Electromagnetism before Maxwell

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Electromagnetism before Maxwell
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Coulombs Law
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Comments

• Using SI units
• ?0 is known as the permitivity of free space
• The vector r is the vector connecting the two
  points
• In general

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Faraday introduced the concept of lines of Force
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Lines of force
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Oersted demonstrated that there is always a
magnetic field associated with a current
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Concept of Field
Idea Space around electric charge is altered so
if we put Another charge in the neighborhood of
the first it will experience a force.
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Principle of Superposition
The interaction between any two charges is
completely unaffected by the presence of
others. Which means to find the total field at a
point we add the field due to all the charges
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Let us assume Gausss Law is true then consider
a point charge at the origin, construct a sphere
around q then clearly by symmetry the field must
be radial and we have
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• Example

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• Solution

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Conductors
A perfect conductor is a material containing an
unlimited supply of free charges Properties


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• Think of 2 Gaussian surfaces one just inside the
  surface and one just outside
• Since for a conductor there is no field inside
• There is no charge inside, therefore all the
  charge must be on the surface

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• A charged metal ball is lowered into an uncharged
  metal can and a lid added
• While the ball inside the can but not touching
  surface charges are generated on the surface of
  the can

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• If the ball is allowed to touch the can and ball
  form a single conductor and all the net charge
  will be on the outside of can

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Faraday constructed a large metal covered box
which he mounted on insulating supports and
charged with a powerful electrostatic generator
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I went into this cube and lived in it and using
lighted candles, electrometers and all other
tests of electrical states.I could not find the
least influence on them, though all the timethe
outside of the cube was very powerfully charged
and large sparks and brushes were darting off
from every part of its outer surface
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• Capacitance is a purely geometrical quantity
  determined by the sizes shapes and separation of
  the two conductors
• Vis by definition the potential of the positive
  conductor less that of the negative
• Q is the charge on the positive conductor
• Thus C is always positive

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• The unit of capitance is the faradcoulomb per
  volt
• Also micro(10-6) and pico(10-12) farads are used.

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