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Chapter 22 · Circuits · Lesson 13.1

Circuit Symbols & Diagrams

A shared language for circuits

Engineers and scientists record circuits with standard symbols so any complex arrangement can be drawn — and read — clearly and precisely by anyone.

Start learning →Explore the interactive lab →
SYMBOLS · CONNECTIONS · ENERGYRead · Connect · Test

Read the relationship. Check its limits.

Lesson Overview

A circuit diagram uses standard symbols and straight lines to show how components are connected into loops.

A cell, a switch, and a lamp drawn in one rectangle of lines means: close the switch and charge can flow around one complete loop, lighting the lamp.

Course map navigation. These links do not establish lesson suitability or syllabus coverage. Follow the reviewed sections for your course and level; advanced or calculus material is not automatically required by an algebra-based route.

IGCSE ↗A-Level ↗IB DP ↗AP Physics ↗MYP ↗

Learning Objectives

  • Recognise standard component symbols
  • Trace connections and complete loops
  • Place ammeters and voltmeters correctly
  • Distinguish a junction from a crossing

Common Circuit Symbols

Select a symbol to read its meaning.

Cell. One electrochemical cell. The longer line is the positive terminal.

Symbols vary slightly between conventions. Use the version specified by your course; the component’s function and connections remain the same.

Interactive Circuit Builder

Change one input. Compare the graph and its readout before drawing a conclusion.

Supply 6.0 VAopen1: 12 ΩVOpen switch: no steady current

Equivalent load resistance12 Ω

Supply current0 A

Voltage across load 10 V

Load 1 · current / power0 A / 0 W

Guided builder: add up to four loads, then choose a series chain or parallel branches. Readings update immediately. With the switch open, the voltage across a connected load is 0 V; the supply voltage is across the switch. Lamps use fixed resistance here, so their colour indicates current, not a calibrated brightness. Use this on-screen model only; real circuit ratings and hazards are not simulated.

Guided ideal DC circuits with zero-resistance wires, an ideal supply and meters. Lamps are modelled as fixed resistances; a real filament warms up and changes resistance. Never test meter-placement faults on a real supply.

Key Formulae

Complete loop
supply → components → supplyCharge can only flow when there is an unbroken path from one terminal of the supply, through the components, and back to the other terminal.Current needs an unbroken conducting path
Ammeter placement
in seriesAn ammeter counts the charge flowing through a point, so it sits in the path of the current — in series with the component.An ideal ammeter has zero resistance
Voltmeter placement
in parallelA voltmeter compares the energy per charge between two points, so it connects across a component — in parallel with it.An ideal voltmeter has infinite resistance

Use the units shown by each control. Convert millimetres to metres for wire calculations and minutes to seconds for energy.

Key Concepts

Practice Questions

Build the physical model.

A circuit diagram is a map, not a picture. Each standard symbol stands for a component; straight lines stand for connecting wire assumed to have no resistance. A circuit only works when the loop from one terminal of the supply to the other is complete.

Sources

A cell is one unit; a battery is two or more cells. The longer line marks the positive terminal. A power supply symbol stands for mains-driven sources.

Control and protection

Switches break the loop; fuses melt to break it automatically when current is too large; variable resistors adjust current smoothly.

Measurement and output

Ammeters read current in series, voltmeters read potential difference in parallel, and lamps, motors, buzzers, and resistors transfer energy out of the circuit.

Sensing and one-way components

A thermistor is a resistor rectangle with a diagonal bar across it. The kind used at this level is NTC: its resistance falls sharply as it gets hotter. A light-dependent resistor is the same rectangle with two arrows pointing in at it, and its resistance falls as the light on it grows brighter. Both are input components — they turn a temperature or a light level into a resistance the rest of the loop can respond to. A diode is a triangle pointing into a bar: it passes conventional current only in the direction the triangle points, and blocks it the other way, which is what makes it useful for turning alternating current into direct. A light-emitting diode is the same symbol with two arrows pointing away from it.

Know what the model can tell you.

Assumptions & boundaries

Guided ideal DC circuits with zero-resistance wires, an ideal supply and meters. Lamps are modelled as fixed resistances; a real filament warms up and changes resistance. Never test meter-placement faults on a real supply.

The lines in a diagram are not drawn to scale and their length does not matter. Connecting wire is treated as resistance-free, so only the components and how the loops connect them change the physics.

How must an ammeter and a voltmeter be connected to measure a lamp's current and potential difference?

Choose an answer to test the idea.

Worked Examples

State the assumptions, convert units, calculate and check the physical meaning. Check which ideal-circuit assumptions apply.

EasyName the two meters in a test circuit and how each must be connected to a lamp.
  1. The ammeter reads current: in series, in the lamp's loop.
  2. The voltmeter reads p.d.: in parallel, across the lamp.

Ammeter in series, voltmeter in parallel

MediumDraw a circuit to measure the resistance of a lamp using a cell, a switch, an ammeter, and a voltmeter.
  1. Draw one loop: cell → switch → ammeter → lamp → back to the cell.
  2. Connect the voltmeter in parallel across the lamp only.
  3. Close the switch, read I and V, then use R = V/I.

Series loop of cell, switch, ammeter, and lamp, with the voltmeter across the lamp

HardTwo wiring faults, each made on its own. (a) The ammeter is connected in parallel with the lamp. (b) In a separate attempt, the voltmeter is connected in series in the loop. Predict the meter readings and the lamp in each case, treating the faults separately.
  1. Fault (a) alone: the ammeter's near-zero resistance short-circuits the lamp, so almost all the current bypasses it. The lamp goes out and the ammeter carries a dangerously large current, limited only by the cell and the leads.
  2. Fault (b) alone: the voltmeter's near-infinite resistance now sits in the only loop, so the current falls to almost nothing. The lamp stays dark and the voltmeter reads close to the full e.m.f., because nearly all of it is dropped across the voltmeter itself.
  3. Judge each fault on its own. If both were made at the same time the series voltmeter would still choke the loop, so the parallel ammeter would not draw a large current — the danger in (a) exists only while the rest of the circuit is wired correctly.

(a) lamp out, ammeter passes a dangerously large current; (b) current ≈ 0, lamp dark, voltmeter reads ≈ the e.m.f.

ChallengingDesign a circuit to measure a lamp's resistance at three different brightnesses using one cell, one variable resistor, an ammeter, and a voltmeter. Explain each component's role.
  1. Series loop: cell → variable resistor → ammeter → lamp → cell; voltmeter across the lamp only.
  2. The variable resistor sets the loop current, giving three operating points; the meters record (I, V) pairs at each.
  3. R = V/I at each point — expect R to rise with brightness as the filament heats.

Rheostat in series to set current; R = V/I from each (I, V) pair, rising as the lamp brightens

Interactive circuit graph

Read the topology before the values.

Use this connectivity map alongside 13.1 Circuit Symbols & Diagrams. Change the network, then select a node or branch to see what the graph preserves.

Diagram mode
Nodes
Points where electrical connections meet.
Branches
Lines or general impedances connecting two nodes.
Simplification
Component details, such as specific resistor values, are hidden to focus purely on connectivity.
Series loop circuit-topology graphEach junction leads into the next branch, so the components share one continuous route. It has 3 nodes and 3 branches. Component names and example values are shown.R110 ohm resistorR220 ohm resistorVs12 volt sourceABC
Series loopEach junction leads into the next branch, so the components share one continuous route. It has 3 nodes and 3 branches. Component names and example values are shown.

Series loop: 3 nodes and 3 branches. Select a node or branch to investigate it.

Exam diagrams for this topic2 figures to inspect and practiseQuestions, hints and marking points in one compact subsection.

See it. Read it. Work it.

These figures come from GioPhysics practice papers. Open one, decode the drawing, work the guided questions, then follow its link to the full paper question.

  1. 01

    InspectRead the figure comment.

  2. 02

    TraceFollow labels, arrows and axes.

  3. 03

    AnswerWork one part at a time.

  4. 04

    CheckReveal hints and marking points.

IGCSE

01Fig. 3.1Electric circuitsIGCSE
A 6.0 ohm resistor and a 3.0 ohm resistor connected in parallel6.0 Ω3.0 Ω

Figure comment

Fig. 3.1A circuit diagram of two resistors connected side by side between the same pair of points. A wire arrives from the left and reaches a junction, marked with a dot, where it divides into two branches. The upper branch contains a resistor labelled 6.0 ohm and the lower branch a resistor labelled 3.0 ohm. The two branches rejoin at a second junction dot on the right, from which a single wire continues away to the right.

Read the comment once, then trace every arrow, label, axis or component in the drawing before opening the questions.

4.3 Electric circuitsOpen IGCSE question · Electricity and magnetism Q3
Guided questions 5 parts

Reading cue. Both resistors lie between the same two junction dots, so they have the same p.d. across them; it is the current, not the voltage, that divides at the left-hand dot.

  1. aState State which of the two resistors in Fig. 3.1 carries the larger current, and give a reason.

    recall2 marks

    Check answer 2 marks
    1. the 3.0 Ω resistor
    2. both branches lie between the same two junctions and so have the same p.d. across them, and I = V/R, so the smaller resistance carries the larger current

  2. bDetermine The potential difference between the two junction dots in Fig. 3.1 is 12 V. Determine the current in each resistor and the current in the single wire arriving from the left.

    routine3 marks

    Check answer 3 marks
    1. current in the 6.0 Ω resistor = 12/6.0 = 2.0 A
    2. current in the 3.0 Ω resistor = 12/3.0 = 4.0 A
    3. current in the wire from the left = 2.0 + 4.0 = 6.0 A

  3. cDetermine Each resistor in Fig. 3.1 is rated at a maximum power of 12 W. Determine the largest potential difference that may safely be applied between the two junction dots, and the current drawn at that potential difference.

    demanding4 marks

    Check answer 4 marks
    1. for the 3.0 Ω resistor, V = √(PR) = √(12 × 3.0) = 6.0 V
    2. for the 6.0 Ω resistor, V = √(12 × 6.0) = 8.5 V, so the 3.0 Ω resistor is the one that limits the pair
    3. largest safe p.d. = 6.0 V
    4. current drawn = 6.0/6.0 + 6.0/3.0 = 3.0 A

  4. dExplain A student claims that connecting a third resistor between the same two junction dots in Fig. 3.1 must raise the total resistance, because there is then more resistance present. Explain why the student is wrong, and state the largest total resistance the arrangement could ever have once a third resistor is added there.

    top of the paper4 marks

    Check answer 4 marks
    1. a resistor connected between the same two dots is in parallel with the others and gives the current an extra path
    2. for the same p.d. the total current is therefore larger, and R = V/I, so the total resistance falls
    3. the pair alone gives 1/R = 1/6.0 + 1/3.0, so R = 2.0 Ω
    4. adding a third branch can only bring the total below 2.0 Ω, so 2.0 Ω is a value the arrangement approaches but never reaches

Transfer challenge

Three identical lamps, each of resistance 240 Ω, are connected in parallel across a 240 V supply. Calculate the total current drawn, and state the effect on the other two lamps if one filament breaks.

Check answer 3 marks
  1. current in each lamp = 240/240 = 1.0 A
  2. total current = 3 × 1.0 = 3.0 A
  3. the other two lamps still have the full 240 V across them and are unaffected, and the total current falls to 2.0 A
02Fig. 7.1Electrical quantities · Electric circuitsIGCSE
A 40 ohm resistor in series with a parallel pair of 30 ohm and 60 ohm resistors40 Ω30 Ω60 Ω12 V

Figure comment

Fig. 7.1Circuit diagram. From the positive terminal of the 12 V battery, drawn at the foot of the circuit with its long plate on the left, the wire runs round to a resistor labelled 40 ohm. Beyond that resistor the circuit reaches a junction and divides into two parallel branches, the upper one containing a resistor labelled 30 ohm and the lower one a resistor labelled 60 ohm. The branches rejoin at a second junction, and a single wire returns from there to the negative terminal of the battery.

Read the comment once, then trace every arrow, label, axis or component in the drawing before opening the questions.

4.2 Electrical quantities4.3 Electric circuitsOpen IGCSE question · Electricity and magnetism Q7
Guided questions 5 parts

Reading cue. The 40 Ω sits before the first junction dot, so it carries the whole battery current; only past that dot is the current shared between the 30 Ω and the 60 Ω.

  1. aDetermine The current drawn from the battery in Fig. 7.1 is 0.20 A. Determine the potential difference across the 40 Ω resistor.

    recall2 marks

    Check answer 2 marks
    1. the 40 Ω resistor lies before the junction, so it carries the whole 0.20 A
    2. V = IR = 0.20 × 40 = 8.0 V

  2. bDetermine Determine the power dissipated in the 40 Ω resistor in Fig. 7.1, and the total power supplied by the battery.

    routine2 marks

    Check answer 2 marks
    1. power in the 40 Ω resistor = I²R = 0.20² × 40 = 1.6 W
    2. total power from the battery = VI = 12 × 0.20 = 2.4 W

  3. cDetermine The 40 Ω resistor in Fig. 7.1 is replaced by a variable resistor. Determine the resistance it must be set to for the potential difference across the parallel pair to be 6.0 V.

    demanding3 marks

    Check answer 3 marks
    1. the parallel pair has a combined resistance of (30 × 60)/90 = 20 Ω, so the current is 6.0/20 = 0.30 A
    2. p.d. across the variable resistor = 12 − 6.0 = 6.0 V
    3. R = V/I = 6.0/0.30 = 20 Ω

  4. dExplain The 30 Ω resistor in Fig. 7.1 is replaced by a filament lamp whose resistance rises as it warms up. Explain what happens to the potential difference across the 40 Ω resistor as the lamp warms, and state whether the current in the 60 Ω resistor rises or falls.

    top of the paper4 marks

    Check answer 4 marks
    1. the resistance of the lamp rises, so the combined resistance of the parallel pair rises
    2. the total resistance of the circuit rises, so the current drawn from the battery falls
    3. the p.d. across the 40 Ω resistor is that current × 40, so it falls
    4. the parallel pair therefore takes a larger share of the 12 V, so the current in the 60 Ω resistor, equal to that p.d. divided by 60, rises

Transfer challenge

A 9.0 V battery is connected to a 200 Ω resistor in series with a thermistor whose resistance is 400 Ω at room temperature. Calculate the potential difference across the 200 Ω resistor, and state how it changes when the thermistor is warmed.

Check answer 3 marks
  1. total resistance = 200 + 400 = 600 Ω, so I = 9.0/600 = 0.015 A
  2. p.d. across the 200 Ω resistor = 0.015 × 200 = 3.0 V
  3. warming lowers the resistance of the thermistor, so the current rises and the p.d. across the 200 Ω resistor rises

Put the ideas to work.

From diagrams to circuits

Trace the path from each supply terminal before assembling a low-voltage circuit.

Reading a schematic

Component symbols describe function and connectivity, not physical size or position.

Before you move on.

  1. Recognise standard component symbols.
  2. Trace connections and complete loops.
  3. Place ammeters and voltmeters correctly.

Quick Quiz

Five questions. Answers stay in this open page only.

0/5 answered
1. Which line of a cell symbol marks the positive terminal?
2. An open switch breaks the only conducting loop. What happens after the brief transient?
3. What does a fuse do if excessive current flows for long enough?
4. Two ideal diagrams show identical connections but different drawn wire lengths. How do they compare?
5. How must an ammeter and a voltmeter be connected to measure a lamp's current and potential difference?

Answer all five questions to check your score.

Downloads & Resources

Related Content

Previous lesson12.6 Parallel Plates and Capacitors →Next lesson13.2 Electric Current & Charge →Exam-style problemsMembers · Apply this lesson →

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Circuit Rush

Connect the components and put circuit reasoning into practice.

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Cambridge 9702 · focused teaching and practice

Complete the objective

Open a topic for its explanation, worked example and original practice. The AS and additional A-Level requirements remain separate. Practical preparation does not replace supervised laboratory work.

AS · 10.1.1 / 10.1.2

Use the complete Cambridge circuit-symbol set

Teaching, example & practice

General browsing: select Cambridge AS or A2 and your examination year for a recommended route.

These 27 entries are the complete circuit-symbol inventory on pages 61–62 of both reviewed Cambridge syllabuses. Draw the component symbols, not pictures of laboratory equipment. The graphic and text key must be used together: a name-only list is not enough to learn the shapes.

A continuous wire joins points into one electrical node; a filled dot explicitly marks a junction. Crossings without a junction dot must not be assumed connected. Circuit shape and wire length are schematic. A cell's longer plate is positive; the battery symbol repeats cells. The general power-supply symbol does not by itself specify mains connection or a voltage.

A variable resistor uses two terminals and an arrow through the resistor. A potentiometer has two end terminals plus a separate sliding contact, so it can provide an adjustable fraction of the end-to-end voltage. The earth symbol identifies the stated reference/earth connection; it is not a missing return wire.

Place ammeters in series and voltmeters/oscilloscope voltage inputs across the two points being compared. A galvanometer detects small current and its direction, particularly around a null. The generator is a rectangle with G; the galvanometer is a circle with an arrow, so they must not be interchanged.

An LDR has arrows arriving at it; an LED has arrows leaving. The diode's bar is its cathode: forward conventional current travels toward that bar. Include a series current-limiting resistor for an LED connected to a voltage supply. A thermistor is crossed by a bent non-arrow line; do not mistake it for an adjustable resistor.

Practise both recognition and connection: copy one symbol correctly, cover its label and name it, then put it into a labelled circuit with a clear source, return path and measurement points. The seven drawing tasks below collectively use every prescribed symbol; supplementary symbols elsewhere on the site are not additional entries in this specific inventory.

Prescribed entryDrawing cueMeaning/connection
cellone long and one short parallel platelong plate is positive
battery of cellsrepeated long/short plate pairs; optional broken extension between end pairstwo or more cells
power supplytwo open terminal circles with outer leadssource terminals; label voltage/polarity if needed
a.c. power supplytwo open terminal circles with sine-wave mark betweenalternating supply
junction of conductorswire meeting with a filled dotconnected node
lampcircle with diagonal crosslight-producing component
fixed resistorplain rectangle with two leadsfixed resistance
variable resistorrectangle with diagonal arrow passing throughadjustable two-terminal resistance
thermistorrectangle with diagonal slash and a bent horizontal foot, no arrowheadtemperature-dependent resistance
light-dependent resistorrectangle with two arrows pointing toward itlight-dependent resistance
heaterrectangle subdivided by four interior vertical strokeselectrical heating output
potentiometerrectangle plus separate third lead ending in arrow at its middletwo ends and movable contact
switchbreak with pivoted sloping contactopens/closes the conducting path
earthvertical lead ending in three successively shorter horizontal barsearth/reference connection
electric bellupper semicircular dome, flat bottom and two leads downbell output
buzzerlower semicircular cup, flat top and two leads downbuzzer output
microphonecircle beside a vertical diaphragm line, two leads to opposite sidesound input
loudspeakerrectangular body widening into a cone, two leadssound output
motorcircle containing M with the small underline shown in the referenceelectrical-to-mechanical device
generatorrectangle containing Gmechanical-to-electrical source
ammetercircle containing Aseries current measurement
voltmetercircle containing Vparallel potential-difference measurement
galvanometercircle containing upward pointer arrowsmall current/null indication
oscilloscopecircle containing an angular rising tracevoltage-time measurement
diodeopen triangle pointing at a transverse barbar marks cathode
light-emitting diodediode plus two arrows pointing awaylight-emitting diode
capacitortwo equal parallel plates separated by a gapcharge-storage component; not a cell

Cell

Long line is the positive terminal; short line negative.

Battery of cells

Repeated long-short pairs; a dashed gap may abbreviate intervening cells.

Power supply

Two supply terminals; label polarity or voltage when needed.

A.C. power supply

A tilde identifies alternating supply.

Junction of conductors

A filled dot means the crossing wires connect.

Lamp

A cross inside a circle, not a resistor rectangle.

Fixed resistor

The rectangular convention used in this syllabus.

Variable resistor

A diagonal arrow crosses the resistor.

Thermistor

A diagonal stroke with a short foot; no arrowhead.

Light-dependent resistor

Incoming arrows indicate incident light.

Heater

A divided rectangular heating element.

Potentiometer

Three terminals: two ends plus the sliding contact.

Switch

An open gap interrupts the current path.

Earth

Connection to earth/reference; not automatically the supply negative.

Electric bell

Dome above a horizontal edge, with two leads beneath.

Buzzer

Bowl below a horizontal edge, with two leads beneath.

Microphone

Circle beside a diaphragm line; two electrical leads.

Loudspeaker

A cone attached to a small rectangle, with two leads.

M

Motor

Circle labelled M with a short underline.

G

Generator

Rectangle labelled G in the Cambridge symbol table.

A

Ammeter

Circle labelled A; connected in series to measure branch current.

V

Voltmeter

Circle labelled V; connected across the two measurement points.

Galvanometer

Circle containing a pointer arrow, not merely a G label.

Oscilloscope

Circle containing a sweep trace; connected across the signal.

Diode

Triangle points towards the cathode bar; conventional forward current follows that direction.

Light-emitting diode

Diode with outgoing arrows for emitted light.

Capacitor

Two equal parallel plates separated by an insulating gap.

All 27 entries in the section-6 guide for both reviewed editions (printed pages 61–62). Practise drawing them as well as naming them. Layout is not circuit connectivity: follow the wires, terminals and junction dots. A crossing without a dot is not a junction.
Switch, ammeter and resistor in series; voltmeter across the resistorPositive supply terminal goes up to a closed switch, then through ammeter A to node P. Resistor R connects P to Q. Node Q returns to the negative supply terminal. A separate high-resistance branch connects P down through voltmeter V and back to Q. The voltmeter is not in the series current path. Constructed readings 2.40 volts and 24.0 milliamperes give approximately 100 ohms when voltmeter loading is negligible.Measure resistance: trace the wires and junctionsAV+−0–3 V d.c.school supplyS: closedR = 100 Ω, ≥1 WPQIConstructed readings: V = 2.40 V; I = 24.0 mA.R ≈ 2.40 / 0.0240 = 100 Ω; power = 0.0576 W.

The current meter reads the main-loop current. The calculation treats the voltmeter current as negligible. A real instrument's range, socket, polarity and rating must be checked before connecting power. Interpreting this diagram is preparation; independent physical setup remains a supervised practical skill.

Complete measuring connections: supply positive → switch S → ammeter A → node P → resistor R → node Q → supply negative. The voltmeter joins P to Q and therefore measures only the resistor's potential difference. Filled dots identify the two parallel-branch junctions. The switch is shown closed for a reading; turn the supply off and open S before wiring. Use a teacher-approved 0–3 V current-limited school supply and a resistor rated at least 1 W. Never put the ammeter directly across the supply.
Connected fixed-resistor and LDR potential dividerThe positive terminal of a 3.0 V school d.c. supply connects to the top of a 10 kilohm resistor. Its lower end connects to output node P. An LDR connects from P to return node Q and the supply negative terminal. A voltmeter connects between P and Q, in parallel with the LDR. Filled dots mark both voltmeter junctions. Incoming arrows identify illumination of the LDR. Output falls as light increases.Sensor divider: measure the LDR voltageV+−3.0 V d.c.school supply10 kΩLDRP: outputQ: 0 V returnlightVout

Keep the same supply voltage and illumination geometry. Measure the output between P and Q, not across both resistors. Do not connect an ammeter between P and Q.

A connected light-sensor divider. The fixed 10 kΩ resistor is above the output node P; the LDR is below it. A high-input-resistance voltmeter measures P relative to the 0 V return Q. Vout = Vs RLDR/(10 kΩ + RLDR). With Vs = 3.0 V, constructed values RLDR = 30 kΩ and 10 kΩ give 2.25 V and 1.50 V. Thus brighter light, which lowers LDR resistance, lowers this output voltage. Interchanging the two resistors reverses the response. These are illustrative calculations, not measured observations.

Worked example

Draw a cell, switch, variable resistor, ammeter and lamp in one series loop, with a voltmeter measuring lamp p.d. Explain what happens if the ammeter is instead placed directly across the cell.

  1. Place cell, switch, variable resistor, ammeter and lamp successively around one closed path.
  2. Connect the voltmeter to the two nodes either side of the lamp, creating a separate high-resistance branch.
  3. An ammeter has very low resistance; across a source it would draw an excessive current and can damage equipment. Turn off power before rewiring.

Ammeter in the lamp's current path; voltmeter across the lamp; never put the ammeter directly across a voltage source.

Try the original practice · 31 marks →Objective and coverage record
Scope and sources

Original GioPhysics teaching and practice, checked against the syllabus. This section does not certify unrelated videos, downloads or the rest of a shared lesson.

Extra challenging questions

Take the ideas further with course-specific extension questions, supplied-data graphs and worked solutions.