Lecture 20
Feb. 26. 2016.
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 an atom has a central nucleus of protons and neutrons surrounded by electrons
 Each element in the periodic table has a different combination of protons and neutrons. Hydrogen just has one proton and one electron.
 Electrons and protons carry electric charge and it is the force between the charges that hold the atom together.
 Charge comes in both negative (electrons) and positive
(protons) and each carry one unit of charge .
 Normally objects have zero net charge but it is possible for an object to have charge which is a multiple of the unit charge either positive or negative.
 It is quite easy to separate electrons from atoms.
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 Almost every action in our everyday lives is due to the electromagnetic force caused by electric charges.
 Everything that is not due to gravity is electromagnetism.
 All transmission and use of electric power is due to the flow of electrons through wires.
 A camera flash is an example of separating positive and negative charge, which requires work. This work is then released as energy by letting the charges combine (lightning)
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If there is a separation of charge between two objects there will be a very strong attractive force which will transfer charge until the objects are neutral
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Charge only occurs as a multiple of the unit charge on an electron or proton which is 1.6 x 10 -19 coulombs
So any charge q is an integer multiple
And N can be positive or negative q = N x 1.6 x 10 -19 coulombs
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Opposite charges attract
Same sign charges repel
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Coulomb’s Law
F = kq
1 q
2
/r 2 (k = 9 x 10 9 N.m
2 /C 2 )
The force between two charges of one coulomb each separated by 1 meter is
9 x 10 9 Newtons (your weight ~ 1000N)
The gravitational force between two 1 kg masses 1 meter apart is 6.67 x 10 -11 Newtons a difference of ~ 10 20 .
F r
F F
To separate a negative charge from a positive charge requires work which is stored as potential energy. When the two charges are released this energy will be liberated as kinetic energy
Held in place
F
F ext
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In order to separate electrons from atoms requires work and frictional forces are sufficient to do this.
Simple actions like rubbing two objects together or walking across a carpet will often result in a transfer of charge.
This transfer is caused by stripping electrons from the atom since every atom has a few electrons which are weakly bound.
Electrons can also be more easily moved since
(mass electron)/(mass proton) ~ 1/2000
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 There are two broad classes of substances related to electrical phenomena.
Conductors – electrons are free to move
Insulators – electrons are not free to move
 If we bring a charged object near a neutral object the electrons and the protons will feel the force and in a conductor the charges will separate.
 Water is a very good conductor so for example we usually only see static sparks in the winter when the air is very dry and objects can hold any excess charge that accumulates .
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5A-01 Static Electricity
Demonstration that positive and negative charge exists
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What information does the electroscope provide ?
Rubbing the hard rubber rod with fur produces a negative charge on the rod
Rubbing the glass with silk produces a positive charge on the glass
If we bring a charged rod close to the electroscope it will repel opposite charge and the moving arm will be displaced. If we touch the electroscope with a charged rod then charge will be transferred
THE ELECTROSCOPE CAN’T DETERMINE THE SIGN OF THE CHARGE BUT
IT DOES SHOW CHARGE EXISTS IN TWO KINDS + AND -
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5A-03 Two-by-Four Electroscope
Showing the strength of the electromagnetic force.
How does the charged ebonite rod move the board ?
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Rubbing the rod produces a surface charge. When it is brought close to the wood it attracts the opposite sign charge in the wood and there is an attractive force. This is the same effect as “static cling”
THE GRAVITATIONAL FORCE IS MUCH WEAKER THAN THE ELECTRICAL
FORCE. THE RATIO OF THE TWO FORCES IS ~10 -39 .
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The difference between conductors and insulators
What happens to the balls ?
The apparatus with the dome produces electric charge.
If the rods connecting the balls are conducting the balls become charged and repel eachother
If the rods are insulators the balls remain uncharged
All materials contain equal amounts of positive and negative charge. In conductors charge, generally electrons, is free to move. In insulators the charges on the constituent atoms or molecules is tightly bound and does not move. Dry air is a very good insulator but water is a very good conductor and damp air is also a good conductor. Even insulators can
“break down” for example lightning.
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Using an electroscope to see the effects of charge
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How do we see charge ?
When the rod is brought close it attracts negative charge meaning the pivot rod becomes positively charged.
If the top plate is now touched negative charge flows out and the whole electroscope becomes positively charged.
The movement of charge, usually electrons, is called a current and all electrical systems work using currents.
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 Any distribution of charges will produce a force field. That is if a test charge q is placed anywhere it will feel a force.
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 By definition the test charge is positive and both the electric field and the force are in the same direction
 The electric field at a point tells you what force a charge +q would feel both in magnitude and direction.
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