Download Electric Fields and Potential

E-Fields and Potential
Electric Fields and Potential
Electric Fields
The space around every charged object is filled with an Electric
Field.
This is a very abstract concept and one that is very difficult to
understand. Electric Fields can’t be seen so there is only one way
to prove that they exist that is to put a test charge in the field and
observe what happens to it. A test charge is a very small positive
charge that is put into the electric field of an object.
Question:
Why is it very important that this test charge is very small?
A:
When a cold thermometer is put into warm liquid what happens?
These two situations are exactly the same. If a cold object is put
into a warm liquid the liquid becomes colder. The same thing is
true for the test charge; if the test charge is large then it will affect
the field that it is trying to detect. The measure of the strength of
this field is in N/C.
Formula: Electric Field =
E
=
/
/
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E-Fields and Potential
Question:
1. What is the Electric Field that effects a 2 C test charge that is
affected by a 500 N force?
Visualizing Electric Fields
There is a specific orientation of each a positive and a negative
charge. Below draw the orientation of a positive test charge.
Draw in the orientation of a negative object?
__
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E-Fields and Potential
Now draw in what you think the Electric field will be between the
positive and negative charge below.
What is the Electric Field between two positive charges?
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E-Fields and Potential
On perfectly circular object the charges are distributed evenly
around the outside of the object. When the object is not perfectly
circular, there is an uneven distribution of charges on it. Draw in
the charges on the positively charged objects below.
**Notes on Charge distribution**
 Charges concentrate at the pointy parts of an object.
 The straighter the edge the less charge accumulates there.
 The tighter the curve the more charge accumulates there.
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A very good example of field lines is parallel plates with opposite
charges.
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How about a plate and a hollow disk.
**Please notice that there is not a field inside the circle.**
Electric Shielding
The electric field inside a conductor is usually zero. This is true
because on a conductor there is the possibility for the negative
electrons to move around the surface freely. This means that the
test charge in the middle of the circle has forces pulling it in all
directions so that the net force on the test charge is zero.
Because of this ability of the electrons to spread out on a
conductor, a person inside a car that is struck by lightning is safe.
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E-Fields and Potential
Electric Potential Energy
This is a very similar idea to that of potential energy. When
dealing with potential energy the farther the object separated from
the earth, the greater the potential energy is. When dealing with
Electric Potential Energy, the separation distance is the
determining factor for the size of the electric potential energy.
a)
b)
F
Exert a force on the small charge in the first picture and increase
the electric potential energy by doing work against the electric
field of the larger charge.
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E-Fields and Potential
Electric Potential Energy-The energy that a charge possesses by
virtue of its location.
Electric Potential
In the previous section, we dealt with the electric potential energy
of a group of charges in one place relative to another. In this
section we will find it better to deal with electric potential energy
in terms of electric potential energy per charge.
Electric Potential - Electric potential energy per charge.
Electric Potential = ________________/_____________
The units for electric potential are the volt.
1volt = 1 _____/ _____
Since electric potential is measured in volts, it is commonly known
as voltage.
Electrical Energy Storage
Electrical energy is stored in a device known as a Capacitor.
A simple capacitor is a very simple design. It is two conductive
plates that are evenly spaced a short distance away from each
other. Capacitors are used in many different electrical devices.
Computer motherboards, and on/off switches in TV’s are just a
couple of examples. The way that a capacitor works is when the 2
different parallel plates are hooked up to different voltages one
becomes more negative and the other one becomes more positive.
This difference is what causes the charge to be stored. A capacitor
is discharged when there is a conducting path between the two
conducting plates. This is when people can get hurt. The larger the
plates, smaller the distance, and greater the voltage difference all
put together determine the amount of charge that a capacitor can
hold. In a TV set, the capacitor that controls the on/off switch can
produce massive amounts of charge even when they are turned off.
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E-Fields and Potential
Question:
What is the electric field between the two conducting plates of a
capacitor.
Van de Graff Generator
This is the device that we used to have the hair on your head stand
on its end. Van de Graff generators can build up a very large
electric potential, around 1 million volts for a generator. Below
draw a simple diagram of how a Van de Graaff works, and how it
effects a string attached by tape to the top of the generator.
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