Electric Potential Energy
Electric Potential and Parallel Plates
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http://hyperphysics.phy­astr.gsu.edu/hbase/electric/vandeg.html
The Van de Graaff generator is an impressive electrostatic generator that is capable of producing enormously large static electric potentials. In fact, giant Van de Graaff generators can produce miliions of volts leading to awesome displays of corona and lightning. More modest "class room" sized Van de Graaff generators typically produce 100,000 V to 500,000 V.
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Consider out previous work with potential energy
Fapp
Δd
We know from previous studies, that work
is done when we move an object in the direction of the force
Mass
W = FgΔd
Fg
In fact in this case we would change the gravitational potential of the book from 0J
to a value of
ΔEp = FΔd
How about we consider this from an electrical point charge of view
Work certainly would be required to
"work" if we wanted to move the positive
charge away from the negative charge
Electric potential energy is the energy stored in the the system of two charges a certain distance apart. Electric potential Energy equals the work done to move a small charge:
THINK
When is the monkey working?
Can you think of other situations that would involve work?
Example #1
Moving a small charge from one position in an electric field
to another position requires 3.2 x 10­19 J of work. How much
potential energy will be gainedby the charge?
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Electric Potential
Suppose two positive charges are pushed towards positive plate
+
+
+
+
+
wire
Twice as much work is done, and twice as much electric potential energy is stored in the system. However, just as much electrical
potential energy is still stored per charge.
Storing 10 J of energy in 1 charge
is the same as storing 5 J in 2 charges
It is often convenient to consider just the electric
potential energy per unit charge at a location
We have a new term for this type of energy
Electric Potential or Voltage
The charge in electric potential energy stored pre unit charge
change in electric potential energy
Electric potential = V = ΔE q
charge
V = W
q
b/c work is energy
Example #2
Moving a small charge of 1.6 x 10­19 C between two parallel plates
increases its electrical potential energy by 3.2 x 10­16 J. Determine
the electrical potential difference between the two plates. 4
With Your partner try to solve these two questions
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Electric Potential Difference
When a charge moves from one location to another, it experiences a change in electric potential between the two points. This change is referred to as an electric potential difference
ΔV = Vf ­ Vi
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Electric Field between Parallel Plates
The field lines between two large, flat, parallel, oppositely charged conducting plates are perpendicular to the plates:
The Electric field between plates is UNIFORM No matter where you place a charge the electric field will be the same numerical value!
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Calculating Electric Field Strength between parallel plates:
In the case of parallel plates having a potential difference, V, separated by a distance, d, the field strength |E| is
V
|E| = Δ
d
OR
Example Calculate the electric field strength between two parallel plates that are 6.00 x 10­2 m apart and the potential difference on the plates is 12.0 V
Charged parallel plates
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BONUS PROJECT
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Example
To calculate the magnitude of the electric field between the plates,
use the equation
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Consider this example
A balloon is rubbed with with fur. The balloon aquires
an electric potential of a few thousand volts. In other words
the electric potential stored per coulomb of charge on the balloon
is a few thousand volts
p
written as V = Δq
Check and Reflect Section
11.2
Now suppose the balloon were to gain 1 C of charge during rubbing
In order for the voltage to stay the same, a few thousand joules of work would be needed to produce the electrical energy that would allow the balloon to maintain a constant voltage
Section 11.2
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