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Physics 1252 Exam #3D (Make-Up) Instructions:

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Physics 1252 Exam #3D (Make-Up) Instructions:
Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
Physics 1252
Exam #3D (Make-Up)
Instructions:
This is a closed-book, closed-notes exam. You are allowed to use a clean print-out of your
formula sheet, any scientific calculator, and a ruler. Do not write on your formula sheet,
except for your name: it must be handed in, signed but clean, with your exam.
There is space after each question to show your work; if you need more space, you may use
the back of the page, or request more paper. Please clearly indicate where your work for
each problem is. Underline or draw a box around your final answer.
The exam consists of four sections. Read all the questions at the start so that you can
allocate your time wisely. Do easy ones first!
You may not share your calculator. The use of cell phones or any other electronic devices (besides calculators) is prohibited. All such gadgets must be turned off and put away
throughout the exam.
• Do not open the exam until told to begin.
• You have the one entire class period to finish the exam.
• Put your last name on every page of the exam and on the formula sheet.
• You must provide explanations and/or show work legibly to receive full credit for
Sections II and III.
• Make sure that your answers include appropriate units and significant digits. (Note:
For intermediate steps in your calculation, it’s best to carry more significant digits.)
• Fundamental constants and unit prefixes are on the Formula Sheet, last page.
By signing below, you indicate that you understand the instructions for this exam and agree
to abide by them. You also certify that you will personally uphold the university’s standards
of academic honesty for this exam, and will not tolerate any violations of these standards by
others. Unsigned exams will not be graded.
Signature:
UGACard #:
c 2015 University of Georgia. Unauthorized duplication or distribution prohibited.
Copyright Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
Section
Score
II
/
/35
c 2015 University of Georgia.
Copyright III
/35
2
Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
II: Electron Between Charged Surfaces (35 points)
Two large, planar, parallel,
square surfaces, S1 and S2 , each of area A = 4.0m2 , and unknown
√
spacing d (with d A) carry unknown charges Qs and −Qs , respectively, uniformly spread
out over each surface. Their electric potential difference is V2 − V1 = −8000V, where V1 and
V2 are the electric potentials on the top and bottom surface, S1 and S2 , respectively. An
electron is shot through a small hole in S1 , into the space between S1 and S2 .
Reminder: Between S1 and S2 , the electric potential, Vs (y), due to the surface charges, Qs
and −Qs , varies linearly with y: Vs (y) = V2 − Ey y, where Ey is a constant.
y
S1 
S2 
Fig. 2.35
x
(a) What is the minimum speed, vmin , the electron must have, as it passes through S1 , in
order to be able to reach the surface S2 ?
(b) If the electron is shot through the hole at some angle from the y-axis, its point of closest
approach to S2 is at a distance of yc = (0.6)×d from S2 , and its speed is vc = 26500km/s
at that point. What was the electron’s speed, v1 , as it passed through the hole at S1 ?
(c) Assuming Qs = +3.540µC on S1 , what is the spacing, d, between the two surfaces?
(d) A point charge, Qp , is now added at a distance of yp = 6.0cm from S2 , on the y-axis.
Assume the same spacing d as in (c) and the same two surface charges, ±Qs , as before,
remaining uniformly spread out over S1 and S2 , respectively. If the electron is shot
through the hole, at some angle from the y-axis, with initial speed v1 = 84000km/s at
S1 , it will impact on S2 with a speed v2 = 95000km/s at x2 = −4.5cm. Find Qp !
c 2015 University of Georgia.
Copyright 3
Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
Work and Drawing Space for Problem II:
c 2015 University of Georgia.
Copyright 4
Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
III: Circuit Analysis (35 points) In the circuit shown below, assume that Io = 6A,
I1 = 14A, Vb − Vy = +32V , R1 = 4Ω, R3 = 6Ω, C = 75µF, and the charge, Q, on the
left-hand capacitor plate is Q = +6mC. Also assume that the capacitor is fully charged, by
the voltage drop across it, and there is no current
flowing to or from either capacitor plate.
Fig. 3.63
a
Io
I1
I2
R1
C
IC
x
I3
R3
y
I4
E
R2
Ro
R4
b
(a) Find the battery voltage E.
(b) Find the current IC .
Hint: Use a loop to find I3 , then a junction (node) to find IC .
(c) Find the resistance R2 .
Hint: Use a junction to find I2 , then a loop to find R2 .
(d) Find the resistances Ro and R4 .
c 2015 University of Georgia.
Copyright 5
Physics 1252 Exam #3D (Make-Up)
Thu., 23 April 2015
Name:
Work Space for Problem III:
c 2015 University of Georgia.
Copyright 6
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