Download PHYS208 - Review Problems for Exam#1 –Chapters

PHYS208 - Review Problems for Exam#1 –Chapters 21,22, and 23
These are the problems that you and a team of other 4-6 students have to solve during the review session of
today. These problems give you and your colleagues the opportunity to work together for reviewing the
concepts and the problem-solving methods for your PHYS208 exam that is around the corner.
By explaining something that you know already to your colleagues, you build a stronger confidence in
answering. This confidence will serve you well during the class exams. By making mistakes and learning how
to correct them, you will learn much better problem-solving skills than by memorizing solved examples. Team
interaction will develop your critical-thinking skills and will prepare you for the comprehensive exams of this
class.
There are 120 minutes assigned for this review session: 50 minutes in MPHY334 then 70 minutes in the lab
room. You are expected to take full advantage of this extended recitation time. Now it is the moment to turn
back to those problems in the previous ICEs that your team did not finalize, to those HW problems that you did
understand, and to the previous PHYS208 exams posted at http://ene208.physics.tamu.edu/prev_exams.shtml
that you have not practiced yet.
There is no recitation grade assigned for the review sessions. Just focus on learning and on practicing.
Good luck with your exam!
a)
b)
c)
d)
e)
1) Square Dancing: Two point charges with values of q1= –5 μC and q2= +2 μC are placed at adjacent
corners of a square with sides of 10 cm. Point A is located at the center of the square and point B is
located at the empty corner closest to q2. Let the reference point where the electric potential is
defined to be zero be located at a distance very far away from both charges. Assume that no other
forces are acting on the charges.
SOLVE ALL PARTS SYMBOLICALLY BEFORE PLUGGING IN NUMBERS
What is the electric potential at point A due to the two charges q1 and q2?
What is the potential energy of a point charge of q3= -1 μC when placed at point A?
What is the potential energy of the point charge q3 when placed at point B?
How much work is done by the electric field on the charge q3 when it is moved from point A to point B?
2) Electric Explosion: Two solid plastic spheres, each carrying charge that is uniformly distributed
throughout its interior, are placed in contact and then released. One sphere has a diameter of D1=
4cm, a mass of m1= 6g, and contains q1= +6μC of charge. The other sphere has a diameter of D2=
2cm, a mass of m2= 2g and contains q2= +2μC of charge. Assume that no other forces are acting on
the spheres.
SOLVE ALL PARTS SYMBOLICALLY BEFORE PLUGGING IN NUMBERS
a) Find the maximum speed achieved by the sphere of mass m1.
b) Find the maximum speed achieved by sphere of mass m2.
3) Force on Plate: The plates of a plan parallel capacitor of area A carry the total
charge Q as shown in the figure.
SOLVE ALL PARTS SYMBOLICALLY BEFORE PLUGGING IN NUMBERS
a)
Calculate the total force on the left plate due to the electric field from the right
plate using Gauss’s law.
b)
How much work you would do for pulling the plates apart against their
attraction until the distance between them doubles?
c)
The atmospheric pressure is 14.7lbf/in2 (100,000Pa). How large would the Efield between the plates have to be to produce this force per unit area? Would the air
breakdown at this value of the electric field?
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PHYS208 - Review Problems for Exam#1 –Chapters 21,22, and 23
4) Plates - Challenge Problem: Two thin, insulating plates of infinite area are located on either side of
the origin at a distance of d = 5 cm away from it. They carry uniformly-distributed surface charges
with the values given in the first diagram below.
y
a
d
b
d
a = - 4 C/m2
b = +2 C/m2
d = 5 cm
x
uncharged
metal slab y
b
a
d
d
x
2 cm
SOLVE ALL PARTS SYMBOLICALLY BEFORE PLUGGING IN NUMBERS
a) What is the electric potential Vb of the right-hand plate?
b) An uncharged metal slab with thickness of 2 cm is now placed parallel to the two plates and centered at
the origin, as shown in the second diagram above. What is the potential difference between plates a and b in
this situation?
5) Cylinder -Challenge Problem: A very long, solid insulating cylinder with radius R= 16mm has a
cylindrical hole with radius a =4mm bored along its entire length. The axis of the hole is a
distance b= 8 mm from the axis of the cylinder, where a<b<R.
SOLVE SYMBOLICALLY BEFORE PLUGGING IN NUMBERS
The solid material of the cylinder has a uniform volume charge density ρ= 10-2 C/m3. Find the magnitude
and direction of the electric field 𝐸⃗ inside the hole.
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