Circuit diagrams and circuit components
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Circuit Symbols
Circuit diagrams use standard symbols to represent components - you need to be able to draw and interpret each of these.
Standard circuit symbols you need to be able to draw and recognise
| Component | Symbol Description | Function |
|---|---|---|
| Cell | Long line (+) and short thick line (−) | Provides e.m.f. (single unit) |
| Battery | Two or more cells in series | Provides larger e.m.f. |
| Power supply (d.c.) | Circle with + and − terminals | Variable d.c. source |
| Generator | Circle with G inside | Produces a.c. or d.c. |
| Switch (open) | Break in line with contacts | Controls circuit on/off |
| Fixed resistor | Rectangle | Limits current (constant R) |
| Variable resistor | Rectangle with arrow through | Adjustable resistance |
| Potential divider | Rectangle with arrow touching | Splits voltage |
| Lamp | Circle with cross inside | Converts electrical to light energy |
| Heater | Rectangle with diagonal lines | Converts electrical to heat energy |
| Motor | Circle with M inside | Converts electrical to kinetic energy |
| Bell | Semicircle shape | Produces sound |
| Ammeter | Circle with A inside | Measures current (in series) |
| Voltmeter | Circle with V inside | Measures p.d. (in parallel) |
| Fuse | Rectangle with line through | Melts to break circuit if current too high |
| Thermistor (NTC) | Rectangle with diagonal line through | Resistance decreases as temperature increases |
| LDR | Rectangle with arrows pointing at it | Resistance decreases as light intensity increases |
| Magnetising coil | Coiled line | Produces magnetic field when current flows |
| Relay | Coil with switch contacts | Electromagnetic switch |
| Transformer | Two coils with lines between | Changes a.c. voltage |
Diodes and LEDs
Extended Only
Diode
- A diode is a component that only allows a current to flow through it in one direction
- this is the direction the triangle points in the symbol
- diodes are occasionally drawn with a horizontal line running through the middle of them too
The circuit symbol for a diode
Current flows when travelling in the same direction as the diode; if the diode is flipped, it blocks the current
- Forward bias (current travelling the same way as the triangle points): current flows easily (low resistance)
- Reverse bias (diode flipped): no current flows (very high resistance)
- Diodes have a threshold voltage (about 0.6 V for silicon) before they start conducting
LED (Light-Emitting Diode)
- An LED behaves in exactly the same way as a diode, but also emits light when current flows through it
- its symbol is a diode symbol with two small arrows pointing outward, representing the light it gives off
The circuit symbol for an LED
An LED only lights up when forward biased, just like a diode only conducts when forward biased
Thermistors and Light-Dependent Resistors
Thermistor (NTC)
- A Negative Temperature Coefficient (NTC) thermistor has resistance that decreases as temperature increases
- cold → high resistance → low current
- hot → low resistance → high current
- Used in temperature sensors and thermostats
Light-Dependent Resistor (LDR)
- An LDR has resistance that decreases as light intensity increases
- dark → high resistance → low current
- bright → low resistance → high current
- Used in automatic lights and light meters
Interactive revision notes, videos and practice questions load below.