ASPhysicsOCRRevision Notes4. Electrons, Waves & Photons4.1 Charge & Current4.1.6 Circuit Symbols & Diagrams

Circuit Symbols & Diagrams (OCR AS Physics)

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Circuit Symbols

  • The diagrams below show the various circuit symbols that could be used in circuit diagrams
  • The most common symbols are:

Circuit Symbols 1, downloadable AS & A Level Physics revision notesCircuit Symbols 2, downloadable AS & A Level Physics revision notes

  • The function of the most common components are:
    • Cell / battery / D.C supply: Provides the circuit with a source of e.m.f. A battery is two or more cells
    • Switch: Turn the circuit on (closed), or off (open)
    • Fixed resistor: A resistor limits the flow of current. A fixed resistor has a resistance it cannot change
    • Variable resistor: A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls
    • Thermistor: The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa
    • Light-dependent resistor (LDR): The resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa
    • Diode: A diode allows current to flow in one direction only. They are used to convert AC to DC current
    • Light-emitting diode (LED): This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays (TVs, road signs)
    • Ammeter: Used to measure the current in a circuit. Connected in series with other components
    • Voltmeter: Use to measure the potential difference of an electrical component. Connected in parallel with the relevant component

  • Each of these components have an electrical resistance that may impact the current in the circuit
    • However, the resistance of the ammeter and voltmeter are taken as negligible in exam questions

Examiner Tip

You must be able to recognise and draw all of these circuit symbols for the exam, as you will rarely be told within the exam question itself.

Circuit Diagrams

  • Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic
  • Electric circuit diagrams require the following to work effectively:
    • An energy source – This is a source of e.m.f so a current can flow. This can be a cell, battery, or a power supply (D.C, A.C or variable)
    • A closed path or a complete circuit – Electrons need to flow in a complete loop for a current to flow. A circuit can be open and closed using a switch
    • Electrical components – These could act as sensors that respond to the environment (LDR, thermistor), or measure a value (ammeter, voltmeter), or transfer electrical energy to other forms of energy (LED, lamp). These must be drawn with the correct circuit symbol

  • The key rules to remember are:
    • An ammeter is always connected in series
    • A voltmeter is always connected in parallel to the component the voltage is being measured
    • The direction of current flow is always from the positive to the negative terminal of the power supply

Worked example

Which circuit diagram correctly represents a circuit with current flowing through?

    • For a circuit to be connected, the switch must be closed
      • This is either circuit B or D

    • The other circuit symbol is a diode
      • Diodes only allow current to flow in one direction

    • Since current flow is from positive to negative, a forward-biased diode must point in this direction in order for the current to flow
      • This is seen in circuit B

Examiner Tip

When asked to draw a circuit diagram, make sure to draw the wires as straight lines with a straight edge or a ruler and make it as neat as possible, especially the circuit symbols.If the diagram is too small or there is ambiguity as to what a symbol represents, the examiner may not award you full marks!

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    Ashika

    Author: Ashika

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    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.