Electric Fields and
Electric Potential
Electric Fields
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Electric Field – A region in space around a
charged object in which a stationary charged
object experiences an electrical force because of
its charge.
E = F/q
F is the electrostatic force
q is the charge on the small test particle.
F is a vector therefore E is also.
Cont’d
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The field is basically the force acting on a test
charge based on the size of that test charge
Units are therefore N/C
The direction of the field is the direction of the
force on a small positive test charge.
Must be small so as not to move the charged
object from its original location.
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The arrows show the direction of the field
and the length shows the magnitude. A +
object will repel a positive test and a –
object will attract a positive test.
Alternative View of E
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Electric field lines can be drawn to
represent an electric field.
Electric field lines NEVER cross each other.
The arrows show direction and the distance
between the lines shows the magnitude.
Always away from a positive charge and
toward a negative charge.
Charged Plates
The electric field
between
two plates is uniform.
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Field still in the direction
of positive to negative.
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Electric Potential Energy
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When two unlike charges are moved away
from each other, work must be done
This work takes the form of stored potential
energy.
This work done in an electric field to move
a charge is called the Electric Potential
Difference.
Electric Potential Difference
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Electric Potential is also known as a Volt
(V)
V = W/q
Other units??
V for like charges is positive because work
is done to bring the charges together.
V for unlike charges in the same situation
would be negative.
Cont’d
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A car battery has a potential difference of 12 volts
from the positive terminal to the negative terminal.
AKA 12 joules/Coulomb
For every Coulomb of charge that passes to the
other terminal 12 joules of work are done on the
surroundings.
You alternator does work on the battery to move
those charges back hence storing them as
potential energy again.
The Electron Volt
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The energy gained by an electron or proton
as it passes through a potential difference
of 1V.
V = PE/q
1J/C = PE/ 1.6x10-19
So PE equals 1.6x10-19J or 1eV
More on this in modern physics
Electric Fields and Electric Potential
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E = F/q
Multiply each side by ∆d
E∆d = F∆d/q
E∆d = W/q but
W/q = V so
E∆d = V or
E = V/∆d
Cont’d
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E = V/∆d
The electric field in a given point in space is
equal to the rate at which the potential
difference changes over distance.
Equipotential Lines
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Just as electric field lines represent E
around a charge, equipotential lines
represent the electric potential around a
charge.
Cont’d
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Equipotential lines are ALWAYS
PERPENDICULAR to the E field lines.
The potential difference between any two
points on an equipotential line is zero.
No work needs to be done to move a
charge between these two points
Millikan’s Oil Drop
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Oil passed through an atomizer is given a net
charge from friction. When between two
charged plates, the oil drop would move toward
the positively charged plate.
Findings
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By adjusting E the oil drop would float.
By finding the weight of the oil drop, he was
able to find the electrostatic force.
He would ionize the air using x-rays and
could remove electrons from the oil drops.
The change in the the charge was always a
multiple of 1.6x10-19C