Potential dividers, thermistors and LDRs
Watch the whole lecture free
Make a free account to watch every lecture to the end: 3 worked examples, 4 practice questions, 2 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.
That was the teaching part. The rest of this lecture is free with an account: 3 worked examples, 4 practice questions, 2 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.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 4 practice questions, 2 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
- Circuits that sense
- The potential divider
- The rule
Read the transcript of the teaching part
Last time
Before we start: last time, in Circuits, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Circuits that sense
A street light comes on at dusk. A fire alarm sounds when a room gets too hot. Neither has a person watching. A small circuit senses the light or the temperature and switches something on. It is built from a potential divider with a special resistor.
The potential divider
A potential divider is two resistors in series across a supply. The supply voltage is shared between them. The bigger resistor gets the bigger share of the voltage. The output voltage is taken across one of the two resistors.
The rule
In series, the current is the same through both. So the p.d.s are in the same ratio as the resistances. The output across the second resistor equals V in, times R two over R one plus R two. If the two resistances are equal, the output is half the supply.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Potential dividers and sensing circuits · Potential dividers.
The whole chapter: Electric Circuits, Electric Circuits.
Chapter 4.3: Electric circuits
- Circuit diagrams, series and parallel circuits 6 min
- Potential dividers, thermistors and LDRs 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.