Physics 220 Class 2

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Physics 220 Class 2

• Today you will learn
• why balloons are never repelled by walls.
• two basic approaches to explain action at a
  distance.
• the basics of the thread model of
  electromagnetic interactions.

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But first if you werent here Wednesday

• Go to the course website http//www.physics.byu.e
  du/faculty/rees/220
• Read the course outline and look over the
  schedule.
• Get a transmitter and a MasteringPhysics Student
  Access Kit from the Bookstore.
• Look at the Power Point slides for Class 1.
• Get started in Mastering Physics.
• Get a Course ID slip from me and activate it
  online.
• Catch up on your reading.

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What happens when we put a charged rod near a
stream of water?
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What happens when we put a charged rod near a
stream of water?

• Know what we mean by
• Polarization of water molecules.
• Electrostatic induction.

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Late at night, and without permission, Reuben
would often enter the nursery and perform
experiments in static electricity. -- Gary Larson
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How does an electrophorus work?


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Charges in the conducting plate become
polarized.Charges on the insulating plate remain
where they were.


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Electrons from your finger flow into the plate,
neutralizing the top surface.
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Negative charge redistributes itself on the metal
plate.
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When you touch it again, electrons flow off the
plate into your finger.
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Physics and Truth

• As physicists we deal with models.
• Models require 1) symbols and 2) rules for
  manipulating symbols.
• These symbols and rules have a 1-to-1
  correspondence with nature.
• Physicists can say relatively little about truth
  at a fundamental level.

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so is light a wave or a particle?
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so is light a wave or a particle?

• sometimes it acts like a wave
• sometimes it acts like a particle

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so is light a wave or a particle?

• sometimes it acts like a wave
• sometimes it acts like a particle
• light is neither wave nor particle -- its
  light.

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Action at a distance.

• action force
• If the earth is 93,000,000 miles from the sun,
  how does it know it should be attracted to the
  sun?

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Virtual Particle Theories

• charges constantly emit and reabsorb virtual
  particles that carry energy and momentum.
• if another charge absorbs the virtual particle,
  a force is exerted.

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Virtual Particle Theories
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Virtual Particle Theories

• electromagnetic force QED
• quantum electrodynamics
• weak force electroweak theory
• strong force QCD
• quantum chromodynamics

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Geometrical Theories

• matter modifies the curvature of space-time.
• curved space-time affects the motion of matter.

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Geometrical Theories

• gravity General Relativity
• electromagnetic force KK
• Kaluza-Klein Theories in
• 5-dimensional space.

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Thread Model

• valid except in quantum mechanical regime.
• helps us understand the relationship between
  electricity and magnetism.
• gives quantitative results.
• useful for visualizing forces, radiation,
  magnetic fields.

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Thread Model

• Youre the first class to ever see it!

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Thread Model
Field particle Feels force
Source particle Causes force
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Thread Model
Threads are emitted by source particles at rest
isotropically and in huge numbers. Threads are
very short. Threads have heads and tails. Threads
travel at the speed of light radially outward.
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Thread Model
Threads point away from positive sources and
toward negative sources. Threads are not modified
by the presence of charges.
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Thread Model

• The force on positive field charges in the
  direction of the threads.
• The force on negative field charges is opposite
  the direction of the threads.

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Thread Model

• Threads carry no energy or momentum. They exert a
  force by modifying space near a field particle.
• To calculate the force on a field particle, we
  need to know
• The charge of the field particle.
• The density of threads near the field particle.
• The length of the threads.
• The direction of the threads.

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Thread Model
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Thread Model

• Online animation
• http//www.physics.byu.edu/faculty/rees/220/Graphi
  cs/v0thread.gif