Download Electric field and electric forces

Physics 272
January 16
Spring 2014
http://www.phys.hawaii.edu/~philipvd/pvd_14_spring_272_uhm.html
Prof. Philip von Doetinchem
[email protected]
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Summary
Source: http://en.wikipedia.org/wiki/Electric_charge
Coulomb's law
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Summary
Superposition of electric force
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Electric field and electric forces
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How do electric charges know of each others
existence? What is an electric field?
A single charge causes an electric field in the
surrounding space
If you drop a second charge in this field the two
charges communicate through their fields
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Eletric field
http://www.youtube.com/watch?v=7vnmL853784
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Electric field and electric forces
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The fields are responsible for exerting the electric
force on the other charge
An electric field creates an electric force on a test
charge q0
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Electric field and electric forces
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The electric field concept is again analogous to the
gravitational field
Electrical field is useful because it does not depend
on the charge of the body on which the electric
force is exerted.
Calculation of electric field becomes more
complicated if the charged object is not point like.
Field strength and direction will depend on the
relative position to the object.
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Sharks
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Living objects produce electric
fields due to, e.g, muscle
contraction
Ocean currents also produce
electric fields
Sharks use electroreception
Source: http://en.wikipedia.org/wiki/Shark
sensors are called the ampullae of
Lorenzini: jelly filled canals ending
in pores: small electric fields cause
charge flow → trigger to nervous
system
Multiple sensors allow 3D sensing
Also used for navigation
Source: http://en.wikipedia.org/wiki/Shark
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Sharks
A bonnethead shark biting
actively and vigorously at
various electric dipoles while
ignoring the live fish
swimming around with it.
Dr. Stephen M. Kajiura,
Elasmobranch Research
Lab, Boca Raton, FL
http://www.youtube.com/watch?v=4aYPHeK1Tyo
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Electric field of a point charge
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Electric field always points away from a positive charge and toward a
negative charge
Associated with one charge you do not have only one single vector, but a
whole set of vectors
→ vector field as a function of location
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Region of the same electric field strength are called: uniform
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In other words: electric charges act as sources for electric fields
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Conductor:
electric field in a conductor leads to motion of charges
→ electrostatics = no motion of charges
→ electric field in a conductor is 0
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Electric field of a point charge
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What is the magnitude of the electric field at a
distance for a point charge?
We know the distance of the point to the charge and
we know the magnitude of the charge:
If you would place a second charge in the field:
multiplying this result with the value of the second
charge gives you the force on the second charge.
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Electron in a uniform field
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Release of an
electron in a
uniform electric
field
Concepts:
–
electric fieldelectric force
relation
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Force and
acceleration
We know the
field, mass, and
charge
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Electron in a uniform field
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Release of an
electron in a
uniform electric
field
Concepts:
–
electric fieldelectric force
relation
–
Force and
acceleration
We know the
field, mass, and
charge
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Electric field calculations
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How to calculate fields if the charges are not pointlike?
Important for many applications: trajectories of
charges particles and ions, computer chips, etc.
Approach: sum up fields of many point-like charges
(electrons are very point-like and protons are
extremely small)
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Charged line segment
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What is the electric field at point P along the x axis?
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Charged line segment
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How large is the electric field in y direction along x?
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Field of a ring of charge
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Integrate over may tiny rings:
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Field of a ring of charge
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Field of a uniformly charged disk
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Nonconducting disk with positive surface charge
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if the disk is large R>>x → electric field is independent of x
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