Current and Charge


Electric Current

  • The rate of flow of charge
  • Symbol I
  • Measured in amperes ( A )
  • Normally a flow of electrons in metals
  • Or flow of ions in electrolytes
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  • Base SI unit of electric current
  • Symbol (A)
  • Defined as the current flowing in two parallel wires in a vacuum 1m apart such that there is an attractive force of 2 x 10 -7 N per metre length of wire between them
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One type of electric charge, attract negative repel other positives

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A type of electric charge such that it repels other negative charge and attracts positive charge.

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Coulomb (C)

  • The derived SI unit of electric charge. 
  • 1 coulomb of electric charge passes a point each second when there is a current of one amp
  • 1 coulomb = 1 amp x second
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Electric Charge

  • Physical property (symbol of q or Q)
  • Positive or negative
  • Measured in coulombs or as a relative charge
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  • An atom that has either lost or gained electrons 
  • Has a net charge
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Relative Charge

  • A simplified measurement of the electric charge of a particle or object
  • Measured in multiples of the elementary charge (q or e)
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  • The availabilty of some quantities only in certain discrete values
  • Examples are elementary charge
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Conventional Current

  • A model used to describe electric current in a circuit
  • Travels from positive to negative
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Free Electrons

  • An electron in a metal that is not bound by an atom and is free to move.
  • AKA a delocalised electron
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  • A liquid containing ions that are free to move and so conduct electricity
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Ionic Solutions

  • An ionic compound dissolved in a liquid
  • This forms an electrolyte
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A positively charged ion attracted to a cathode. (Which are negatively charged)

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A negatively charged ion attracted to an anode.

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Cathode and Anode

Cathode - a negatively charged electrode

Anode - a positively charged electrode

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  • A device used to measure electric current
  • Placed in series in a circuit and ideally has zero resistance
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  • Property of a component calculated by divided the potential difference across it by the current in it. 
  • Symbol R, unit ohm 
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Potential Difference

  • Defined as the energy transferred from electrical to other forms per unit charge
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Conservation of charge

  • States electric charge cannot be created or destroyed 
  • Total charge in any interaction is the same before and after said interaction
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Kirchhoff's First Law

  • At any point at a circuit the current entering is the same as the current leaving 
  • Electrical charge is conserved
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Number Density

  • The number of free electrons per cubic metre of material.
  • Symbol n, unit 1/m-3
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  • A material with a lower number density than a typical conductor. 
  • Number density typically of order 10 to the 17 1/m3
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Mean Drift Velocity

  • The average velocity of electrons as they move through a wire
  • Symbol v, units metres per second
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