Circuit Components, Diagrams and Sensors
Drawing and reading circuit diagrams is a skill in its own right. The symbols must be standard, and the meters must go in the right places — both are marked directly.
1. The standard symbols
You need to recognise and draw:
| Component | Symbol description |
|---|---|
| Cell | one long line, one short line |
| Battery | two or more cells in a row |
| Switch | a break in the line with a hinged arm |
| Lamp | a circle with a cross |
| Fixed resistor | a plain rectangle |
| Variable resistor | a rectangle with a diagonal arrow through it |
| Ammeter | circle containing A |
| Voltmeter | circle containing V |
| Thermistor | rectangle with a line bent at right angles |
| LDR | rectangle in a circle with two arrows pointing in |
| Diode | a triangle pointing at a line |
| LED | a diode with two arrows pointing out |
| Fuse | a rectangle with a line through it |
A fixed resistor is a plain rectangle — never a zigzag. The zigzag is an older or American convention and can lose the mark.
Draw components with a ruler, connected by straight lines, with no gaps at the corners.
Learn the sensor symbols apart from each other. Confusing a thermistor with a potential divider was recorded, and the LDR and thermistor symbols are easily muddled — the LDR has arrows (light going in); the thermistor does not.
2. Ammeters and voltmeters
Ammeters go in SERIES with the component whose current you want. Voltmeters go in PARALLEL across the component.
Why:
An ammeter measures the current through something, so the current must flow through the meter — hence in series. It has very low resistance so it doesn’t change the circuit. A voltmeter measures the potential difference across something — the difference between two points — so it must be connected to those two points, in parallel. It has very high resistance so almost no current flows through it.
An ammeter cannot be placed anywhere in the circuit. A recorded error assumed it could. In a series circuit the current is the same everywhere so its position doesn’t matter — but in a circuit with parallel branches, an ammeter reads only the current in the branch it sits in. Position it deliberately.
Never connect an ammeter in parallel across a component — its low resistance would create a short circuit.
3. Variable resistors
A variable resistor (rheostat) changes the resistance in a circuit, and so controls the current.
Increasing its resistance decreases the current everywhere in that series loop, dimming a lamp or slowing a motor.
Used in dimmer switches, volume controls, and in experiments to vary the current so several pairs of readings can be taken.
4. Sensors
Thermistor
A thermistor’s resistance DECREASES as the temperature INCREASES.
This is the opposite of a metal wire, whose resistance rises with temperature. Keep the two apart.
Used in thermostats, fire alarms and temperature-sensing circuits.
Light-dependent resistor (LDR)
An LDR’s resistance DECREASES as the light intensity INCREASES.
So it has high resistance in the dark and low resistance in bright light.
Used in automatic street lights, burglar alarms and camera light meters.
Both sensors work the same way: more of the stimulus, less resistance. That single sentence covers both, and both are examined through potential divider circuits.
5. Potential dividers
A potential divider is two resistors in series across a supply, used to produce a fraction of the supply voltage.
The supply voltage splits between the two resistors in proportion to their resistances:
V₁ / V₂ = R₁ / R₂ V_out = V_supply × R₂ / (R₁ + R₂)
Example: a 12 V supply across a 2 kΩ and a 4 kΩ resistor. The output across the 4 kΩ:
- V_out = 12 × 4000/(2000 + 4000) = 12 × 2/3 = 8 V
Convert kilo-ohms to ohms (× 1000) — or keep both in kΩ consistently. Mishandling this conversion was a recorded error.
Label which resistor is R₁ and which is R₂ before substituting. Confusion over which was which was recorded, and it inverts the answer.
Sensor circuits — how they actually work:
Replace one resistor with a thermistor or LDR. As the temperature or light changes, that component’s resistance changes, so the share of the voltage changes, and the output voltage rises or falls — which can then switch something on.
Worked reasoning for a street light: as it gets dark, the LDR’s resistance increases, so it takes a larger share of the supply voltage, and the voltage across it rises — triggering the lamp.
Trace the chain: stimulus → resistance → share of voltage → output. That is the full-mark answer, and skipping the middle steps loses marks.
6. Reading circuit diagrams
- Identify the supply and trace the complete path
- Decide which components are in series and which in parallel
- Note where the meters are and what each therefore measures
- Apply the series and parallel rules
Later parts of a question usually build on earlier ones. A recorded error missed that a potential difference found in one part was needed in the next — check whether you have already calculated something you need.
7. Mistakes that cost marks
Ammeter in parallel, or voltmeter in series.
Assuming an ammeter’s position never matters.
Drawing a resistor as a zigzag.
Confusing the thermistor and LDR symbols.
Saying a thermistor’s resistance rises with temperature.
Not converting kΩ to Ω.
Mixing up R₁ and R₂ in a potential divider.
Giving only the start and end of a sensor circuit explanation.
Freehand, gappy circuit diagrams.
Frequently asked questions
Where does an ammeter go? In series with the component — it measures the current through it.
Where does a voltmeter go? In parallel across the component.
Why does an ammeter have low resistance? So it does not change the current it is measuring.
What does a variable resistor do? Changes the resistance, and so controls the current.
What happens to a thermistor’s resistance when it gets hotter? It decreases.
What happens to an LDR’s resistance in the dark? It increases.
What is a potential divider? Two resistors in series that split the supply voltage in proportion to their resistances.
How do I calculate the output voltage? V_out = V × R₂/(R₁ + R₂).
How does an automatic street light work? As it darkens, the LDR’s resistance rises, so its share of the voltage rises, switching the lamp on.
How is a resistor drawn? As a plain rectangle.
Quick revision checklist
- I can draw and recognise all the standard symbols
- I draw resistors as rectangles, with a ruler
- I know ammeters go in series and voltmeters in parallel
- I can explain why, using their resistances
- I think about where an ammeter sits in a branched circuit
- I know what a variable resistor does
- I know a thermistor’s resistance falls as temperature rises
- I know an LDR’s resistance falls as light rises
- I can tell their symbols apart
- I can calculate a potential divider output
- I convert kΩ to Ω
- I can explain a sensor circuit as a full chain
- I check whether earlier parts of a question feed into later ones
These notes cover circuit components, diagrams and sensors in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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