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

Published by Patrick Mutisya · 8 days ago

IGCSE Physics 0625 – Circuit Diagrams and Components

4.3.1 Circuit Diagrams and Circuit Components

Learning Objective

Students must be able to:

  • Draw accurate circuit diagrams that include cells, batteries, power supplies, generators, potential dividers, switches, resistors (fixed and variable), heaters, thermistors (NTC only), light‑dependent resistors (LDRs), lamps, motors, bells, ammeters, voltmeters, magnetising coils, transformers, fuses and relays.

  • Interpret a given diagram to identify the function of each component and predict the overall behaviour of the circuit.

Standard Symbols

The following table lists the conventional symbols used in IGCSE circuit diagrams and a brief description of each component’s electrical behaviour.

ComponentSymbol (suggested diagram)Typical Behaviour in a Circuit
Cell (single)
Suggested diagram: short line with a longer line parallel to it
Provides a constant emf; polarity indicated by long (+) and short (–) lines.
Battery (multiple cells)
Suggested diagram: series of cell symbols
Higher emf than a single cell; terminals marked + and –.
Power supply / Generator
Suggested diagram: circle with + and – signs
Provides a controllable emf; can be AC or DC.
Potential divider (two resistors in series)
Suggested diagram: two resistors in series with a tap
Divides the total voltage proportionally to the resistances.
Switch (single‑pole, single‑throw)
Suggested diagram: open/closed gap in a line
Opens (breaks) or closes (completes) the circuit.
Fixed resistor
Suggested diagram: zig‑zag line
Opposes current flow; obeys \$V = IR\$.
Variable resistor (potentiometer)
Suggested diagram: zig‑zag line with an arrow across
Resistance can be adjusted; used for controlling current or voltage.
Heater (resistive element)
Suggested diagram: rectangle with wavy line
Converts electrical energy to heat; \$P = I^{2}R\$.
Thermistor (NTC)
Suggested diagram: resistor symbol with a “T” inside
Resistance decreases with temperature; \$R = R_{0}e^{-\beta T}\$ (approx.).
Light‑dependent resistor (LDR)
Suggested diagram: resistor symbol with a photo‑cell symbol
Resistance falls as illumination increases.
Lamp (incandescent)
Suggested diagram: circle with a filament inside
Acts as a resistor that brightens with current; filament temperature rises with \$I^{2}R\$ heating.
Motor (DC)
Suggested diagram: circle with the letter “M”
Converts electrical energy to mechanical rotation; draws a back‑EMF proportional to speed.
Bell (electromagnet)
Suggested diagram: coil with a hammer symbol
Produces a sound when current flows; behaves like a coil with a moving armature.
Ammeter
Suggested diagram: a circle with “A” inside
Connected in series; low internal resistance; measures current \$I\$.
Voltmeter
Suggested diagram: a circle with “V” inside
Connected in parallel; high internal resistance; measures potential difference \$V\$.
Magnetising coil (inductor)
Suggested diagram: series of loops
Opposes changes in current; stores energy in magnetic field \$U = \frac{1}{2}LI^{2}\$.
Transformer (ideal)
Suggested diagram: two coils with a core
Steps voltage up or down; \$ \frac{V{p}}{V{s}} = \frac{N{p}}{N{s}}\$, \$ \frac{I{s}}{I{p}} = \frac{N{p}}{N{s}}\$.
Fuse
Suggested diagram: a rectangle with a line through it
Protects circuits; melts when current exceeds a rated value.
Relay
Suggested diagram: coil with a set of contacts
Electrically operated switch; coil energises to move contacts.

Series and Parallel Connections

Understanding how components combine is essential for interpreting diagrams.

  1. Series connection: Same current flows through each component.

    • Total resistance: \$R{\text{total}} = R{1}+R{2}+ \dots +R{n}\$.

    • Voltage divides: \$V{k}=I R{k}\$.

  2. Parallel connection: Same voltage across each branch.

    • Reciprocal total resistance: \$\displaystyle \frac{1}{R{\text{total}}}= \frac{1}{R{1}}+\frac{1}{R{2}}+\dots+\frac{1}{R{n}}\$.

    • Current divides: \$I{k}= \frac{V}{R{k}}\$.

Behaviour of Selected Components in Circuits

The following points summarise how the listed components respond to changes in voltage, current, temperature or illumination.

ComponentEffect of Increasing \cdot oltageEffect of Temperature / Light (where applicable)
Fixed resistorCurrent increases linearly (\$I = V/R\$).Negligible change (unless temperature coefficient is large).
Variable resistorCurrent depends on set resistance; can be limited.Resistance set manually; no intrinsic temperature effect.
Thermistor (NTC)Current rises faster as \$R\$ falls with heating.Resistance decreases exponentially with temperature.
LDRCurrent rises as resistance falls under illumination.Resistance drops when light intensity increases.
Lamp (incandescent)Brightness and filament temperature increase with \$I^{2}R\$.Filament resistance rises with temperature, giving a non‑linear \$V\$–\$I\$ curve.
MotorSpeed increases with voltage, but back‑EMF reduces net voltage across the armature.Friction and armature resistance cause heating; excessive current may damage windings.
Transformer (ideal)Primary voltage determines secondary voltage via turn ratio.Core losses increase with temperature; real transformers are not ideal.

Worked Example

Consider the circuit shown below (suggested diagram). It contains a 12 V battery, a switch, a fixed resistor \$R{1}=4\;\Omega\$, a variable resistor \$R{2}\$, an ammeter \$A\$, and a voltmeter \$V\$ across \$R_{2}\$.

Suggested diagram: 12 V battery → switch → series \$R{1}\$ → series \$R{2}\$ → back to battery; ammeter in series with \$R{1}\$; voltmeter connected across \$R{2}\$.

When the switch is closed and \$R_{2}\$ is set to \$6\;\Omega\$, determine:

  1. The total current \$I\$ flowing through the circuit.

  2. The reading on the voltmeter \$V\$.

  3. The power dissipated in each resistor.

Solution

Series total resistance:

\$R{\text{total}} = R{1}+R_{2}=4\;\Omega+6\;\Omega=10\;\Omega\$

Current from Ohm’s law:

\$I = \frac{V{\text{battery}}}{R{\text{total}}}= \frac{12\;\text{V}}{10\;\Omega}=1.2\;\text{A}\$

Voltage across \$R_{2}\$ (voltmeter reading):

\$V = I R_{2}=1.2\;\text{A}\times6\;\Omega =7.2\;\text{V}\$

Power in each resistor:

\$P{1}=I^{2}R{1}= (1.2)^{2}\times4=5.76\;\text{W}\$

\$P{2}=I^{2}R{2}= (1.2)^{2}\times6=8.64\;\text{W}\$

Common Mistakes to Avoid

  • Placing ammeters in parallel – they must be in series to measure current.

  • Connecting voltmeters across a component that already has a high‑resistance path – the voltmeter’s internal resistance must be much larger than the component’s resistance.

  • Forgetting polarity on cells, batteries and power supplies; reversing them changes the direction of current.

  • Assuming a thermistor behaves like a fixed resistor – its resistance varies strongly with temperature.

  • Neglecting the back‑EMF of a motor when calculating current; the effective voltage is \$V{\text{applied}}-E{\text{back}}\$.

Quick Reference – Symbol Cheat‑Sheet

When drawing diagrams, use the symbols listed in the table above. Keep the following layout conventions in mind:

  1. Power sources are drawn on the left or top of the diagram.

  2. Current direction is indicated by an arrow on the ammeter or by a labelled arrow on the diagram.

  3. Switches are drawn open when the circuit is to be shown as “off”.

  4. Connect measuring instruments (ammeter, voltmeter) with their terminals clearly marked.

Summary

Mastering circuit diagrams involves recognising symbols, understanding how each component behaves under different conditions, and applying series‑parallel analysis to predict currents, voltages and power. Practice by sketching circuits, labeling each element, and checking calculations against the expected behaviour of the components.