GCSE Physics, Unit 2, Lesson 6 of 6

Sensors and the resistance practical

GCSE Physics, Electric circuits, 25 minutes

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LDRs and thermistors in sensing circuits, how the meter readings change, and the required practical on the resistance of a wire.

What you will learn

  • Describe how the resistance of an LDR and a thermistor changes
  • Predict how the readings in a sensing circuit change
  • Plan and analyse the resistance required practical

LDRs and thermistors

Two resistors change with their surroundings:

  • LDR (light-dependent resistor): more light, lower resistance. Dark means high resistance. Used in automatic night lights and streetlights.
  • Thermistor: hotter, lower resistance. Cold means high resistance. Used in thermostats, fire alarms and engine sensors.

Put one in series with a fixed resistor and the battery's p.d. is shared between them. Two components in series that share a supply p.d. like this make a potential divider:

V6 VLDRfixed resistor
  1. The room gets darker
  2. The LDR's resistance goes up, so the total resistance goes up
  3. Same battery, more resistance: the current goes down
  4. The fixed resistor's p.d. is I × R with R unchanged: it goes down
  5. The LDR takes the rest of the p.d.: its share goes up

Two readings, two answers: across the sensor and across the fixed resistor always change in opposite directions, because together they add up to the battery p.d.

To read a sensor graph, go up from the temperature (or light level) to the curve, then across to the resistance. If the axis is in kΩ, multiply by 1000 before you use I = V ÷ R: a thermistor of 4.0 kΩ = 4000 Ω on a 12 V supply takes 12 ÷ 4000 = 0.003 A = 3 mA.

The rest of this lesson and its 8 practice questions are locked.

In this lesson

  1. LDRs and thermistors
  2. Designing a sensing circuit
  3. Required practical: resistance of a wire
  4. Try it: Reason through a sensing circuit

8 practice questions, marked as you go; about 25 minutes in all.

Key terms

Lower

In this unit: Electric circuits

Circuit symbols, charge and current, V = IR, I-V graphs, series and parallel circuits, and sensing circuits with LDRs and thermistors.

All of Electric circuits

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