processing results and safety

Getting a reliable result

Graph

• Graph Plot resistance on the y-axis against temperature or light intensity and draw a smooth best-fit curve;
  • the line slopes downward in both cases.

Reliability

• Reliability Repeat each reading and calculate a mean;
  • wait for the reading to settle so the component matches its surroundings.

Safety

• Safety Take care with hot water and heating equipment, keep electrical connections away from water, and do not let the lamp overheat.

Section 6 of 7Key terms

Term	Meaning
Independent variable	The quantity the investigator deliberately changes.
Dependent variable	The quantity that is measured to see the effect of the change.
Control variable	A factor kept the same so that it cannot affect the result.
Resistance	Calculated from R = V ÷ I and measured in ohms (Ω).
Best-fit curve	A smooth line drawn through the trend of the plotted points.

Section 7 of 7Exam technique

1

Putting the voltmeter in series or the ammeter in parallel — the voltmeter is always across the component.

2

Taking readings immediately after changing the temperature, before the thermistor has reached the same temperature as the water.

3

Using a high supply voltage, which heats the component and changes its resistance during the experiment.

4

Joining the plotted points dot to dot instead of drawing a smooth best-fit curve.