Circuits (OCR A Level Physics)

• Constructing, designing and checking circuits are an essential practical skill in physics
• There are some important points to consider with all circuits:
  • If the current is to be measured, an ammeter must be connected in series
  • If the potential difference is to be measured, a voltmeter must be connected in parallel to the required component
  • If the current is to change between two paths (eg. charging and discharging a capacitor) a switch must be present
  • A diode must always be facing the direction of the current flow
  • Make sure each component is able to handle the amount of current in the circuit. Otherwise, they could fuse, be damaged, or spark

Circut with corresponding circuit diagram

Polarity

• The polarity of an electrical component means that it will function differently depending on the direction in which it is connected
  • In other words, a non-polarised components, such as a lamp can be connected in any direction it will still light up

• A polarised component can only be connected in a circuit in one direction
  • If not, it will either not work, work incorrectly or break

• Common polarised electrical components include:
  • Cell / batteries / any power supply
  • Diodes
  • LEDs (light-emitting diodes)
  • Electrolytic capacitors

• The positive and negative terminal on a battery is normally clearly marked
  • If not, the negative side has a larger metal area

• LEDs, and diodes consist of two pins or 'legs'
  • The longer leg is the positive (anode) pin
  • The shorter leg is the negative (cathode) pin

• The polarity can sometimes be checked by multimeters and leads connected to the appropriate pins

Polarity of an LED is shown by the longer or shorter pin