Download Lecture 5

PHY 2049: Physics II
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Tutoring Center is open in room NPB 1215,
M-F 12:00AM -4:00PM. It is free.
Hopefully all homework problems have been
solved. Please see me immediately after
the class if there is still an issue.
PHY 2049: Physics II
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About bicycle riding: If some one shows
you a trick, it means that,
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It is possible to do that trick
The person who showed you, can do the trick.
It does not mean that you can do the trick.
Try it a lot of times and you can too.
PHY 2049: Physics II
What have we learned
 Coulomb’s law, electric field, Gauss’
theorem and electric potential.
 All comes together in the concept of a
capacitor.
Capacitor: A device to store charge.
PHY 2049: Physics II
PHY 2049: Physics II
•Calculate the Electric Field at P
•Calculate the el. potential at P
PHY 2049: Physics II
•Point charge
•Distribution of charges
•Line charge at an edge
•Disc on an axis through
the center
q
V k
r
qi
 k
i ri
l  l 2  d 2 
 k ln 

d




( z 2  R 2  z)
2 o
PHY 2049: Class Quiz
If 500 J of work are required to carry a charged
particle between two points with a potential
difference of 20V, the magnitude of the charge on
the particle is:
A. 0:040C
B. 12:5C
C. 20C
D. cannot be computed unless the path is given
E. none of these
Some old business:
What is the electric field
of a shell
of a uniform spherical charge distribution?
What is the potential for each of the above?
PHY 2049: Physics II
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E
R
V
r
The potential of a
uniform spherical
charge.
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V
Some old business
Calculate electric
field, integrate it to
get the potential
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Two particles with charges Q and -Q are
fixed at the vertices of an equilateral
triangle with sides of length a. The work
required to move a particle with charge q
from the other vertex to the center of the
line joining the fixed particles is:
A. 0
B. kQq/a
C. kQq/a2
D. 2kQq/a
E. 1.4kQq/a
PHY 2049: Physics II
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a= 39 cm
q1 = 3.4pC
q2 = 6 pC
E at the center?
What is the
potential at the
center?
PHY 2049: Physics II
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Capacitors: A device to hold charge.
It is held by Coulomb force between the
charges. +q and –q on two separate
places.
V : potential difference: q= CV
C: capacitance units farad.
PHY 2049: Physics II
PHY 2049: Physics II