5.2.1 Circuits

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

Common circuit symbols

Functions of Most Common Components

• Switch: it turns the circuit on (closed), or off (open)
• Fixed resistor: it limits the flow of current
  • As electrons flow through a resistor, they transform electrical potential energy into other forms of energy (i.e. 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

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

• Light-dependent resistor (LDR): a resistor whose resistance depends on the light intensity
  • As light intensity increases, the resistance of a LDR decreases and vice versa

• Diode: it allows current to flow in one direction only
  • It is often use for rectification - i.e. conversion of AC into DC

• Light-emitting diode (LED): it emits light when a current passes through it
• Light bulb: a resistor that transforms electric potential energy into such large thermal energy that energy is dissipated as light emitted to the surroundings
• Ammeter: it measures the current in the circuit
• Voltmeter: it measures the potential difference of an electrical component

• 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

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 whose voltage is being measured
  • This means that the voltmeter must be hooked across the terminals of the component

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

• When electrons flow through a component in the circuit (e.g. a resistor):
  • They transfer some of their electrical potential energy to it
  • The component transforms this electrical potential energy into some other form of energy (e.g. thermal)

• Measuring the potential difference across a component means comparing the value of the electric potential energy of the electrons before they enter the component to when they leave it