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10th week Lectures
March 20. 2017. Chapter 12
Electric charge.
Conductors and Insulators
Coulomb law
Electric field
Electric Potential
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Physics 214 Spring 2017
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Electric charge
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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Electromagnetism
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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Positive and negative
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
Opposite charges attract
Same sign charges repel
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Coulomb’s Law
Coulomb’s Law
The force between two charges of one
coulomb each separated by 1 meter is
9 x 109 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 ~ 1020.
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
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Physics 214 Spring 2017
r
F
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
F
F
F
Fext
Held in place
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Everyday charge
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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Conductors and insulators
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
++++++++
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 3/20/2017
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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 ?
++++++++++
-------------
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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5 A 04 Conductors and insulators
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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5 A 06 Static electric charge and induction
Using an electroscope to see the effects of charge
++++++++++++
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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Electric field
Any distribution of charges will produce a
force field. That is if a test charge q is placed
anywhere it will feel a force.
Electric field E = F/q0
or F = q0E
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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Electric field
Any distribution of charges will produce a
force field. That is if a test charge q is placed
anywhere it will feel a force. This force acts at a
distance.
Charges do not have to be in contact. Electric
charges modify the space surrounding them
Electric field E = F/q0
or F = q0E
This concept is called electric field just like
Newton introduced the gravitational field.
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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Electric potential
Let us produce a uniform electric field E
Separate two conductors by insulating
material like air. Let us charge the two
conducting plates with uniformly distributed
opposite charges +q and –q. This device is
called a capacitor.
Just like a potential energy is associated with
gravitational interaction or with a compressed
spring, the test charge experiences an
external force in the direction of the electric
field. If we apply an external force to move the
charge in the opposite direction, it increases
the potential energy of the charge. This
process is similar to what happens when we
lift an object against the gravitational force.
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Electric potential
Electric potential difference is the change
in potential energy per positive charge.
It is the work done or energy released in
moving unit charge in an electric field.
The change in electric potential is equal to
the change in electrostatic energy per unit
of positive charge
The unit is
ΔV = ΔPotential Energy/q (joules/coulomb
= volt)
In a battery charge is separated and
potential energy is stored. When it is
connected in a circuit, charge flows and the
energy is released.
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Voltage
In a uniform field the work done in
moving a charge q a distance d
W = qEd = ΔPotential Energy
and the voltage difference is
ΔV = ΔPotential Energy/q = Ed
So if a positive charge q is moved
toward the positive plate the voltage
increases.
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Basic Force
Two point particles with electric charge
feel a force and the force between two
charges q1q2 is
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
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F
r
F
F F
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Work and Energy
F
Objects with electric charge
also have mass and everything
we have done on work and
energy is the same. In fact
when I push an object across
the floor the force is
electromagnetic.
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Physics 214 Spring 2017
d
F =ma
W =Fd
KE = 1/2mv2
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5A-10 Motion in an Electric Field
The effects of transferring charge
+
-
+
-
What is the
movement of
the balls ?
THE BALL IS ATTRACTED TO ONE TERMINAL THEN RECEIVES A
CHARGE AND THEN IS REPELLED TO THE OTHER TERMINAL, WHERE
IT PICKS UP THE OPPOSITE CHARGE AND IS REPELLED.
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5A-23 Electric Wind
The emittance of electrically charged particles from highly charged object
What causes
the arms to
turn ?
The metal arms are charged by an electrostatic generator and the forces
are greatest at the tips so charged particles are driven off by repulsion.
Conservation of momentum makes the arms turn in the “electric wind”
The “wind” can be indirectly seen by the extinguishing of a candle.
Before lighting strikes there is charge build up and lightning rod
conductors (invented by Benjamin Franklin) have sharp tips to
“attract” the lightning. The sun also has large electric and magnetic
fields and emits the “solar wind”
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