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First exams 2025

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Electric Current (CIE A Level Physics)

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Katie M

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Defining electric current

  • Electric current is the flow of charge carriers and is measured in units of amperes (A) or amps
  • Charge carriers can be either positive or negative
  • When a conductor is exposed to a potential difference, charge carriers will flow

Charge flowing between conductors

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

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

  • 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 the discovery of the electron

Conventional current and electron flow

5-1-3-direct-current-in-a-simple-circuit_sl-physics-rn

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

  • Current is measured using an ammeter
  • Ammeters should always be connected in series so that the charged particles flow through the ammeter

An ammeter in a circuit

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 carried by charged particles is quantised
  • Charge is carried in discrete amounts or quanta (sometimes referred to as packets)
  • Charge is measured in coulombs (C)
    • 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 value is known as the elementary charge
  • In this way, the quantity of charge is dependent on how many protons or electrons are present; charge always occurs in multiples of the magnitude of the elementary charge

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.