Quantum ElectroDynamics

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Electricity Magnetism IDr Chris Booth
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Organisation
Two lectures per week (for 6 weeks) Wednesdays
1100 Hicks LT02 Thursdays 1200 Hicks LT02
Syllabus (see sheet) 7 topics (plus revision
session) 1 or 2 lectures per topic
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TOPIC 1Electric ChargeCoulombs Law
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Introduction
Electric charge is everywhere! Atoms positive
nucleus negative electrons Unit of charge
Coulomb (C) very large! Charge on proton e
1.6?1019 C Charge on electron e (So gt 6?1018
electrons to make 1 C!)
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Normally, matter is neutral equal numbers of e
p Separated charge ? electrostatics Moving
charge ? magnetic effects (as well) Rub plastic
(with fur) ? plastic gains negative charge Rub
glass (with silk) ? glass gains positive
charge In each case transfer of
electrons. (Positive charge deficit of
electrons)
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Conductors Insulators
Conductors e.g. metals Some atomic electrons
free to move within body of conductor Insulators
electrons tightly bound to atom or
molecule (Semiconductors small number of free
charges depends on impurity level)
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Forces Induction
Like charges ? ? ? ? repel Unlike
charges ? ? attract Conductor Add extra
electrons at one end. They repel and spread
throughout material Induction Use one charged
body to charge another see illustrations.
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Insulating rod Isolated conducting
sphere Macroscopic view charge on rod repels
positive charge on sphere, attracts
negative. Microscopic view some free (negative)
electrons in sphere are attracted, leaving excess
positive charge behind.
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Connect sphere with wire to ground Macroscopic
view positive charges escape to
ground Microscopic view electrons are attracted
from ground, neutralising positive charge.
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Coulombs Law
Force between 2 charges Q and q Proportional to
product of charges Qq Proportional to 1 / square
of separation Q r q Const. of proportionality
k 8.99?109 N m2 C2 (Check the units!)
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Normally written ?0 8.85?1012 C2 N1 m2 (Why
4?? See later!) Vector form Force on q due
to Q if points from Q to q.
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Superposition

• The force due to one charge is unaffected by
  others.
• ? Just add vectors
• Ftot F1F2F3...

• Example
• Charges Q, Q and Q are arranged at the
  vertices of an equilateral triangle as shown.
  What would be the force on a charge q at the
  centre of the triangle?

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Example 2 Charges 9Q and Q are a distance ?
apart. At what point would there be no net force
on a test charge q? Example 3 What is the ratio
of the gravitational to the electrostatic force
between the electron and proton in a hydrogen
atom? Mass of proton 1.67?1027 kg mass of
electron 9.11?1031 kg Magnitude of electron
proton charges 1.6?1019 C