# Circuit symbols and components

Standard symbols draw circuits. A cell or battery supplies voltage. A resistor limits current. A diode lets current flow one way only. An LDR's resistance falls in light. A thermistor's falls as it gets hot.

Physics (school) · Part 06: Electric circuits · Card 8 of 8 · Last updated 2026-10-19

## Analogy

Like road signs: everyone reads them the same way.

## In practice

Thermistors work in fire alarms and thermostats. LDRs switch on street lights at dusk.

## Practice

### Flashcards

- **What happens to an LDR's resistance in bright light?** It goes down.

## Questions people ask

### What does an LDR do?

A light-dependent resistor (LDR) has a high resistance in the dark and a low resistance in bright light. Put it in a circuit with a lamp and it can switch the lamp on at dusk. It is the sensor that turns street lights on automatically.

### Why is a diode useful?

A diode lets current flow in one direction only. It blocks current in the other direction. This protects circuits that must be connected the correct way round, and it is used to turn alternating current, which changes direction, into direct current that flows in one direction.

## Related concepts

- [Ohm's law](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-4)
- [Series circuits](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-5)
- [Parallel circuits](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-6)

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