Electricity

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Electricity Magnetism

• Seb Oliver
• Lecture 2 Microscopic Description of Charge

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Why Study Electricity Magnetism?
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Why Study Electromagnetism

• Abstract
• The search for a Grand Unified Theory
• Gravity
• Electricity
• Magnetism
• Weak Nuclear
• Strong Nuclear

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Why Study Electromagnetism

• Practical
• most every-day of forces
• all chemical properties of elements boil down to
  electromagnetism
• most physical properties of materials
• Thermal conductivity
• Mechanical properties
• Optical properties
• Electrical properties

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Different Types of Materials
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Different Types of Material

• Conductors
• Within a conductor charge can move freely
• Non-Conductors or Insulators
• Within insulators charge does not move much
• semi-conductors
• Conducting properties vary dramatically depending
  on impurities and charge present

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Conductors Insulators
Demo. Insulator
Demo. Conductor
Schematic of Conductors Insulators
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Microscopic Description of Charges
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Quantification of Electrostatics

• Microscopic explanation of "charging"
• Units of "charge"

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Microscopic explanation of charging

• Previously we have looked at the macroscopic
  behaviour of charge (e.g. balloons)
• Need to look at a simple model of the atom, the
  Bohr model

Orbiting around this are electrons
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Bohr model

• Electrons have -ve charge
• Protons have (equal opposite) ve charge
• Neutrons have no charge
• Same number of electrons protons in a neutral
  atom
• similar number of protons to neutrons
• -ve electrons attracted to ve nucleus
• A neutral atom which loses an electron becomes a
  ve ion
• A neutral atom which gains an electron becomes a
  -ve ion

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Atoms Ions
3 ve Protons 3 ve electrons means a neutral
atom
Removing an electron Leaves us with a net ve ion
Adding an electron Leaves us with a net -ve ion
Atoms Ions
polarization
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Atomic Explanation for Macroscopic Behaviour

• Some atoms e.g. in glass easily lose electrons
  and become ve charged
• These electrons are gained by other atoms e.g. in
  silk which become -ve charged
• Charge is quantised and the basic unit of charge
  is the charge of the electron -e
• Charging is an exchange of electrons

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Units of charge

• Since charge is a fundamental property of matter
  (like mass) it needs a unit
• A natural unit would be e, the charge of the
  electron
• The SI unit is the Coulomb (C)
• e 1.60219 x 10-19 C
• i.e. 1C 6.2 x 1018 e

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Properties of Atomic Particles
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Micro-scopic to Macro-scopic

• Definitions
• Normal Carbon 12C has 12 nucleons
• 12g (i.e. 0.012 kg) of Carbon is one Mole
• 1 Mole is 6.023 1023 atoms, (Avagadro's number)
• Calculations
• Charge (q) in a baloon is typically 10-6 C
• q 10-6 C 10-6 6 1018 e 6 1012 e
• i.e. Around 1 in 1021 extra electrons

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Summary Lecture 2

• Why study electromagnetism?
• It is fundamental, in abstraction and practically
• Model of atom has electrons orbiting nucleus
• Charge is carried by electrons

• Properties of matter
• Conductors
• non-conductors
• semi-conductors