Electricity and Magnetism

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Electricity and Magnetism
electron, electricty, electronics
(electrostatics) from Greek word
elektron(amber) - attracts bits of straw when
rubbed with silk magnet, magnetism
(magnetostatics) after a Greek region Magnesia
- attracts iron
Electricity
Magnetism

• Link found by Oersted (1820)
• electricity could produce magnetism
• Experiments by Faraday (1831)
• magnetism could produce electricity
• Maxwell (1873)
• unified theory of electricity and magnetism

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Unity of Electricity Magnetism
Changing electric and magnetic fields are linked

• change in magnetic flux produces an electric
  field
• change in electric flux produces a magnetic field

• Similarity of Motors and Generators
• Motors use magnetic forces on currents
• Generators use electric fields induced by
  rotating magnets
• Light is an Electromagnetic wave
• Speed of light can be found from electrical and
  magnetic measurements
• EM is evident everywhere
• Macroscopic scale -- Lightning, main power
• Microscopic scale -- Atoms, nuclei

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Maxwells Equations - Differential Form

• Gausss Law
• Gausss law for Magnetism
• Amperes Law
• Faradays law

where
and
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Applications of Maxwells Equations

• Electromagnetic Wave Equation
• Charged Particle Equation of Motion

where
and
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Electric Charge

• In nature () and (-) charges cancel each other
  to an incredible degree.
• Electrostatic effects are due to a slight
  imbalance.

Electrostatics Study of charges at rest or
moving slowly with respect to each other.
Basic Rule Like charges repel and unlike charges
attract
Applications

• Xerox process
• fly ash precipitators (power plants, etc)
• home dust precipitators
• ink-jet printers

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Quantization of Charge
Many elementary particles are made of quarks p
(uud) n (udd) e
Elementary charge
e 1. 602 X 10-19 C
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Conservation of Charge

• Individual charges may be created or destroyed
• there is no change in the total net charge.
• Annihilation of particle and anti-particle
• e e- --gt ? ?
• Pair production
• ? --gt e e-
• Nuclear decay
• 238U --gt 234Th 4He

Does not happen !
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Conductors and Insulators

• Good conductor
• electrons weakly attached to atoms, free to move
  around in material (Cu or Al)
• Insulator (poor conductor)
• electrons strongly bound to atoms, can distort
  the atoms (polarize) but hard to free up the
  electrons (glass, teflon)
• Semiconductor (in between)

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Charging up an object

• Direct contact / transfer
• demo - charged rubber/ glass rods
• transfer electrons between objects
• Charging by induction
• redistribute the electrons on a conductor using
  an external charged object
• use ground as a source of electrons

Charge density on an object Use this concept
when charges are so close together they look like
a continuous distribution of charge.. ? (C/m), ?
(C/m2), and ? (C/m3)
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Coulombs Law

• Force between two charges
• Coulombs law
• k1/4??o8. 99x109 Nm2/C2
• ?o8. 85x10-12 C2/Nm2 permittivity constant of
  free space
• q - coulomb (abbreviation C), r - meters
• Fundamental law of electric force between two
  charges q (not derived by observation)
• Vector quantity !
• Inverse square law, like gravitational force
• Attraction/repulsion in sign of q1q2
• Strong force !

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Gravitation and Electrostatics
Compare the gravitational and electric forces of
attraction between electron and proton in the
Hydrogen atom. Assume r 0. 53x10-10 m
me 9. 11x10-31 kg mp 1. 67x10-27 kg
qe -qp -1. 60x10-19 C G 6. 7x10-11 Nm2/kg2
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Coulombs Law (Vector Form)

• is force on 1 duo to the presence of 2
• is vector from 2 (origin of force) to 1

Principle of Superposition
Vector sum
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Principle of Superposition
Coulomb Force , acting on a charge q is
Coulomb Force for Charge Distributions
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Shell Theorem for Electrostatics

• A uniform SPHERICAL shell behaves for external
  points as if its charge is concentrated at the
  center
• A uniform SPHERICAL shell of charge exerts no
  force on a charged particle placed inside the
  shell.
• These theorems allow problems to be simplified
  tremendously. Radial charge density can vary !