Circuit Symbols & Diagrams (OCR A Level Physics)

Circuit Symbols

• The diagrams below show the various circuit symbols that could be used in circuit diagrams

• The most common symbols are:

• 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

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