Represent d.c. circuits with the conventions of positive and negative…

The idea

A circuit diagram is a shared drawing code. Draw the long line of a cell as the positive terminal and the short thick line as the negative terminal, so the supply pushes conventional current from positive round the outside of the circuit to negative. Use the standard symbols: a cell is one long and one short line, a power supply is drawn as several cells or a labelled supply, a diode is a triangle with a bar and conducts one way only, an LDR is a resistor box with arrows pointing in, an NTC thermistor is a resistor box marked with a small t or a diagonal line, a filament lamp is a circle with a cross, an ammeter is a circle with A placed in series, a voltmeter is a circle with V placed in parallel across a component, a fixed resistor is a plain rectangle, a variable resistor is a rectangle with an arrow through it, and a switch is a break in the line with a hinged arm.

What earns the marks

• Draws the long line of a cell as the positive terminal and the short thick line as the negative terminal.
• Places an ammeter in series with the component whose current is measured and a voltmeter in parallel across the component whose potential difference is measured.
• Selects the diode symbol of a triangle with a bar to show a component that conducts in one direction only.
• Uses a rectangle with an arrow through it for a variable resistor and a plain rectangle for a fixed resistor.
• Labels an LDR with arrows pointing towards the resistor box and an NTC thermistor with its identifying mark.

Common mistakes and how to put them right

• Draws the ammeter in parallel with a component.

  Put it right: Place the ammeter in series instead.

• Uses a plain rectangle for a variable resistor.

  Put it right: Add the arrow through the rectangle to show it is variable.

• Reverses the cell convention by making the short thick line positive.

  Put it right: Keep the long line as positive and the short thick line as negative.

• Draws a diode as a plain resistor box.

  Put it right: Use the triangle and bar symbol that shows one-way conduction.

Exam tips

• Learn each symbol as a small sketch you can reproduce in under five seconds, then check the direction of the diode triangle and the cell lines.
• When a question asks where a meter goes, decide first whether it measures current through a component or potential difference across it, then place it in series or parallel accordingly.
• Read the whole symbol list in the question before choosing an answer, because distractors often swap a fixed resistor for a variable one or an LDR for a thermistor.