Transcript Electric Potential - K

Electric Potential
SPH4U – Grade 12 Physics
Unit 1
Quick Review Mini Quiz



The symbol

Field lines show the direction of the electric force on a
positive/negative test charge placed at every point in
the field.

Electric field strength is measured in: _____

The electric field between two parallel plates of charge is
uniform/non-uniform and parallel/perpendicular to the
plates.
stands for: _______________
Quick Review Mini Quiz



The symbol

Field lines show the direction of the electric force on a
positive/negative test charge placed at every point in
the field.

Electric field strength is measured in: N/C

The electric field between two parallel plates of charge is
uniform/non-uniform and parallel/perpendicular to the
plates.
stands for: Electric Field
Electric Potential Videos
by Derek Owens

Electricity concepts can be difficult for us to wrap our
minds around. Today we will watch a few videos by
Derek Owens to get more of a visual concept of what is
happening.

It is probably a good idea to take notes of these videos,
because it might help your understanding. All of these
video links will be posted to our Wiki-classroom for you
to view again later.
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http://www.youtube.com/watch?v=wT9AsY79f1k
http://www.youtube.com/watch?v=LkIai_KXGxg
http://www.youtube.com/watch?v=NrpFMyVLSck
http://www.youtube.com/watch?v=A9TDzsnYkHo
Electric Potential and Fields

Recall: An electric field is defined as the region
in which a force is exerted on an electric charge.

We know that a charged particle will feel a force
if it is in the presence of an electric field (either
repulsion or attraction). The magnitude of this

force is given by: F  q
E
Electric Potential and Fields
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Several things are implied by this:
 If there is a net force acting on a charge, that charge
must be accelerating. This is due to Newton’s
second law.
 If there is a force being exerted on the charge over

some distance, then by W  Fd we know that
work is done on the charge.
 If work is being done on the charge, the charges must
be given energy, because energy is the ability to do
work.
Electric Potential and Fields

Let us define electric potential energy EE as the
energy stored in a system of two charges a
distance d apart.

We can also define EE as the energy stored in
an electric field that can do work on a positively
charged particle.
Electric Potential and Fields
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We also know that if the electric force does an
amount W of work on a charged particle, the
change in the electric potential energy is
EE  W
EE  FE d
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Combining everything we get:
EE  qd
Electric Potential and Fields
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This gives us the change in the potential energy
as the charge moves through a displacement d
in a region where the electric field is parallel to
the displacement. Note that the path taken does
not effect E .It is dependent on the starting
E
and ending locations only.
Electric Potential and Fields

For ΔEE > 0, work is done against the field. (W). This results in energy stored in the field.
Electric Potential and Fields

For ΔEE < 0, work is done by the electric field
(+W) on the particle, which typically increases
the potential energy of the particle.
Example 1

An electron enters a uniform electric field of
145N/C pointed toward the right. The point of
entry is 1.5m to the right of a given mark, and
the point where the electron leaves the field is
4.6m to the right of that mark.
(a) Determine the change in the electric potential
energy of the electron
(b) The initial speed of the electron was 1.7 x 107m/s
when it entered the electric field. Determine its final
speed.
Example 1

An electron enters a uniform electric field of
145N/C pointed toward the right. The point of
entry is 1.5m to the right of a given mark, and
the point where the electron leaves the field is
4.6m to the right of that mark.
(a) Determine the change in the electric potential
energy of the electron (Ans: 7.2 x 10-17 J)
(b) The initial speed of the electron was 1.7 x 107m/s
when it entered the electric field. Determine its final
speed. (Ans: 1.1 x 107 m/s)
Electric Potential

Electric Potential (V) is the value in volts of the
potential energy EE per unit positive charge for a
given point in an electric field.
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Electric potential is also a measure of how much
electric potential energy is associated with a
specific quantity of charge at a particular location
in an electric field.
Electric Potential
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Electric Potential is measured in volts.
1V = 1J/C.

Electric Potential is calculated using the
equation:
EE
V 
V = Electric Potential (Volts)
q
EE = Electric potential Energy
(Joules)
q = charge (Coulombs)
Electric Potential
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Electric Potential difference ΔV is the amount
of work required per unit charge to move a
positive charge from one point to another in the
presence of an electric field. It is often referred
to as “voltage”.
E E
It is given by the equation: V 
q
Electric Potential
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If we have a uniform electric field ε, we can also
use the following equation for an electric
potential difference: V  d
Electric Potential
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Rearranging this formula, we see how a nonuniform electric field must depend on the electric
potential difference and the change in position in
the field:
 V

d
Electric Potential
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The equation tells us that the electric field is
largest in regions where V is large and changes
rapidly with small changes in displacement.
The electric field is zero in regions where V is
constant.
Also, because of the negative sign in the
equation, the electric field points from regions of
high potential to regions of low potential.
Example 2

An old cathode ray tube creates a potential
difference of 1.6 x 104V across the parallel
accelerating plates. These plates accelerate a
beam of electrons toward the target phosphor
screen. The separation between the plates is
12cm. Calculate the magnitude of the electric
field.
Example 2

An old cathode ray tube creates a potential
difference of 1.6 x 104V across the parallel
accelerating plates. These plates accelerate a
beam of electrons toward the target phosphor
screen. The separation between the plates is
12cm. Calculate the magnitude of the electric
field. (Ans: 1.3 x 105 N/C).
Homework
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Read Sections 7.4 & 7.5
 Make
additional notes to supplement the
lesson notes.

Complete the following questions:
 Pg.
354 # 1, 2, 4, 5