Circuit Elements

In a series circuit, the components are connected along one loop, with no branches. A cell or battery supplies a potential difference, and the loop must be complete for current to flow. An open switch breaks the loop; closing it joins the contacts and completes the path.

Recognising the standard symbols

Each component has its own symbol. Using the same symbols makes a circuit understandable even when the real components look different.

A cell is represented by one long line and one short line. A battery has repeated pairs of these lines. A lamp has a cross inside a circle, while a resistor is shown as a rectangle. A variable resistor has an arrow through the resistor symbol, showing that its resistance can be adjusted.

An ammeter has an A inside a circle and measures current. A voltmeter has a V inside a circle and measures potential difference across a component. The open and closed switch symbols show whether the contacts are separated or joined.

Some symbols include details that distinguish related components. A light-dependent resistor (LDR) is drawn as a resistor inside a circle, with two arrows pointing towards it to represent incoming light. Its resistance depends on the light falling on it. A thermistor is a component whose resistance depends on temperature. Its symbol is a resistor crossed by a sloping line that bends into a short horizontal tail at one end; unlike the variable resistor’s line, it has no arrowhead. Both components can act as sensors.

A diode allows current in only one direction. Its symbol has a triangle-shaped part pointing towards a bar, inside a circle. A light-emitting diode (LED) uses the diode symbol with two arrows pointing outwards, representing emitted light. These outward arrows contrast with the inward arrows on an LDR.

Positive and negative terminals

On a cell symbol, the long line is the positive terminal and the short line is the negative terminal. The same convention identifies the outer terminals of a battery. The symbols can be rotated, so positive does not always mean the left-hand side or the top of the diagram.

Conventional current flows through the external circuit from the positive terminal towards the negative terminal. To follow its direction, start at the long line of the cell and trace the connected wires and components back to the short line.

For a diode, conventional current can pass through it towards the bar, but not in the reverse direction. A complete loop alone therefore does not guarantee current: a diode connected the wrong way round can prevent it.

Drawing and reading a complete circuit

Suppose a circuit contains a cell, a closed switch, a lamp and an ammeter, with a voltmeter measuring the potential difference across the lamp.

The ammeter is in series: it lies along the same path as the lamp, so the lamp’s current passes through it. The voltmeter is connected in parallel across the lamp: its two connections go to opposite sides of the lamp. It is a measuring branch, not another component placed along the main series path.

To draw a circuit, work systematically around its connections. Draw the cell symbol, then add each component’s symbol in sequence, joining them with straight wires. Complete the return connection to the other terminal. Add a voltmeter, if needed, across the component being measured.

To interpret a diagram, use the reverse process: identify the symbols, locate the supply’s terminals, then trace the path. Check for open switches or gaps and inspect any diode’s orientation. Moving a symbol to another side of the drawing does not change the circuit if its connections remain the same.