Potential dividers with thermistors and LDRs
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- A fire alarm that knows the temperature
- The potential divider
- The LDR in a divider
Read the transcript of the teaching part
Last time
Before we start: last time, in Series, parallel and bridge networks, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A fire alarm that knows the temperature
A fire alarm needs a signal that changes with temperature. Put a thermistor in series with a fixed resistor across a battery. As the thermistor's resistance changes, the share of the battery voltage across each part changes, and that voltage is the signal.
The potential divider
Two resistors in series across a battery divide its voltage. The current is the same, so the p.d. across each is proportional to its resistance. The p.d. across resistor R two is the e.m.f. times R two over the sum of the resistances. A larger resistance takes a larger share of the p.d. With twelve volts across two thousand and one thousand ohms, the shares are eight volts and four volts.
The LDR in a divider
A nine volt battery is in series with an LDR and an eighteen hundred ohm fixed resistor. A voltmeter across the fixed resistor reads five point four volts. The current is five point four over eighteen hundred: three milliamperes. The LDR has nine minus five point four, three point six volts. Its resistance is three point six over three milliamperes: twelve hundred ohms. If the light increases, the LDR resistance decreases, the current increases, and the voltmeter reading increases.
Chapter 10.4: Potential dividers
- Potential dividers with thermistors and LDRs 7 min
- LDR circuits and parallel networks 7 min
- Potential divider design 5 min
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