Download 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
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