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1
Fri. Oct 8
Phy107 Lecture 14
From Last Time…
• Coulomb force between charged particles
– Same form as gravitational force
• Electric field lines: path followed by charged particle
• Electric current: flow of charged particles
• Electrostatic potential:
–
Measured in volts.
–
Analogous to gravitational potential.
• Magnetic field:
–
arises from electric currents (moving charges)
–
also results in force on an electric current
• Faraday effect:
–
changing magnetic field
induces
electric current
–
Magnetic field from induced currents opposes change in
applied field
Fri. Oct 8
Phy107 Lecture 14
James Clerk Maxwell
•
Electricity and magnetism
were originally thought to
be unrelated
•
in 1865, James Clerk
Maxwell provided a
mathematical theory that
showed a close relationship
between all electric and
magnetic phenomena
Fri. Oct 8
Phy107 Lecture 14
Maxwell’s Starting Points
•
Electric field lines originate on positive charges and terminate
on negative charges (Gauss’s law for E)
•
Magnetic field lines always form closed loops – they do not
begin or end anywhere (Gauss’s law for B)
•
A varying magnetic field induces an emf and hence an electric
field (Faraday’s Law)
•
Magnetic fields are generated by moving charges or currents
(Ampère’s Law)
Fri. Oct 8
Phy107 Lecture 14
Maxwell’s Predictions
•
Maxwell used these starting points and a corresponding
mathematical framework to prove that
electric and magnetic
fields play symmetric roles in nature
•
He hypothesized that a changing electric field would produce
a magnetic field
•
Maxwell calculated the speed of light to be 3x10
8
m/s
•
He concluded that visible light and all other electromagnetic
waves consist of fluctuating electric and magnetic fields, with
each varying field inducing the other
Fri. Oct 8
Phy107 Lecture 14
Hertz’s Confirmation of
Maxwell’s Predictions
• Heinrich Hertz was the
first to generate and
detect electromagnetic
waves in a laboratory
setting
Fri. Oct 8
Phy107 Lecture 14
Hertz’s Experimental
Apparatus
• An induction coil is
connected to two large
spheres forming a
capacitor
• Oscillations are
initiated by short
voltage pulses
• The inductor and
capacitor form the
transmitter
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Fri. Oct 8
Phy107 Lecture 14
Hertz Transmitter
•
Hertz's radio wave generator
(transmitter). The free standing
structure on the right was a two
meter high reflector with a spark gap
and short dipole antenna at its focal
point. The apparatus on the table was
an induction coil to generate a high
voltage spark at the gap. Figure 1
diagrams adapted from "Electric
Waves", by Heinrich Hertz,MacMillan
&Co. (1900)
•
http://www.newscotland1398.net/nfl
d1901/marconinfld.html
Fri. Oct 8
Phy107 Lecture 14
Hertz Trans & reciever
Magnified view of the spark
gap and dipole transmitting
("feed") antenna at the focal
point of the reflector. The high
voltage spark jumped the gap
between the spherical
electrodes. The electrical
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 Spring '08
 n/a
 Charge, Current, Force

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