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PHYS 1112 In-Class Exam #3B Thu. April 8, 2010, 11:00am-12:15pm

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PHYS 1112 In-Class Exam #3B Thu. April 8, 2010, 11:00am-12:15pm
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
PHYS 1112 In-Class Exam #3B
Thu. April 8, 2010, 11:00am-12:15pm
This is a closed-book, closed-notes exam, but you are permitted to bring and use a copy
of the official Formula Sheet for this exam, which you should have printed out from the
PHYS1112 web page.
The exam consists of 12 multiple-choice questions. Each question is worth one raw score
point. There will be no penalty for wrong answers. No partial credit will be given. I
recommend that you read all the questions at the start so that you can allocate your time
wisely. (Answer the easy questions first!)
You may use a scientific calculator for arithmetic only; your calculator must be non-graphing,
non-programmable, and non-algebraic. You are not allowed to share your calculator. The
use of cell phones, pagers, PDAs, or any other electronic devices (besides calculators) is
forbidden. All such gadgets must be turned off and put away; distractions caused by these
devices will not be tolerated.
• Do not open the exam until you are told to begin.
• Make sure the scantron sheet has your name and your UGA Card ID (810-...)
number filled in. Make sure you also have entered your name, UGA Card ID number
and signature on the exam cover page (this page!) below.
• At the end of the exam period you must hand in both your scantron sheet and this
exam cover page, signed and with your name and UGA Card ID (810-...) number
filled-in. You may keep all other pages (pages 3+) of this exam paper.
• Your exam will not be graded, and you will receive a score of zero, if you do not hand
in both a properly filled in scantron sheet and this properly filled-in and signed exam
cover page.
• You have until the end of the class period (i.e. until 10:45am for Period 2 Class, until
12:15pm for Period 3 Class) to finish the exam and hand in the required exam materials
described above.
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 standards
of academic honesty for this exam, and will not tolerate any violations of these standards by
others. Unsigned exams will not be graded.
Name (please print):
UGACard ID (810-...) #:
Signature:
1
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
WORKSPACE
2
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
Conceptual Problems
Problem 1: A wire of some length 20m and with a circular cross-section of diameter D has
a resistance R. A second wire of length 360m and also of circular cross-section, made from
the same material, has, at the same temperature, only half the resistance of the first: 12 R.
What is the diameter of the second wire?
(A) 36D
1
(B) 36
D
(C) 12 D
(D) 6 D
(E) 16 D
Problem 2: Three different circuits, Z, Y and X, are built with the same three resistors,
R1 > 0, R2 > 0, and R3 > 0, and the same battery of battery voltage E, as shown in Fig.
3.33 (notice the order!). Compare and rank the magnitude of the voltage drops V1 across
R1 , observed in the three different circuits.
Fig. 3.33
Io
I3
I2
E
I1
Io
R3
R2
Io
R1
I2
E
R1
(Z)
(A)
(B)
(C)
(D)
(E)
I1
I3
R2
I2
R2
I3
E
I1
R3
(Y)
(X)
V1 (X) > V1 (Y ) > V1 (Z)
V1 (Y ) > V1 (X) > V1 (Z)
V1 (Z) > V1 (Y ) > V1 (X)
V1 (Z) > V1 (X) > V1 (Y )
V1 (X) > V1 (Z) > V1 (Y )
3
R3
R1
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
Problem 3: A molecular ion beam containing four different types of ions, called P , Q, R
and S here, enters a uniform magnetic field, as shown in Fig. 3.37, with B 6= 0 above the
~ perpendicular to, and pointing into, the plane of the drawing.
lower horizontal line, and B
The incident beam, below the lower horizontal line, is in the plane of the drawing.
Fig. 3.37
B (into)
S
R
Q
P
B=0
~
All ions have the same mass and speed and their semi-circular trajectory radii in the B-field
are in a ratio
|rP | : |rS | : |rR | : |rQ | = 4 : 3 : 2 : 1 .
Given the trajectories as shown in Fig. 3.37, what could be possible charges qP , qQ , qR and
qS of the four different ion types in the beam? (e > 0 is the elementary charge).
(A)
(B)
(C)
(D)
(E)
qP
qP
qP
qP
qP
= −1e,
= +1e,
= −4e,
= +6e,
= −6e,
qS
qS
qS
qS
qS
= +2e,
= +2e,
= +3e,
= −8e,
= +8e,
qR
qR
qR
qR
qR
= +3e,
= −3e,
= +2e,
= −12e,
= +12e,
qQ = −4e,
qQ = −4e,
qQ = −1e,
qQ = +24e,
qQ = −24e,
Problem 4: A very long thin straight wire running parallel to the z-axis carries a current
I as shown in Fig. 3.38.
I
(A)
Fig. 3.38
(B)
y
(E)
(C)
(D)
z
P
x
Which arrow drawn at point P in the x − y-plane could correctly represent the magnetic
4
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
~ produced by by I at P ?
field vector B
(A)
(B)
(C)
(D)
(E)
Numerical Problems
Problem 5: A wire made of a very poorly conducting metal alloy, with a circular crosssection of 0.09mm diameter and a length of 920m, produces 36J of heat when connected to
a 20V battery for 20 hours. The area of a circle with radius r is πr2 .
What is the resistivity of that metal alloy?
(A)
(B)
(C)
(D)
(E)
1.16 × 1017 Ω · m .
2.89 × 1016 Ω · m ;
2.21 × 10−5 Ω · m ;
5.53 × 10−6 Ω · m ;
1.38 × 10−6 Ω · m ;
Problem 6: How many electrons flow through the metal alloy wire described in Problem 5
during the 20 hours of being connected to the battery?
(A)
(B)
(C)
(D)
(E)
2.81 × 1018
5.63 × 1018
11.25 × 1018
8.89 × 10−20
5.55 × 10−21
Problem 7: In circuit Y shown in the middle panel of Fig. 3.33 (see Problem 2), the
resistances are R1 = 5 Ω, R2 = 4 Ω, R3 = 7 Ω and the battery voltage is E = 24V.
What is the current Io flowing through the battery?
(A)
(B)
(C)
(D)
(E)
1.41A
3.18A
6.98A
14.2A
181A
Problem 8: If I1 = +5A and the voltage drop VR ≡ Va − Vb across the resistor R = 4Ω is
is VR = +28V, in the circuit fragment shown in Fig. 3.10, what is the magnitude |I2 | of the
5
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
2nd current and in which direction is it flowing?
I1
I2
Fig. 3.10
a
I3
R
b
(A)
(B)
(C)
(D)
(E)
12A, flowing towards a
12A, flowing away from a
7A, flowing away from a
2A, flowing away from a
2A, flowing towards a
Problem 9: Suppose there was no gravity, no friction and no air resistance on earth and
earth’s magnetic field was 70T pointing horizontally due north everywhere at and above the
earth’s surface. Standing on top of a very tall building, how fast and in which direction
would you have to throw a charged billiard ball of mass 0.28kg and charge −0.5mC so that
it reaches a maximum distance of 80m horizontally due west from you.
(A)
(B)
(C)
(D)
(E)
10.m/s,
10.m/s,
5.0m/s,
5.0m/s,
5.0m/s,
thrown
thrown
thrown
thrown
thrown
northward
downward
upward
downward
southward
Problem 10: A current flows in a thin straight horizontal metal rod, of length 4m and mass
650g, through a uniform horizontal magnetic field of 3.2T; the direction of the current flow
is at an angle of 55o from the direction of the magnetic field vector; and the rod is suspended
against gravity (with g = 9.81m/s2 ) by the magnetic force. Find the current in the rod.
(A)
(B)
(C)
(D)
(E)
0.608A
0.869A
1.61A
2.95A
3.02A
6
Physics 1112
Spring 2010
University of Georgia
Instructor: HBSchüttler
Problem 11: You are standing on top of a very long thin horizontal straight wire which is
carrying a current of 68A in eastward direction. The tip of your nose is exactly 1.7m above
the wire. What is the strength and direction of the magnetic field produced by that wire at
the tip of your nose ?
(A)
(B)
(C)
(D)
(E)
~ pointing upward
16.0µT , B
~ pointing westward
8.0µT , B
~ pointing southward
8.0µT , B
~ pointing downward
4.0µT , B
~ pointing northward
8.0µT , B
Problem 12: A very thin circular ring with radius R = 4m is centered at the coordinate
origin O ≡ (0, 0, 0), as shown in Fig. 3.35. The ring lies in the y − z-plane with current
I1 = (28/π)A flowing around the ring in the direction indicated in panel (B) of Fig. 3.35.
(A)
(B)
y
Fig. 3.35
z
I2
I2
I1
x
y
y
z
x-y-Plane View
z
x
x
y
y-z-Plane View
An infinitely long straight wire carries a current I2 parallel to the z-axis and crosses the
y-axis at y = +6m, as also shown in Fig. 3.35. If the total magnetic field, from I1 and I2
combined, is zero at the center of the ring, what is the magnitude and the direction of I2 ?
(A)
(B)
(C)
(D)
(E)
42A, in −z-direction;
(42/π)A, in −z-direction;
84A, in −z-direction;
π×42A, in +z-direction;
42A, in +z-direction;
7
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