Transcript Chapter 14: Maxwell’s Theory By Katie Zerr and Kristin Walker

Chapter 14:
Maxwell’s Theory
By Katie Zerr and Kristin Walker
Topics that will be covered:
Electromagnetic Theory
Displacement Current
Michelson-Morley Experiment
LIGO
Precursors to Relativity
James Clerk Maxwell
• Born in Edinburgh,
Scotland in 1831
• Attended Edinburgh
Academy, The
University of
Edinburgh, and
Cambridge University
• Published by the
young age of 14
More on Maxwell
• Maxwell differed from his contemporaries
in the nineteenth century
• Faraday & Ampere contributed to
Maxwell’s theories
• Much of his important work was
accomplished between the ages of 29 and
35
Aether
• Appendix of Chapter 13
“When we observe one body acting on another at a distance . . . we
generally inquire whether there is any material connection between the two
bodies . . . we prefer to explain the action by means of these intermediate
connections, rather than to admit the notion of direct action at a distance.”
Maxwell’s Contributions
•
•
•
•
Analysis of color perception
An explanation of Saturn’s rings
Analysis of the theory of fluids and solids
Helped father modern statistical mechanics and the
molecular theory of gases
• Theory of Electromagnetism –
Maxwell’s equations
Maxwell’s Equations
• Gauss’s Law, no monopoles, Ampere’s Law
and Faraday’s Law
Prior to the Displacement Current
Magnetic field generated solely by electric charge in
motion – Ampere’s Law:
 B = (4/c)j
It was also thought (and still is) that electric charge cannot
be created or destroyed – the continuity equation:
 J + /t = 0
 Combined, give the result that the amount of
electric charge at any particular place never changes –
mathematical contradiction
Displacement Current
1st story
Conflict could be resolved by modifying Ampere’s
Law so that both electric current and displacement
current generate the magnetic field:
Displacement Current
2nd story
For a time dependent electric field, a material medium
would become polarized, just as a dielectric does
E
-+
 For a constant E field, each pair of charges soon
equilibrates as shown above
 If the E field varies with time, then the charge
configurations are constantly in motion – displacement
current
The Final Classical Theory
• 1865 paper “A Dynamical Theory of the
Electromagnetic Field” did not discuss
vortices and idle wheels
• Classical mechanics was used as model
• After consistent defeat, mechanical models
abandoned
• Electromagnetic theory distinct branch of
physics
Albert Michelson (1852-1931)
• American experimental physicist
• Devoted his life to making extremely
accurate measurements of the speed of light
• Nobel Prize in Physics in 1907 for his
efforts
• 1887 collaborated with Edward Morley to
perform an experiment sensitive enough to
detect the earth’s motion through the aether
Michelson – Morley Experiment
• Swimmers race across the river
Michelson-Morley Animation
• http://galileoandeinstein.physics.virginia.ed
u/more_stuff/flashlets/mmexpt6.htm
LIGO
• LIGO stands for Laser Interferometer
Gravitational-wave Observatory
• Similar apparatus to Michelson-Morley
• Run into same problem- trying to block out
high frequency sounds that disrupt results
• They hang apparatus while Michelson and
Morley suspended theirs in mercury
Precursors to Relativity
• FitzGerald (1851-1901) could avoid negative
result of Michelson-Morley experiment
• Contraction hypothesis was independently
postulated by Hendrik Lorentz
• Lorentz manipulated equations and are called
Lorentz transformations today
• Lorentz-FitzGerald contraction hypothesis
Precursors to Relativity (con’d)
• Poincaré postulated that
“optical phenomena depend only on the relative motions of
the material bodies”
• He questioned the existence of aether
• In 1904 he determined his principle of
relativity
• Lorentz came up with a theory of electrons
consistent with Poincaré’s principle but still
left room for the aether
Conclusion
• Maxwell provided us with modern physical
and mathematical equations
• Many contributions to physics even though
his belief in the existence of aether was not
valid
• Michelson-Morley experiment proved aether
wrong
• LIGO today uses similar apparatus and
encounters similar problems