Download Lecture 5.1:

Lecture 5.1:!
Review Session for Exam 1
Major Topics:!
Electric Charge!
Electric Force!
Electric Field!
Electric Potential!
Gauss’s Law!
!
Textbook Material:!
Ch. 25, 26, 27, 28
Feb. 10, 2015
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Announcements
!
•No Mastering Physics assignment this week…study!!
•Recitation on Wed. will be focused on exam review.!
•Exam #1 in class on Thursday.!
•Bring a calculator and your 1 sheet of notes.!
•I will provide values of fundamental constants…you
are responsible for writing any equations you don’t
think you can remember on your sheet of notes.!
•Seating assignment will be posted at the entrances to
Stolkin.!
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Last Week...
Potential energy shared by two point charges
Looks like Coulomb’s Law, but it’s different!
3
Last Week…
Since Energy is always conserved, a charged object that is gaining
Potential Energy must lose the same amount of Kinetic Energy.
V = Potential Difference, or Voltage.
U =q V
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Last Week...
A Positive charge increases in potential energy as it
approaches the positive side of a capacitor. Since energy is
conserved, its kinetic energy must simultaneously decrease.
Analogous to lifting an object above the earth to
increase its potential energy.
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Last Week…
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Last Week…
Equipotential Surfaces are
surfaces with the same
value of V at every point.!
!
Where are the
equipotentials in this
drawing?
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Potential of Point Charges
What is the potential at the point indicated?
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Exam #1: Overview
!
•Electric Charge is a property of matter!!
•Like charges repel, opposite charges attract.!
•We can describe interactions of charged objects in
terms of Forces (Coulomb’s Law) or Fields.!
•We can also consider energy, or potential, of a
system with electric charge(s)/fields.!
•For charge distributions with symmetry, Gauss’s
Law can greatly simplify the determination of the
Electric Field.!
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Clicker #1
A rod attracts a positively charged hanging ball.
The rod is
A.  Positive.
B.  Negative.
C.  Neutral.
D.  Either A or C.
E.  Either B or C.
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Clicker #2
Metal spheres 1 and 2 are touching. Both are initially neutral.
a.  The charged rod is brought near.
b.  The spheres are separated.
c.  The charged rod is then removed.
Afterward, the charges on the
sphere are:
A. 
B. 
C. 
D. 
E. 
Q1 is + and Q2 is +.
Q1 is + and Q2 is –.
Q1 is – and Q2 is +.
Q1 is – and Q2 is –.
Q1 is 0 and Q2 is 0.
Net charge is obtained if contact is
broken while the spheres are polarized.
This is charging by induction.
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Clicker #3
Which is the direction
of the net force on the
charge at the lower left?
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Clicker #4
Which is the direction
of the net force on the
charge at the top?
What about the E-field
direction at the position
of the charge at the top?
Given an arrangement of point charges, know how
to calculate various Force and Electric Field vectors!
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Clicker #5
The direction of the force
on charge –q is
A.  Up.
B.  Down.
C.  Left.
D.  Right.
–Q is slightly closer than +Q.
E.  The force on –q is zero.
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Clicker #6
At which point is the electric
field stronger?
A.  Point A.
B.  Point B.
C.  Not enough information to tell.
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Clicker #7
Which is the electric field at the dot?
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Clicker #8
When r >> d, the electric field
strength at the dot is
A.
B.
C.
D.
Looks like a point charge 4Q at the origin.
E.
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Clicker #9
Two protons, A and B, are
next to an infinite plane of
positive charge. Proton B is
twice as far from the plane
as proton A. Which proton
has the larger acceleration?
A.  Proton A.
B.  Proton B.
C.  Both have the same acceleration.
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Clicker #10
A proton is moving to the right in a
vertical electric field. A very short
time later, the proton s velocity is
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Clicker #11
The electric flux through the shaded surface is
A.  0.
N m2/C
2/C
B.  200 N m
m/C.
C.  400 N m2/C.
D.  Flux isn t defined
for an open surface.
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Clicker #12
The electric flux through the shaded surface is
A.  0.
N m2/C
2/C
B.  200 N m
m/C.
C.  400 N m2/C.
D.  Some other value.
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Clicker #13
The electric flux through the shaded surface is
A.  0. N m2/C
B.  400cos20° N m2/C.
C.  400cos70° N m2/C.
D.  400 N m2/C.
E.  Some other value.
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Clicker #14
These are cross sections of 3D closed surfaces. The top and
bottom surfaces, which are flat, are in front of and behind the
screen. The electric field is everywhere parallel to the screen.
Which closed surface or surfaces have zero electric flux?
A. 
B. 
C. 
D. 
E. 
Surface A.
Surface B.
Surface C.
Surfaces B and C .
All three surfaces.
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Clicker #15
Which spherical Gaussian surface
has the larger electric flux?
A.  Surface A.
B.  Surface B.
C.  They have the same flux.
D.  Not enough information to tell.
Flux depends only on the
enclosed charge, not the radius.
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Clicker #16
Spherical Gaussian surfaces of
equal radius R surround two
spheres of equal charge Q.
Which Gaussian surface has
the larger electric field?
A. 
B. 
C. 
D. 
Surface A.
Surface B.
They have the same electric field.
Not enough information to tell.
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Clicker #17
A spherical Gaussian surface
surrounds an electric dipole.
The net enclosed charge is
zero. Which is true?
A.  The electric field is
zero everywhere on
the Gaussian surface.
B.  The electric field is not
zero everywhere on the Gaussian surface.
C.  Whether or not the field is zero on the surface
depends on where the dipole is inside the sphere.
The flux is zero, but that doesn t require the field to be zero.
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Clicker #18
A cylindrical Gaussian surface
surrounds an infinite line of charge.
The flux Φe through the two flat
ends of the cylinder is
A.  0.
B.  2 × 2πrE.
C.  2 × πr2E.
D.  2 × rLE.
E.  It will require an integration to find out.
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Clicker #19
A cylindrical Gaussian surface
surrounds an infinite line of charge.
The flux Φe through the wall
of the cylinder is
A.  0.
B.  2πrLE
C.  πr2LE.
D.  rLE.
E.  It will require an integration to find out.
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Clicker #20
A point charge q is located distance r
from the center of a neutral metal
sphere. The electric field at the
center of the sphere is
A. 
.
B. 
.
C. 
.
D.  0.
E.  It depends on what the metal is.
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Clicker #21
Charge +3 nC is in a hollow cavity
inside a large chunk of metal that
is electrically neutral. The total
charge on the exterior surface
of the metal is
A.  0 nC.
B.  +3 nC.
C.  –3 nC.
D.  Can t say without knowing the shape
and location of the hollow cavity.
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Clicker #22
Two positive charges are
equal. Which has more
electric potential energy?
!
A.
B.
C.
D.
Charge A.
Charge B.
They have the same potential energy.
Both have zero potential energy.
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Increasing PE
Clicker #23
Two negative charges are
equal. Which has more
electric potential energy?
Increasing
PE for
negative
charge
!
A.
B.
C.
D.
Charge A.
Charge B.
They have the same potential energy.
Both have zero potential energy.
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Clicker #24
A positive charge moves
as shown. Its kinetic
energy
!
Increasing PE
Decreasing KE
A. Increases.
B. Remains constant.
C. Decreases.
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Reminders
!
•Study!!
•You’ll get more practice in recitation tomorrow.!
•I have office hours today from 3-5pm…please stop
by if you have any questions about anything.!
•I will be in my office tomorrow, and free before
11:30am, and after 2:45pm.!
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