Electric Fields
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Transcript Electric Fields
21.2 Applications of Electric Field
If you do work to lift a ball against gravity the
PE of the ball will increase.
The same works with charges.
How do you get unlike charges apart?
The farther apart the charges are moved the
more work that is done and more energy is
gained.
Electric Potential Difference, V, is defined as
the work done in moving a positive test charge
between two points in the electric field.
V
Wonq'
q'
The difference in electrical potential is the ratio of
the work needed to move a charge to the strength
of that charge.
Units of volt (J/C = V)
Equipotential positions
Equipotential lines are similar to altitude lines
on a map
The Electric Potential in a
Uniform Field
Use 2 plates, one charged
positive and one negative
The electric field is constant
between the plates and the
direction is from positive to
negative
Place a positive test charge,
q’ in the field and move it a
distance d towards the
positive plate
Won q’ = Fd
Won q’ = Vq’
So, V=Fd/q’=(F/q’)d
V=Ed
E=V/d
So the units for E can be either N/C or V/m
The electric potential increases in the
direction opposite of the electric field
direction
The electric potential is higher near the
positively charged plate
By dimensional analysis the product of the
units of E and d is (N/C)(m)
N.m is a J (Joule)
So 1 V = 1J/C
Millikan’s Oil-Drop Experiment
Used to find the charge on an electron
A fine mist was sprayed into and Electric
Field
Gravity causes the drops to fall
Field increased until drops rise
Field adjusted until drops are suspended
Downward force of gravity equals upward
electric force
Magnitude of E determined form the electric
potential difference
Weight of electron had to be determined
Drop was suspended and then the electric
field was turned off so the drop could fall
Because friction of tiny drop so large terminal
velocity was reached quickly
Using a complex equation the mass was
found
Then using mg the weight was calculated.
Sharing of Charge
All systems come to equilibrium to make the
energy of the system at minimum.
Ball on a hill will end up resting in a valley
If one object is charge and comes in contact
with an uncharged object the charges with
spread out evenly across both objects.
If objects are not of same size then the
charges equal out until voltage is the same
Charges are closer together at sharper
points
Storing Charges: The Capacitor
Lift a book to a shelf and you increase GPE.
In a sense you are storing GPE
A device that stores electric potential energy
is called a capacitor.
As a charge is added to an object the electric
potential difference between that object and
the Earth increases.
For a given situation the ratio of the charge
stored to the electric potential difference is a
constant
That ratio is called
the capacitance, C
The unit of
capacitance is a
farad, F
q
C
V
Farad
Farad, F, named after Michael Faraday
One Coulomb/volt
1 C is a very large charge so 1 F is also very
large
Often
F or pF
Uses
Storage of charge
Memory capacitors to
prevent losses of
memory in computers
Televisions have very
large capacitors
Power giant lasers
Small ones power a flash
of a camera