Use one standard symbol for each component, draw it the same way every time, and place meters correctly. An ammeter goes in series with a component, and a voltmeter goes in parallel across it.
This lesson builds on the direction of conventional current, because the arrows and the terminals on a diagram only make sense if the symbols are right.
Which symbols do I need?
The standard set for this topic covers a cell, a battery, a switch, a lamp, a fixed resistor, a variable resistor, an ammeter, a voltmeter and a fuse. The list you must know depends on your syllabus year, so check the current Cambridge IGCSE Physics page.
Some tips for reading them:
- The longer line on a cell is the positive terminal.
- A lamp is a circle with a cross inside.
- An ammeter is a circle with A, and a voltmeter a circle with V.
- A resistor is a rectangle. A variable resistor has an arrow through it.
How should a circuit be drawn?
Use a ruler, straight wires and right-angle corners. Junctions are marked with a dot. Do not draw wires crossing without a dot unless you mean that they are not joined.
Keep the same symbol for the same component through the whole answer. Labelling a lamp in one part and drawing a resistor for it in another changes the meaning of the circuit.
Worked example
Draw a circuit with a cell, a switch, a lamp and an ammeter that measures the current through the lamp, and a voltmeter that measures the potential difference across the lamp. (Invented example.)
Step 1, series loop: connect the cell, switch, lamp and ammeter in one closed loop. The ammeter is in series because it must carry the same current as the lamp.
Step 2, voltmeter: connect one wire from the voltmeter to each side of the lamp only. This makes a parallel branch across the lamp. Put dots at the two junctions.
Step 3, check: trace the loop with a finger. There is one path for current through the cell, switch, ammeter and lamp. The voltmeter is on a side branch and does not interrupt that path.
As a check on the idea, in an ideal circuit of 6 V across a 2 Ω and a 4 Ω resistor in series, the current is 6 ÷ 6 = 1 A and the potential differences are 2 V and 4 V. The circuit reasoning simulator lets you confirm readings like this on screen. Potential difference and resistance are the topic of the next module.
The mistake to watch for
A common error is to connect the voltmeter in series, in line with the lamp.
Mistaken diagram: cell, switch, lamp and voltmeter all in one loop.
A voltmeter has very high resistance, so almost no current flows and the lamp stays off.
The correction is to ask, for each meter, “does the current go through this, or is it beside this?” An ammeter is in the loop. A voltmeter is beside the component, joined at both ends.
Check yourself
Try these, then open each answer.
1. In the symbol for a cell, which line is the positive terminal?
Show answer
The longer line.
2. A student wants to measure the current through a resistor. Should the ammeter be in series or parallel?
Show answer
In series, in line with the resistor, so that the same current flows through it.
3. A drawing shows a voltmeter joined at one end of a lamp only, with the other wire going back to the cell. Is it measuring the lamp?
Show answer
No. To measure the lamp it must be joined at both ends of the lamp, so it is in parallel with it.
Where this leads next
Next, distinguish current from charge stored to finish this module, and then try the charge and current practice set.
Drawing circuits from memory under time pressure is a skill that improves with quick feedback. A teacher in online one-to-one Physics tuition can give that feedback as you draw.