Download Exam II from spring 2002

Exam 2
Mar. 7, 2002
Physics 212 Spring 2002
Name________________________
This 100-point exam consists of five questions, some with two or more parts. If you cannot do
one part, move on to the next. Partial credit will be given on all parts of this exam.
This is a closed book exam. You will need a calculator. Please remember to write your name on
every page. Remember to convert to meters and kilograms.
To receive full credit, you must show all of your work or reasoning. At times, you may need to
explain your reasoning in words. When grading I look for quality thinking more than right
answers so do take the time to reason carefully. Please ask if the wording of a question is not
clear to you.
Equations:


FE ,q  qE
Force on a charge, q
KE  21 mv 2

q
A
kinetic energy
E
kq
r2
point charge
E

2 o
single plate
kq
r
potential—point charge or
k = 9 x 109 Nm2/C2
sphere
e = 1.6 x 10-19 C
elementary charge
uniform E-field
mp = 1.7 x 10-27 kg
proton mass
me = 9.1 x 10-31 kg
electron mass
V 
V  ED
charge density—plate
Constants:
o = 8.8 x 10-12 C2/Nm2
PE AB  qVAB
KE AB  PEAB
energy conservation
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Exam 2
Mar. 7, 2002
Physics 212 Spring 2002
Name________________________
1) [20 points total] The following questions all refer to the situation drawn below consisting of
a point charge and an infinite plate.
a) [5 points] Which of the eight choices shown
best describe the direction of the E field at point
C due to the point charge and the plate? Show
your reasoning.
infinite plate with
 = +4.4  10-8 C/m2
A
point charge with
q = -8.0  10-8 C
B
b) [5 points] Is the E field at A greater than, less
than, or the same as the E field at B? Show
your reasoning.
All dotted lines
are 40cm long
C
c) [5 points] Is VAB a negative number, positive number, or zero? (Is one at a higher
potential or are both A and B at the same potential?) Show your reasoning.
d) [5 points] Is VAC a negative number, positive number, or zero? (Is one at a higher
potential or are both A and C at the same potential?) Show your reasoning.
2) [5 points total] A negative charge is observed moving right, slowing down, and turning
around. Which direction (left or right) does the E field point in this region? Show your
reasoning.
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Exam 2
Mar. 7, 2002
Physics 212 Spring 2002
Name________________________
3) [25 points] Three charges are arranged at the corners of a 45-45-90 rt. triangle with q = 1C
and r = 1cm per square. Find answers in terms of k, qo, ro, and a numerical factor. The goal
is to find the E-field magnitude and direction at Q2, but I have divided the work into three
parts. Read the entire problem before you start. Each square = 1cm = ro.
a) [5 points] Find the E-field at Q2 (magnitude and direction) due to Q1 only. Show your
work and draw an arrow at Q2 representing the field due to Q1 indicating direction.
Q1 = +50qo = +50C
8 cm
Q2 = -12qo = -12C
8/2 cm
8/2 cm
b) [5 points] Find the E-field at Q2 (magnitude and
direction) due to Q3 only. Show your work and
draw an arrow at Q2 representing the field due to
Q3 indicating direction.
Q3 = -20qo = -20C
c) [10 points] Find the net E-field at Q2 (magnitude and direction). Show your work and
draw an arrow at Q2 representing the net E-field at the Q2 indicating the angle in degrees.
Q1 = +50q = +50C
8 cm
Q2 = -12q = -12C
8/2 cm
8/2 cm
Q3 = -20q = -20C
d) [5 points] What is the force on Q2, magnitude and direction. Show your work.
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Exam 2
Mar. 7, 2002
Physics 212 Spring 2002
Name________________________
4) [25 points] A battery connects two conducting plates. The electric field
between the plates is 3 x 105 N/C. A +5C charge of mass 2mg is released
from rest near the positive plate and is moving at 40m/s when near the
negative plate.
+
A
B
a) [10 points] What is the potential difference, VAB, between the plates?
Show your reasoning.
Now the battery is disconnected before the plates are moved closer together so
that their separation is exactly half the pervious distance.
b) [5 points] Did the electric field between the plates change as they were
brought together? If so, find the new electric field and show your reasoning.
If not, explain why not.
A B
c) [5 points] Did the potential difference, VAB, between the plates change as they were
brought together? If so, find the new potential difference and show your reasoning. If
not, explain why not.
d) [5 points] How far apart are the plates in part b? Show your reasoning.
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Exam 2
Mar. 7, 2002
Physics 212 Spring 2002
Name________________________
5) [25 points total] Three identical charges are placed in a line spaced one meter apart as shown.
Their masses and charges are given.
m = 2g
q = +4x10-4C
Q1
1m
m = 2g
q = +4x10-4C
1m
Q2
m = 2g
q = +4x10-4C
A
Q3
a) [10 points] Q3 is released from rest. What is its velocity when it is very far away? Show
all your reasoning.
b) [8 points] How fast is it moving when it is at point A, 1m to the right of its initial
position? Show your reasoning.
Now the three charges are re-assembled and Q1 and Q3 are released simultaneously from rest.
Q2 remains in place.
c) [7 points] How fast are Q1 and Q3 moving when they are very far away? Show your
reasoning.
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