CIE AS Physics (9702) exams from 2022

Revision Notes

9.1.1 Electric Current

Defining Electric Current

  • Electric current is the flow of charge carriers and is measured in units of amperes (A) or amps
  • Charge can be either positive or negative
  • When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

Flow of charge, downloadable AS & A Level Physics revision notes

Charge can flow between two conductors

  • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
    • This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood

 

Electric current flow, downloadable AS & A Level Physics revision notes

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

  • There are several examples of electric currents, including in household wiring and electrical appliances
  • Current is measured using an ammeter
  • Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

Ammeter in series, downloadable AS & A Level Physics revision notes

An ammeter can be used to measure the current around a circuit and always connected in series

Quantisation of Charge

  • The charge on charge carriers is quantised
  • Charge comes in definite bits – e.g. a single proton has a single positive charge, whereas a single electron has a single negative charge
  • In this way, the quantity of charge can be quantised dependent on how many protons or electrons are present – positive and negative charge has a definite minimum magnitude and always comes in multiples of that magnitude
  • This means that if we say something has a given charge, the charge is always a multiple of the charge of an electron by convention
    • The charge of an electron is -1.60 × 10-19 C
    • The charge of a proton by comparison is 1.60 × 10-19 C (this is known as the elementary charge, denoted by e and measured in Coulombs (C) )

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