Circuit Diagrams (HL IB Physics)

• Circuit symbols are used to represent electrical components in circuit diagrams
• These symbols are universally recognised by all scientists in any language
• The following symbols are commonly used in circuit diagrams

Circuit symbols

Cells & Batteries

• A device which converts energy from a chemical energy store to electrical store (one battery)
  • The direction of the current flow is from the positive (longer side) to the negative (shorter side) terminal
  • This is the opposite direction to the electron flow

• A battery is simply a power source made up of multiple cells

Switch

• A device which turns the circuit on (closed), or off (open)
• The switch allows or prevents the flow of current

Voltmeters & Ammeters

• A voltmeter is a device which measures the potential difference between two points in a circuit
• An ammeter is a device which measures the current flowing in a circuit

Fixed resistor

• A device which increases resistance to limit the flow of current
• As electrons flow through a resistor, they transform energy from its electric potential energy store into other stores (e.g. thermal energy)

Variable resistor

• A resistor with a slider that can be used to change its resistance
• As the resistance of the variable resistor increases, the current in the circuit decreases and vice versa

Light-dependent resistor (LDR):

• A resistor whose resistance depends on the light intensity
• As light intensity increases, the resistance of an LDR decreases and vice versa

Thermistor:

• A resistor whose resistance depends on its temperature
• As temperature increases, the resistance of a thermistor decreases and vice versa

Potentiometer

• A resistor with a sliding contact to form an adjustable voltage divider

Lamp

• A lighter emitting component consisting of heating a filament inside a glass cover

Light-emitting diode (LED)

• A device that emits light when a current passes through it
• A diode only allows current to flow in one direction only

Heating element

• An element that converts energy from an electrical store into a thermal store through the process of resistance

Motor

• An element that converts energy from an electrical store into a mechanical store 

Earth (ground)

• The point in the circuit which is grounded i.e. connected to the Earth
• A connection to the Earth allows an instantaneous discharge to occur if an appliance malfunctions
• Electrons are transferred directly to the Earth through a low-resistance wire

Drawing Circuit Diagrams

• Circuit diagrams represent the arrangement of components in a circuit
  • This is important, as some components need to be in a certain position in relation to the others to work

• Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic

A circuit diagram must include:

• An energy source
  • This is a source of potential difference so a current can flow
  • This can be a cell, battery, or a power supply

• 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 (using the correct circuit symbol)
  • 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)

Measuring Current

• Electric current is measured using an ammeter
• Ammeters should always be connected in series within a circuit

To measure the current flowing through a light bulb, an ammeter must be connected in series with it

• An ideal ammeter should have zero resistance
  • This way, it will not take any energy from the electrons flowing through it
  • Otherwise, it would alter the value of the current it is trying to measure

Measuring Potential Difference

• Potential difference (or voltage) is measured using a voltmeter
• A voltmeter is always set up in parallel to the component being measured
• This means that the voltmeter must be connected across the terminals of the component

To measure the potential difference of a fixed resistor, a voltmeter must be connected in parallel to it

• A measurement of potential difference represents the difference between the values of electric potential on either side of the component
  • This is because 1 Volt (V) is equivalent to 1 Joule per Coulomb (J C⁻¹)

• Therefore, 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