Topic B.5 — Current & CircuitsPaper 1 & 2Circuit symbolsMeter placement~7 min read
Reading & Drawing Circuit Diagrams
A circuit diagram is a map of how components are wired together — every symbol, wire, and junction has a precise meaning. Before you can analyse currents, voltages, or resistances, you need to be fluent in reading and drawing these diagrams accurately.
📘 What You Need to Know
Circuit symbols are standardised internationally — you must recognise and draw them correctly, but the data booklet supplies them, so memorising isn’t required
A valid circuit needs three things: a source of potential difference, a closed conducting path, and correctly-drawn components
An ammeter always sits in series (in the main loop); a voltmeter always sits in parallel (branching across the component being measured)
An ideal ammeter has zero resistance; an ideal voltmeter has infinite resistance — unless a question states otherwise
Solid dots mark junctions where wires join; wires that simply cross without a dot are not connected
Essential Circuit Symbols
Each symbol represents a specific job in the circuit. Getting the shape, orientation, and proportions right matters — an examiner may withhold marks for a sloppy or ambiguous symbol.
The six symbols you’ll draw most often in circuit diagrams — note the long/short plate convention on the cell (long = positive) and the arrow through the variable resistor.
Beyond these six, you’ll also meet the thermistor and light-dependent resistor (resistor boxes with a diagonal arrow and a temperature or light symbol), the lamp (circle with a cross), the LED (diode with light arrows), and earth/ground (a short vertical line into horizontal bars). All of these are provided in the data booklet — your job is to recognise them instantly and place them correctly.
What Makes a Valid Circuit Diagram
A diagram only “works” as a circuit if charge has somewhere to flow. Three ingredients are non-negotiable:
🔧 Building a Circuit Diagram
Add an energy source — a cell, battery, or power supply, giving the circuit a potential difference to push charge around
Complete the loop — every component must sit on a closed conducting path; a break (open switch, missing wire) stops current everywhere in that loop
Insert components with correct symbols — orientation matters for diodes and LEDs, since they only conduct one way
Placing Ammeters & Voltmeters
This is the single most common diagram-drawing error IB examiners report. The rule never changes, regardless of how complicated the rest of the circuit looks:
Meter placement rule
Ammeter → in series (in the current’s path) Voltmeter → in parallel (branching across the component)
The ammeter sits directly in the main loop; the voltmeter branches off and rejoins on either side of the resistor, so it measures the p.d. across that component alone.
Quick recap: valid circuit = source + closed loop + correct symbols; ammeter in series, voltmeter in parallel, dots mark real junctions.
WE 1
A student draws a torch circuit with a cell, a switch, and a lamp, then adds a voltmeter directly in the main loop (in series with the lamp) to check the lamp’s p.d. What happens to the lamp?
Think about resistance:
An ideal voltmeter has infinite resistance.
Effect on the loop:
Placed in series, that huge resistance dominates the whole circuit.
Current everywhere in the loop drops to almost zero.The lamp goes out — the voltmeter must be moved to a parallel branch across the lamp
WE 2
In a diagram, two wires cross on the page but no dot is drawn at the crossing point. What does this represent?
Convention check:
A junction dot is what tells you two wires are electrically joined.
No dot means the wires simply pass over one another.The wires are not connected — they belong to two separate parts of the circuit
💡 Top Tips
Draw wires as straight lines with a ruler — neat right-angle corners are far easier for an examiner to mark than freehand curves
Keep every symbol a sensible size; a resistor box that’s too small can be mistaken for a wire kink and cost you marks
Always show the long/short plate convention on a cell so the polarity is unambiguous
When redrawing a circuit from a photo or description, build the main loop first, then add parallel branches afterwards
⚠ Common Mistakes
Wiring an ammeter in parallel — this gives it almost zero resistance to work with, which can send a damaging surge of current through the meter
Wiring a voltmeter in series — as in WE1, its huge resistance chokes off the current for the whole loop
Forgetting a junction dot where three or more wires genuinely meet, making the diagram ambiguous
Drawing a diode or LED without checking which way the arrowhead points relative to conventional current
Up next: Electric Current — once you can read a diagram fluently, the next step is understanding exactly what’s flowing through those wires.
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