# Current and Charge

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## 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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## Amperes

• 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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## Positive

One type of electric charge, attract negative repel other positives

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## Negative

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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## Ion

• 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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## Quantise

• 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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## Electrolytes

• 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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## Cations

A positively charged ion attracted to a cathode. (Which are negatively charged)

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## Anion

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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## Ammeter

• A device used to measure electric current
• Placed in series in a circuit and ideally has zero resistance
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## Resistance

• 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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## Semi-conductors

• 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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