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DEVIL PHYSICS
THE BADDEST CLASS ON CAMPUS
IB PHYSICS
TSOKOS LESSON 5-2
ELECTRIC FIELD AND ELECTRIC
POTENTIAL
IB Assessment Statements
 Electric Potential Difference
 5.1.1. Define electric potential difference.
 5.1.2. Determine the change in potential energy
when a charge moves between two points at
different potentials.
 5.1.3. Define the electronvolt.
 5.1.4. Solve problems involving electric potential
difference.
IB Assessment Statements
 Electric Current and Resistance
 5.1.5. Define electric current.
 5.1.6. Define resistance.
 5.1.7. Apply the equation for resistance in the
form where ρ is the resistivity of the material
of the resistor.
IB Assessment Statements
 Electric Current and Resistance
 5.1.8. State Ohm’s Law.
 5.1.9. Compare ohmic and non-ohmic
behavior.
 5.1.10. Derive and apply expressions for
electrical power dissipation in resistors.
 5.1.11. Solve problems involving potential
difference, current and resistance.
Introductory Video: Electric
Fields and Potential
Review
 In 5.1 we learned:
 There is a force between electric charges and
that vector methods must be used to find
the net force on a given charge.
 How to charge by friction and by induction
and the difference between the two.
Review
 In 5.1 we learned:
 What an electroscope is and how to use it.
 That the charge in a conductor resides on
the outside surface of the conductor and
that the net charge on the interior of the
conductor is zero.
Review
 In 5.1 we learned:
 The formula for the electric force between
two point charges is
 Or,
1
Q1Q2
F=
40 r 2
Q1Q2
F=k 2
r
 where
k=
1
40
 8.99 x10 N  m
9
2
C
2
Objectives
 Appreciate that a charge q in an electric
field of magnitude E will experience a
force of magnitude
F  qE
Objectives
 Understand that the electric field of a
point or spherical charge Q a distance r
away has a magnitude of given by
Q
Ep  k 2
r
and is radial in direction. The field is zero
inside the charged conductor
Objectives
 Understand that the electric field inside
parallel plates is uniform and its
magnitude is given by
V
E
d
Objectives
 Understand that the work done in
moving a charge q across a potential
difference V is
W  qV
Objectives
 Understand that a charge q that is at a
point where there is potential V will
have an electric potential energy of
U  qV
Objectives
 Understand that a charge moving in an
electric potential satisfies the law of
conservation of energy,
1 2
1 2
mvA  qVA  mvB  qVB
2
2
Electric Field
 An electric field exists around any
charged object and extends/radiates
either into or out of the object
 By convention, charge flows from positive to
negative so,
 For a positively charged object, the field lines
extend outward
Electric Field
 For a positively charged object, the field
lines extend outward
+
-
 For a negatively charged object, the field
lines extend inward
Electric Field
 The field does not “exist” unless shown
to exist by a charge
 We use a small positive test charge, q, to
determine if a field exists – bring the test
charge close and if it experiences a force,
then a field exists
Electric Field
 Electric field is defined as the force per unit
charge experienced by a small positive test
charge, q,

 F
E
q
F  qE
+
The electric field is a vector with direction
being the same as the force a positive
charge would experience at the given point
Electric Field
 Units for electric field is N/C

 F
E
q
F  qE
Electric Field
 The electric field from
a single point charge,
Q, at a point a
distance r away is

 F
E
q
F  qE
Q1q
F k 2
r
Q1q
qE  k 2
r
Q
Ep  k 2
r
Electric Field
 Likewise, the charge on the surface of
a spherical conductor is given by
Q
Ep  k 2
R
where R is the radius of the sphere.
Inside the conductor the field is zero
Electric Field Lines
 For a positively charged object, the field
lines extend outward
+
 For a negatively charged object, the field
lines extend inward
Electric Field Lines
 A convention for drawing field lines is
that the number of field lines should be
proportional to the strength of the
charge
 The stronger the electric field, the closer
the field lines are to each other
Electric Field Lines
 Field lines will flow toward opposite
charges and away from like charges. For
example, two equal and opposite
charges:
Electric Field Lines
 Two unequal but opposite charges:
Electric Field Lines
 Two equal, but like (+) charges
Electric Field Lines
 Uniform Electric Field exists when the
field has a constant magnitude and
direction such as that generated by two
oppositely charged parallel plates.
Electric Field Lines
 The field lines at the edges begin to curve
 The field is uniform if the length of the field
is large compared to the distance between
the plates
Electric Potential
 Consider an electric field and a positive
test charge q
 In order to move the charge from its
equilibrium position, work must be done
Electric Potential
 If held in that new position, the test
charge now has potential energy like a
compressed spring because it wants to
go back to its equilibrium position
Electric Potential
 “V” is the electric potential and is
defined in terms of the work, W, needed
to bring a positive test charge, q, from
very far away to a position close to the
charged body
Remember that work is
based on displacement
and not distance
travelled!
W
V
q
qV  W
Electric Potential
 The unit of potential is:
1V = 1J/1C
 The potential energy, U, is:
U = qV
 The unit of potential energy is:
(1C) x (1J/1C) = 1J
Potential Difference
 The amount of work
needed to move a
test charge from one
point to another is
equal to the change
in potential energy
of the charge
 Just like gravity
W  U
W  U B U A
W  qVB  qV A
W  q VB  V A 
Charge Moving In A Region of
Electric Potential
 A point charge moving in a region of an
electric potential will have kinetic energy due
to its mass and velocity
 It also has potential energy due to the electric
potential
 As it moves through the region
the potential impacts the
velocity as potential changes
Charge Moving In A Region of
Electric Potential
 Law of Conservation of Energy states that the
sum of the potential energy and kinetic
energy at one point must equal the sum of
the potential energy and kinetic energy at
any other point
1 2
1 2
mvA  qVA  mvB  qVB
2
2
Electric Field between
parallel plates
 The electric field, E, between two parallel
plates is equal to the potential difference
between the plates, V, divided by the
distance between the plates, d
 Note that E is the electric field – E does not stand
for energy!
V
E
d
How Does It Fall?
 Consider a positively
charged metal sphere
with some mass m
falling vertically through
an electric potential
between two plates
 What is the direction of
motion if it starts at point
P?
 What is the direction of
motion if it starts at point
Q?
Electronvolt
 Atomic physics deals with extremely small
amounts of energy where the Joule is not really
appropriate
 The electronvolt, eV, is equal to the work done
when the charge on one electron is moved across
a potential difference of 1 volt
W  qV

1eV  1.6 x10
1eV  1.6 x10
19
19

C 1V 
J
Objectives
 Appreciate that a charge q in an electric
field of magnitude E will experience a
force of magnitude
F  qE
Objectives
 Understand that the electric field of a
point or spherical charge Q a distance r
away has a magnitude of given by
Q
Ep  k 2
r
and is radial in direction. The field is zero
inside the charged conductor
Objectives
 Understand that the electric field inside
parallel plates is uniform and its
magnitude is given by
V
E
d
Objectives
 Understand that the work done in
moving a charge q across a potential
difference V is
W  qV
Objectives
 Understand that a charge q that is at a
point where there is potential V will have
an electric potential energy of
U  qV
Objectives
 Understand that a charge moving in an
electric potential satisfies the law of
conservation of energy,
1 2
1 2
mvA  qVA  mvB  qVB
2
2
IB Assessment Statements
 Electric Potential Difference
 5.1.1. Define electric potential difference.
 5.1.2. Determine the change in potential energy
when a charge moves between two points at
different potentials.
 5.1.3. Define the electronvolt.
 5.1.4. Solve problems involving electric potential
difference.
IB Assessment Statements
 Electric Current and Resistance
 5.1.5. Define electric current.
 5.1.6. Define resistance.
 5.1.7. Apply the equation for resistance in the
form where ρ is the resistivity of the material
of the resistor.
IB Assessment Statements
 Electric Current and Resistance
 5.1.8. State Ohm’s Law.
 5.1.9. Compare ohmic and non-ohmic
behavior.
 5.1.10. Derive and apply expressions for
electrical power dissipation in resistors.
 5.1.11. Solve problems involving potential
difference, current and resistance.
QUESTIONS?
Homework
#1-14