Draw and interpret circuit diagrams containing cells, batteries, power supplies, generators, potential dividers, switches, resistors (fixed and variable), heaters, thermistors (NTC only), light-dependent resistors (LDRs), lamps, motors, bells, ammete

4.3.1 Circuit diagrams and circuit components

Learning objective

Draw, label and interpret circuit diagrams that may contain any of the components listed in the Cambridge IGCSE Physics (0625) syllabus. Recognise the standard IEC/ISO symbols, understand the conventional direction of current, and know how each component behaves in a circuit (series/parallel, voltage‑division, current‑division, etc.).

1. General conventions for circuit diagrams

  • All components are represented by the standard IEC/ISO symbols (see the tables below). Symbols are drawn to scale.

  • Lines represent ideal conductors. A solid dot (·) at a junction means the conductors are electrically connected; an open circle (○) means they are NOT connected.

  • Current direction is shown by an arrow; the conventional direction is from positive to negative.

  • Polarity of cells, batteries, power supplies and generators is shown by a long (+) line and a short (–) line.

  • Switches are drawn open unless the circuit is intended to be closed.

  • Component values (e.g. R = 10 Ω, V = 5 V) are written next to the symbol.

  • When a component has a non‑linear I‑V characteristic (e.g. thermistor, LDR, diode) the symbol is often accompanied by a small label (NTC, LDR, →|) to remind the student of its behaviour.

2. Symbol key – Core components

ComponentSymbolFunction (short description)Behaviour in a circuit
Cell (single)Provides a fixed emf (≈ 1 V for a dry cell).Acts as an ideal voltage source; internal resistance is ignored at IGCSE level. The same current flows through all elements that are in series with the cell.
Battery (multiple cells)Higher emf than a single cell (sum of the individual cells).Same as a cell but with a larger voltage. Polarity is indicated by the outermost long line.
Adjustable DC power supplyProvides a controllable DC voltage.Voltage can be set within a specified range. Internal resistance is negligible; behaves like an ideal source for the chosen setting.
DC generatorGConverts mechanical energy to electrical energy.Emf ∝ speed of rotation. Polarity shown by the brush symbols; the output behaves like a source whose voltage changes with speed.
Potential‑divider (two resistors in series)VoutProvides a fraction of the source voltage.Current through both resistors is the same; the output voltage is taken from the junction:
\(V{\text{out}} = V{\text{s}} \dfrac{R2}{R1+R_2}\). Used as a voltage reference or sensor bias.
Switch (normally open)Opens or closes a circuit on command.Open: infinite resistance → no current. Closed: negligible resistance → current limited only by the rest of the circuit.
Fixed resistorLimits current; converts electrical energy to heat.Linear I‑V relationship: \(V = IR\). In series the same current flows; in parallel the voltage across each branch is the same.
Variable resistor (potentiometer)Adjustable resistance, often used as a sensor or volume control.Resistance can be varied continuously; the same linear law \(V = IR\) applies at any setting. When used as a voltage divider the wiper provides a variable output voltage.
Heater (resistive heating element)HConverts electrical energy into heat (e.g., kettle).Same \(V = IR\) law; power dissipated as heat \(P = I^2R = V^2/R\).
Thermistor (NTC)NTCResistance decreases with temperature.Non‑linear: \(R \approx R_0 e^{-\beta/T}\). Often used in temperature‑sensor circuits; placed in a potential divider to obtain a voltage that varies with temperature.
Light‑dependent resistor (LDR)LDRResistance decreases as illumination increases.Approximate relation \(R \approx k/I_{\text{light}}\). Used in light‑controlled switches or dimmers; also placed in a potential divider.
Lamp (incandescent)Converts electrical energy into light (and heat).Filament resistance rises with temperature; the I‑V curve is slightly non‑linear. In a simple circuit it can be treated as a resistor.
Motor (DC)Converts electrical energy to mechanical rotation.Current flows from + to –; a back‑EMF opposes the applied voltage and limits the speed. The effective resistance is therefore \(R{\text{eff}} = (V - E{\text{b}})/I\).
Bell (electromagnetic)Produces a sound when current flows through a coil.Current creates a magnetic field that moves a striker; when the circuit is opened the striker returns by a spring, producing the characteristic “ding”.
Ammeter (analogue)Measures current (must be connected in series).Low internal resistance so it does not appreciably change the current being measured. Reading is proportional to the current flowing through it.
Voltmeter (analogue)Measures potential difference (connected in parallel).Very high internal resistance; draws only a tiny current, so the circuit voltage is essentially unchanged.
Magnetising coil (inductor)Produces a magnetic field when current flows; stores energy.Opposes changes in current: \(V = L \, \dfrac{dI}{dt}\). In a DC steady‑state the coil behaves like a short circuit.
Transformer (ideal)N₁N₂Changes voltage and current levels between primary and secondary.Ideal relations: \(\displaystyle \frac{V2}{V1}= \frac{N2}{N1}\) and \(\displaystyle \frac{I2}{I1}= \frac{N1}{N2}\). No losses assumed.
FuseFProtects a circuit by melting if current exceeds a rating.Acts as a low‑resistance conductor until \(I > I_{\text{rated}}\); then it opens the circuit.
Relay (electromagnetic switch)Uses a coil to open or close a separate set of contacts.When current flows through the coil a magnetic field pulls the contacts together (or apart). The contacts can switch much larger currents than the coil itself.

3. Supplementary components (extended‑syllabus)

ComponentSymbolFunctionBehaviour
Diode (rectifier)Allows current in one direction only.Forward‑biased: low resistance (~0 Ω). Reverse‑biased: very high resistance (≈ ∞). Used for rectification and protection.
LED (light‑emitting diode)Emits visible light when forward‑biased.Same polarity rules as a diode, but forward voltage is typically 1.8–3.3 V. Current must be limited with a resistor.
Transformer (core‑type, non‑ideal)N₁N₂Same function as the ideal transformer but includes losses.Real transformers have winding resistance and core losses; efficiency is usually 90‑95 % for classroom‑size devices.
Hall‑effect sensor (magnetic switch)HDetects the presence of a magnetic field.Produces a voltage proportional to the magnetic flux density; often used as a non‑contact switch.

4. Circuit analysis basics (formula summary)

Series circuits

  • Total resistance: \(R{\text{S}} = R1 + R2 + \dots + Rn\)

  • Current is the same through every component: \(I{\text{S}} = I1 = I_2 = \dots\)

  • Voltage divides: \(Vi = I{\text{S}} R_i\)

Parallel circuits

  • Reciprocal total resistance: \(\displaystyle \frac{1}{R{\text{P}}}= \frac{1}{R1}+ \frac{1}{R_2}+ \dots\)

  • Voltage across each branch is the same: \(V{\text{P}} = V1 = V_2 = \dots\)

  • Current divides inversely with resistance: \(Ii = \dfrac{V{\text{P}}}{R_i}\)

Potential divider

  • \(V{\text{out}} = V{\text{s}} \dfrac{R2}{R1+R2}\) (output taken across \(R2\)).

Power

  • \(P = VI = I^2R = \dfrac{V^2}{R}\)

Ohm’s law (linear components)

  • \(V = IR\)

5. Practical skills – building and testing circuits

  • Planning: Sketch the circuit on paper first, label all components, and decide on the order of connections (usually from the power source outwards).

  • Breadboard/strip‑board use: Insert components into the same row (or column) for a common connection; check continuity with a multimeter before applying power.

  • Measuring:

    • Connect an ammeter in series with the component whose current you wish to read.

    • Connect a voltmeter in parallel across the component whose voltage you wish to read.

    • Never connect a voltmeter in series – it would act as a large resistance and stop the current.

  • Verification experiment (Ohm’s law):

    1. Set up a simple series circuit: a DC power supply, a known resistor (R), an ammeter and a voltmeter.

    2. Vary the supply voltage (e.g., 2 V, 4 V, 6 V) and record I and V each time.

    3. Plot V (vertical) against I (horizontal). The graph should be a straight line through the origin; the gradient gives R.

6. Safety checklist (mandatory for all practical work)

  • Always switch off the power source before making or breaking connections.

  • Check that all leads are in good condition – no exposed wires or cracked insulation.

  • Use fuses or a current‑limiting device that is rated ≤ 1.5 × the expected circuit current.

  • Never touch live conductors; use insulated tools where possible.

  • Keep the work area dry and free of conductive debris.

  • When using a motor or bell, ensure moving parts are clear of fingers and clothing.

  • After the experiment, switch off the power, disconnect the circuit, and store components safely.

7. Example: Light‑controlled alarm circuit

The diagram below shows a simple circuit that uses an LDR to switch a bell when the room becomes dark.

5 V

LDR

R

How it works: In bright light the LDR resistance is low, so most of the supply voltage drops across the fixed resistor R and the bell receives insufficient voltage to sound. In darkness the LDR resistance becomes very high; the voltage across the bell rises, causing it to ring. This illustrates a voltage‑divider controlling a load.

8. Quick reference – symbols at a glance

SymbolComponent
Cell
Battery
Adjustable power supply
GDC generator
Switch (normally open)
Fixed resistor
Variable resistor (potentiometer)
NTCThermistor (NTC)
LDRLDR
Lamp (incandescent)
DC motor
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