K323 / 2027
D.C. circuits overview

Topic 1 of 6

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.

Recognise the component and its connections

Names and short roles accompany the actual circuit symbols. Light arrows on an LED point out; light arrows on an LDR point in.

Cell

Cell circuit symbolSupplies energy per charge. The longer plate is positive.+-

Supplies energy per charge. The longer plate is positive.

Battery

Battery circuit symbolTwo or more cells form the source shown here.+-

Two or more cells form the source shown here.

Switch

Switch circuit symbolOpens or closes a conducting path; shown open.

Opens or closes a conducting path; shown open.

Lamp

Lamp circuit symbolTransfers electrical energy into light and internal energy.

Transfers electrical energy into light and internal energy.

LED

LED circuit symbolEmits light in forward operation. The bar marks cathode K.K

Emits light in forward operation. The bar marks cathode K.

Fixed resistor

Fixed resistor circuit symbolProvides resistance in the conducting path.

Provides resistance in the conducting path.

Variable resistor

Variable resistor circuit symbolAdjusts resistance in a two-terminal path.

Adjusts resistance in a two-terminal path.

Fuse

Fuse circuit symbolIts link can melt and open the circuit if current is excessive.

Its link can melt and open the circuit if current is excessive.

Ammeter

Ammeter circuit symbolMeasures current through the path in which it is inserted.A

Measures current through the path in which it is inserted.

Voltmeter

Voltmeter circuit symbolMeasures p.d. between the two points to which it connects.V

Measures p.d. between the two points to which it connects.

D.c. supply

D.c. supply circuit symbolProvides a supply with the indicated fixed polarity.+-

Provides a supply with the indicated fixed polarity.

A.c. supply

A.c. supply circuit symbolProvides a supply whose polarity alternates.

Provides a supply whose polarity alternates.

Potentiometer

Potentiometer circuit symbolTwo track ends and a wiper provide three connections.

Two track ends and a wiper provide three connections.

Electric bell

Electric bell circuit symbolProduces sound when operated by the circuit.

Produces sound when operated by the circuit.

LDR

LDR circuit symbolResistance falls as more light reaches it; arrows point in.

Resistance falls as more light reaches it; arrows point in.

NTC thermistor

NTC thermistor circuit symbolResistance falls as its temperature rises.

Resistance falls as its temperature rises.

Each card shows a circuit symbol, its name and its role. A component's symbol identifies it; the wires attached to its terminals determine its place in the circuit.

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.

An a.c. supply reverses its polarity; its symbol is distinct from a d.c. supply's. The resistive-network examples here use a steady d.c. source.

An LDR has light arrows pointing towards it. A thermistor responds to temperature. The sensor circuits page explains their resistance changes. The potentiometer has two track-end connections and a separate wiper connection; that third terminal is essential to its divider use.

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.

Trace connections instead of judging position

Junction dots show connected wires. These two drawings show only the A/B/C resistor network; a source can be connected across its outer terminals A and C.

One drawing of the network

One drawing of the networkR1 joins A to B. R2 joins B to C along the upper path and R3 joins the same B and C nodes along the lower path. The vertical wires at B and C connect the respective upper and lower junctions. There is no plain wire directly from B to C.ABCR1R2R3R1: A/B; R2 and R3: B/C

Move R3 around the outside

Move R3 around the outsideThe same network is redrawn. A joins the left terminal of R1. R1 ends at B. R2 connects directly from B to C. From B, a wire rises to the right terminal of R3; the left terminal of R3 connects around the left and bottom edges to C. The outer path contains R3 and is not a short circuit. The drawing has no unmarked wire crossing.ABCR1R2R3The named connections are unchanged
Both drawings keep R1 between A and B, and R2 and R3 each between B and C. R2 and R3 are therefore parallel. R1 is in series with that whole parallel combination, regardless of where the symbols sit on the page.
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.

Optional check R1 connects A to B. R2 and R3 each connect B to C, with no plain wire directly joining B to C. Which description remains correct when the drawing is rearranged?
R1 connects A to B. R2 and R3 each connect B to C, with no plain wire directly joining B to C. Which description remains correct when the drawing is rearranged?