Circuit diagrams and circuit components
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
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 |
| 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 |
| 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 |
Light-Emitting Diodes (LEDs)
- An LED is a component that only allows a current to flow through it in one direction, and emits light when current flows through it
- the triangle in its symbol points in the direction current must flow, 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 - connected the right way round for current to flow
- Forward bias (current travelling the same way as the triangle points): current flows and the LED lights up
- Reverse bias (LED flipped): no current flows, and the LED stays unlit
Series Circuits
- In a series circuit, components are connected end-to-end in a single loop
- there is only one path for current to flow
- The current is the same at every point in a series circuit
The same current flows through every component in a series circuit
P.d. in Series Circuits
Extended Only
- The total p.d. across the components in a series circuit is equal to the sum of the individual p.d.s across each component:
- V_total = V_1 + V_2 + V_3
Combined e.m.f. and Resistance in Series
- When cells are connected in series:
- same direction → e.m.f.s add up
- opposite direction → e.m.f.s subtract
- Resistances in series add up:
- R_total = R_1 + R_2 + R_3 + ...
Question: Museli connects three 1.5 V cells in series, all facing the same direction, and wants to find the total e.m.f.
AnswerTotal e.m.f. = 1.5 + 1.5 + 1.5 = 4.5 V
Worked Example: Resistors in SeriesQuestion: Muchindu connects a 4 Ω resistor and a 6 Ω resistor in series, and wants to find the total resistance.
AnswerR_total = 4 + 6 = 10 Ω
Parallel Circuits
- In a parallel circuit, components are connected across each other, forming separate branches
- The current from the source is larger than the current in each individual branch
- The combined resistance of resistors in parallel is less than the resistance of the smallest individual resistor
Current splits between branches in a parallel circuit, but the p.d. across each branch is the same
P.d. in Parallel Circuits
Extended Only
- The p.d. across an arrangement of parallel resistances is the same as the p.d. across just one branch of that arrangement
The Junction Rule
Extended Only
- The sum of the currents entering a junction is equal to the sum of the currents leaving the junction
- I_in = I_out
- this is a consequence of the conservation of charge - charge cannot build up or disappear at a junction
Current entering a junction equals current leaving it: I = I_1 + I_2
Combined Resistance in Parallel
Extended Only
- Formula:
- 1R_total = (1/R_1) + (1/R_2)
Question: Muumbe connects a 4 Ω resistor and a 12 Ω resistor in parallel, and wants to find the combined resistance.
write down the equation. (1/R) = (1/4) + (1/12) find a common denominator and add. (1/R) = (3/12) + (1/12) = (4/12) = (1/3) take the reciprocal. R_total = 3 Ω AnswerNotice that 3 Ω is less than either 4 Ω or 12 Ω, as expected for resistors in parallel
Advantages of Parallel Lighting Circuits
- Each lamp receives full mains voltage, so it operates at its correct brightness
- If one lamp fails, the others keep working, since each has its own independent path
- Lamps can be switched on and off independently of each other
- Adding more lamps doesn't reduce the brightness of the existing lamps
Interactive revision notes, videos and practice questions load below.