Science

Circuits

Electrical circuits can be represented by circuit diagrams. The various electrical components are shown by using standard symbols in circuit diagrams. Components can be connected in series, or in parallel. The characteristics of the current [current: Moving electric charges, for example, electrons moving through a metal wire. ] and potential difference (voltage) are different in series and parallel circuits.

Circuit symbols

You need to be able to draw and interpret circuit diagrams.

Standard symbols

The diagram below shows the standard circuit symbols you need to know.

two horizontal lines with a gap where one line is at 45 degrees

Open Switch

two horizontal lines with a join in the centre

Closed Switch

a circle with an 'x' inside, attached to a horizontal line either side

Lamp

two 't' shapes head to head, one with a thicker top

Cell

three t-shapes, the one on left flipped to the right, the two on the right flipped 90 degrees to the left. the two on the right side have a thicker "top".

Battery

a circle with a 'V' in the centre. two horizontal lines come out from either side of the circle.

Voltmeter

a rectangle lying flat with two horizontal lines coming out of its sides

Resistor

a rectangle lying flat with one horizontal line running through the centre of it and out of both sides

Fuse

a circle with an 'A' in the centre with two horizontal lines coming out of the sides of the circle

Ammeter

a rectangle lying flat with an arrow running through it at a 45 degree angle. two horizontal lines run out of the sides of the rectangle

Variable resistor

a rectangle lying flat with two horizontal lines running out either side of it. A 45 degree line runs through the rectangle which bends at the bottom to run parallel with the base of the rectangle

Thermistor

a rectangle lying flat with two horizontal lines running out of either side. a circle runs around the rectangle, and two arrows point downwards at the rectangle, from the top left.

Light dependent resistor (LDR)

Circuit diagrams

Two things are important for a circuit to work:

  • there must be a complete circuit
  • there must be no short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, there is a short circuit and the lamp will not light.

Work out which of these four lamps will light when the switch is closed. Move your mouse over a circuit, then left click to check your answer.

Series and parallel connections

You should know the difference between series and parallel connections in circuits.

Series connections

Components that are connected one after another on the same loop of the circuit are connected in series. The current [current: Moving electric charges, for example, electrons moving through a metal wire. ] that flows across each component connected in series is the same.

Two lamps connected in series

The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light.

Parallel connections

Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.

Two lamps connected in parallel

The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still light.

Which of the circuits below are connected in series, and which are connected in parallel? Move your mouse over each circuit to check your answers.

Current and potential difference

You need to know how to measure the current that flows through a component in a circuit. You also need to know how to measure the potential difference, also called voltage, across a component in a circuit.

Current

A current flows when an electric charge [electric charge: The electrical state of an object, which can be positively charged or negatively charged. ] moves around a circuit. No current can flow if the circuit is broken, for example, when a switch is open. Click on the animation to see what happens to the charge when the switch is opened or closed.

Measuring current:

  • current is measured in amperes
  • amperes is often abbreviated to amps or A
  • the current flowing through a component in a circuit is measured using an ammeter [ammeter: A device used to measure electric current. ]
  • the ammeter must be connected in series with the component
An ammeter in series with a lamp

Potential difference - voltage

A potential difference, also called voltage, across an electrical component is needed to make a current [current: Moving electric charges, for example, electrons moving through a metal wire. ] flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference:

  • potential difference is measured in volts, V
  • potential difference across a component in a circuit is measured using a voltmeter
  • the voltmeter must be connected in parallel with the component
A voltmeter in parallel with the lamp

Cells and circuits

You should know what happens to the potential difference and current when the number of cells in a circuit is changed.

Potential difference

A typical cell produces a potential difference of 1.5V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5V cells are connected in series in the same direction, the total potential difference is 3.0V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow.

Current

When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result.

What will happen to the reading on the ammeter in the simulation below when the number of cells is increased?

Series circuits

You should know the characteristics of the current and potential difference in series circuits.

Current

When two or more components are connected in series, the same current flows through each component. Check your understanding of this by answering the questions about the circuit below:

Potential difference

When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.

Check your understanding of the voltages across components connected in series using this activity.

Parallel circuits

You should know the characteristics of the current and potential difference in parallel circuits.

Current

When two or more components are connected in parallel, the total current flowing through the circuit is shared between the components. Check your understanding of this by answering the questions about the circuit seen here. Assume that both lamps are identical.

Potential difference

When two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12V, the voltage across another lamp connected in parallel is also 12V.

Check your understanding of the voltages across components connected in parallel using this activity:

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