Download Exam 3 Solutions

EXAMINATION #3- Solutions
PHY2054 – STUDIO
April 7, 2010
Department of Physics
Drs. Bindell & Dubey
NAME _________________________________
PHY-2054 College Physics II
Drs. Bindell & Dubey
May 4, 2017
Examination III
Instructions: Answer all questions carefully and completely. Answers with no supporting work will not be graded
even if they are correct. Write legibly; if your work can not be read or followed, it will be considered incorrect. Be
safe: put your name on EVERY page. Good luck.
permeability of a vacuum
permittivity of a vacuum
electron mass
proton mass
electro charge
μ0
ε0
me
mp
e
4π x 10-7 N A-2
8.854 x 10-12 F m-1
9.109 x 10-31 kg
1.6726 x 10-27 kg
1.6 x 10-19 C
_____________________________________________________________________________________
PROBLEM #1 (20 points)
Here we go again. For the third time, explain how this motor works.
Include what the functions of the magnet, the battery, the two posts
and the coil are. Why must only one side of the wire be stripped and
how does it keep rotating. You should have expected this question!
You can be brief. If you need more space, you may use the back of the
page but please indicate so, somewhere near the bottom of the page.
The posts conduct the current to the coil. The magnet provides the
magnetic field. The battery produces the current in the coil (Ohm).
For this problem, two approaches were accepted. The first (and best) is that the field from the
magnet applies a force to the bottom of the loop (torque) if a current flows. The other is that the
loop becomes a magnet and the N pole of the magnet attracts the S pole of the loop.
In both of these cases, once the loop turns ½ turn, the current would be going in the wrong
direction and the direction of rotation would reverse, leaving the motor incapable of a full turn.
It would shake a bit, though. That is why only one side of the wire is shaved. When the current
would normally “reverse” with respect to the field of the magnet, there would be no current and
the device would keep going due to its inertia. The next rotation would give it a kick and it
would continue to spin.
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NAME _________________________________
PROBLEM #2 (30 points – 3 points each part)
In the inductor experiment that you attempted in class (it didn’t
work for everyone), the coil was represented by a combined
resistance R and a coil of inductance L. An additional resistor
r=1 was added to the circuit. The actual circuit is shown on
the right. According to the specifications, R=76 . Assume that
this is correct. The square wave represented turning the source
on and off and goes from +5.0 V to -5.0v. Consider “your” results to be as follows:
1 V/div
200 ms/div
(a) Directly on this graph, sketch the applied voltage. (see graph. The scale is 1 volt per division
so the graph should switch each time the current seems to be switched on or switched off.
(b) What is the frequency of the applied voltages in Hz. (Look at the scales)? Most responses
didn’t notice that the changes didn’t exactly match the even divisions on the graph … I allowed
that. Using the “majority opinion”, the period is about 5 divisions on the time axis or 5 x 200
ms or 1000 ms or 1 second. The frequency = 1/T=1 Hz.
(c) What is the (approximate) time constant for this coil? The current is switched on at roughly
t=0 and raises to its maximum art roughly 0.5 seconds. The time constant is roughly where the
current gets to 63% of the maximum which I allowed you to eyeball. The result is ~100ms or 0.1
seconds. (Again – eyeballing allowed).
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NAME _________________________________
(d) What is the approximate inductance of the coil? We know that =L/R so that L=R=0.1 x
(76+1)=7.7 H. I believe that I accepted 7.6 as well.
(e) What is the purpose of the 1.0  resistor? What is actually measured? (Explain).
The voltage across this resistor is IR and R=1 so the measurement of the voltage across
the resistor would actually be measuring the current through the coil. This was well
explained in the experiment’s directions.
(f) Suppose we change from a square wave to a sine wave. Draw the phase diagram for the
circuit. Draw it for t=0.
I actually meant phasor diagram but since this was my screw-up, everyone got full credit for this
part of the problem.
(g) What is the inductive reactance of the coil?
X L  L  2 fL  48.4
(h) What is the phase angle for this circuit?
tan  
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48.4
 0.63  =32.20
77
NAME _________________________________
(i) What is the impedance of the circuit?
Z  X L2  R 2  90.9
(j) Does the voltage applied to the circuit lag or lead the current?
This is an inductive circuit so the voltage leads the current.
COMMENT: Most students appear to have no idea how to read a graph. This is a necessary
skill that each of you should work on.
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NAME _________________________________
PROBLEM #3 (25 points)
A square coil of wire containing a single turn is placed in a uniform 0.30 T
magnetic field, as the drawing shows. Each side has a length of 0.22 m, and
the current in the coil is 12 A. Determine the magnitude and direction of the
magnetic force on each of the four sides. (WebAssign Problem)
Top
___________________________
Right ___________________________
Bottom ___________________________
Left ____________________________
This is a simple BiL problem where BiL~0.79 N for the two horizontal currents in the loop.
There is no force on the right or the left portion of the loop because B and the direction of the
current (v) are parallel. The force is OUT of the page on the top current and into the paper for
the bottom current as you can verify with the ole Right Hand Rule.
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NAME _________________________________
PROBLEM #4 (5 points each)
(4a) Three particles move through a constant magnetic field and follow the paths shown in the
drawing. Determine whether each particle is positively charged, negatively charged, or neutral.
Particle #1
____________positive____________
Particle #2
_____________neutral___________
Particle #3
_____________negative___________
(4b) The diagram shows two currents coming OUT of the page. The direction of the magnetic
field created by these currents at point A is:
A Up
B Down
C Left
D Right
E Other
ANSWER __________C__________
(4c) A long, straight wire is in the same plane as a rectangular, conducting loop. The wire
carries a constant current I as shown in the figure. Which one of the following statements is true
if the wire is suddenly moved toward the loop?
A)
B)
C)
D)
E)
There will be no induced emf and no induced current.
There will be an induced emf, but no induced current.
There will be an induced current that is clockwise around the loop.
There will be an induced current that is counterclockwise around the loop.
There will be an induced electric field that is clockwise around the loop.
ANSWER _______D_______
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NAME _________________________________
(4d) Which one of the following statements concerning the magnetic force on a charged particle
in a magnetic field is true?
A) The magnetic force is a maximum if the particle is stationary.
B) The magnetic force is zero if the particle moves perpendicular to the field.
C) The magnetic force is a maximum if the particle moves parallel to the field.
D) The magnetic force acts in the direction of motion for a positively charged particle.
E) The magnetic force depends on the component of the particle's velocity that is perpendicular
to the field.
ANSWER: ______E_____________
(4e) Two long, straight wires separated by 0.10 m carry currents of 18 A and 6 A in the same
direction as shown.
Determine the magnitude of the magnetic field at the point P.
A) 2.4 × 10–5 T
B) 4.8 × 10–5 T
C) 7.2 × 10–5 T
D) 9.6 × 10–5 T
E) zero tesla
ANSWER: __________B_________
Here the two currents are parallel and in the same direction so each produces a magnetic field at
point P that is into the paper. The magnitude is given by:

B 0
2
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6  4 x107
 18

120  120  4.8x105 T



2
 0.15 0.05 