Circuit symbols and connections

A circuit diagram shows what is connected to what. Follow the wires and component terminals before deciding which calculation to use.

Current is charge passing per second, measured in amperes. Potential difference is energy transferred per charge between two points, measured in volts. Current is measured through a path; p.d. is measured across two points.

Draw the components

Use the standard symbols below, with lines for connecting wires. Direct current (d.c.) flows in one direction. The worked models here additionally use steady current, whose value remains constant during each measurement.

For a cell, the longer plate is positive and the shorter plate negative. A battery symbol shows repeated cell pairs. Mark the polarity when it matters to the current direction or meter connection.

A variable resistor changes the resistance included in a two-terminal path. Its adjustment arrow is part of the symbol, not a current-direction arrow. A switch opens or closes a path; a fuse is a protective link that melts and breaks the path if the current is sufficiently large for long enough.

An LED is a light-emitting diode. In forward operation, conventional current enters its anode and leaves its cathode, the side marked by the bar. The small arrows pointing outwards represent emitted light. Use an appropriate series current-limiting resistor in a practical LED circuit; different LEDs need different operating conditions.

Identify the connection points

A junction dot marks connected wires. Where wires cross without a connection, draw a clear bridge or gap so the crossing cannot be mistaken for a junction. Do not add a junction simply because two lines pass near each other.

A node is a set of points joined by ideal connecting wire with no component between them. These points have the same potential in the negligible-wire-resistance model. A wire can bend or take a longer route on the page without changing the node it represents.

Series connection

Components share one path, so the current does not split between them. R1 carries the current entering the complete R2/R3 group; it does not necessarily carry the same current as either individual branch.

Parallel connection

Components or groups share both endpoints. R2 and R3 each connect B to C. Sharing just one point is not enough.

To redraw a circuit, first record each component's two endpoints. Keep those endpoints and junctions unchanged as you move the symbols. An extra plain wire joining B directly to C would change this circuit: it would bypass the resistors rather than provide another drawing of the same network.

Place meters for the quantity you need

An ammeter goes in series in the selected path. In a main lead it measures total current; inside one branch it measures that branch's current. Connecting it directly across the source would make a low-resistance bypass, not the intended current measurement.

A voltmeter connects across the two endpoints of the component or group. Across A/C it measures the whole network's p.d.; across B/C it measures the parallel group's p.d. A reading across a pair of series resistors is not automatically the voltage across either one.

We treat the ammeter's resistance as negligible and the voltmeter's current as negligible in the model calculations. Actual instruments approximate these conditions. The measurement examples show how to pair readings for a component or a whole network.