Brainlag PlusLDRs and thermistors in sensing circuits, how the meter readings change, and the required practical on the resistance of a wire.
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:
- The room gets darker
- The LDR's resistance goes up, so the total resistance goes up
- Same battery, more resistance: the current goes down
- The fixed resistor's p.d. is I × R with R unchanged: it goes down
- 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.
The rest of this lesson and its 8 practice questions are in Plus.
Screen 1 of 3, free to read.
- 8 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards