Review

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(No Transcript)
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Review
You have two neutral pieces of tape stuck
together. You rip them apart and notice that each
one is charged (because each one is attracted to
your neutral hand). Without doing an experiment,
we know that they MUST be oppositely charged. Why?
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Electrostatic Force
The observation that like charged particles
repel and unlike charged particles attract
comes from experiment. Its not a memorized fact
nor a definition.
Experiment shows that for two charged particles
if the particles are in a vacuum
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Coulombs Law
q2
Fon 2
Fon 1
q1
The electrostatic force is attractive (toward
each other) for like charges.
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Coulombs Law
Fon 2
q2
Fon 1
The electrostatic force is repulsive (away from
each other) for unlike charges.
q1
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Comparison with Gravitational Force
m1
m2
q1
q2
OR
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At the atomic scale, the electrostatic force
dominates interactions of charged particles
The mass of a proton is 1.7 x 10-27 kg. The mass
of an electron is 9.1 x 10-31 kg. (Note a proton
is roughly 2000 times more massive than an
electron.) Compare the gravitational force and
electrostatic force of a proton on an electron if
they are 0.5 x 10-10 m apart.
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Poll
Two positively charged spheres exert a force of
0.1 N on each other as shown below. What is the
direction of the electrostatic force on Sphere A
(due to Sphere B)?
qA
qB
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Poll
Sphere A has a charge 1 ?C. Sphere B has a charge
10 ?C. On which sphere is the electrostatic force
the largest (in magnitude)?
qA
qB

1. Sphere A
2. Sphere B
3. Neither, because the force on each sphere is the
   same (in magnitude)

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Poll
Given a small positive charge and a large
negative charge, which figure best represents the
forces that the charges place on each other?
1)
4)
2)
5)
3)
6)
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Poll
An assembly is composed of a positive and a
negative charge of equal magnitude connected by a
rigid rod. The assembly is placed near another
negative charge. What is the direction of the net
force on the assembly?
1) The net force is directed toward the
right. 2) The net force is directed toward the
left. 3) The net force is zero.
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Superposition of Forces in 1-D

• If multiple charges are present, the net force on
  any one charge is equal to the vector sum of the
  forces due to each of the other charges.
• If the charges all lie in a straight line
  (x-axis), all of the forces will act along the
  or x-axis.
• Use sign for forces acting in positive
  x-direction.
• Use sign for forces acting in negative
  x-direction.
• Net force will simply be the sum of all forces
  and will either be in the or x-direction.

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Example
Q1 7.5mC
Q2 4.0mC
Q3 -4.0mC
0.20 m
0.20 m
What is the net force on Q1 due to Q2 and Q3?
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Superposition of Forces in 2-D

• If the charges do not lie in a straight line then
  all of the forces must be resolved into x and y
  components before addition.
• Choose orientation of x and y axes.
• Resolve each force into x and y-components.
• Use and signs to indicate whether each
  component is in the positive or negative x (or y)
  direction.
• The x-component of the net force will be the sum
  of the x-components of all the forces.
• The y-component of the net force will be the sum
  of the y-components of all the forces.
• Find magnitude and direction of net force from
  Pythagorean Theorem and tanq(Fy/Fx). (Note q is
  the angle between Fnet and the x-axis.)

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Example
What is the net force on Q1 due to Q2 and Q3?
Q1 -4.5mC
Q3 -8.2mC
Q2 5.0mC
Q1
0.50 m
Q2
Q3
0.50 m
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Electric Field
How does q2 know that q1 is there?
q1
q2
q1 creates an electric field everywhere in space
around it. This electric field exerts a force on
a charged particle.
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Electric field due to a positively charged
particle.
The electric field due to a positively charged
particle points away from the particle and
decreases with distance squared.
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Electric field due to a negatively charged
particle.
The electric field due to a negatively charged
particle points toward the particle and decreases
with distance squared.
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Force on a charged particle by an electric field
The force by an electric field on a charged
particle is
q
If q is positive, then the force on the particle
is parallel to the electric field. If q is
negative, then the force on the particle is
opposite (i.e. antiparallel) to the electric
field.
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Poll
The electric ?eld at some location and instant of
time is in the y direction. In what direction is
the force on a proton at this same location?
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Poll
The electric ?eld at some location and instant of
time is in the y direction. In what direction is
the force on an electron at this same location?
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Poll
The electric ?eld at some location and instant of
time is in the y direction. Which is larger, the
force on a proton or the force on an alpha
particle (i.e. helium nucleus with two protons)
at this same location?
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Poll
The electric ?eld at some location and instant of
time is in the y direction.
1
If this electric field is CREATED by a positive
charge, q, where might that charge be located?
4
2
3
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Poll
The electric ?eld at some location and instant of
time is in the y direction.
1
If this electric field is CREATED by a negative
charge, -q, where might that charge be located?
4
2
3
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Poll
A proton placed at location A experiences an
electric force in the direction shown below. What
is the direction of the electric field at
location A?
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Poll
At location C there is an electric field in the
direction indicated, due to charges not shown. A
chloride ion (Cl-) is placed at location C. What
is the direction of the electric force on the
chloride ion?
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Poll
At location D there is an electric field in the
direction indicated, due to charges not shown. An
electron is placed at location D. What is the
direction of the electric force on the electron?
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Poll
An electron is placed at location A. What is the
direction of the electric field at location B,
due to the electron?
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Poll
A proton is placed at location B. You have
calculated the electric field at location A, due
to the proton. To draw an arrow representing the
electric field you calculated, what should you do?

1. Put the tail at point A.
2. Put the tail at point B.
3. Put the tip at point A.
4. Put the tip at point B.

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Superposition
If there are many charged particles, then the
electric field at a point in space is the sum of
the electric fields created by all of the charged
particles independently.
M
P
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Poll
What is the direction of the net electric field
at point P due to the dipole?
P
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Poll
What is the direction of the net electric field
at point P due to the dipole?
P