Download Physics 222 – Modern Physics

Physics 144 – Chowdary
How Things Work
Spring 2006
Homework Set #7
Due Friday 3/31/06
Problem 40: In the Motors and Generators Lab, you
constructed a simple circuit similar (but not identical) to the
one shown in the sketch. When you momentarily complete
the circuit, you observe that the bent metal wire initially
deflects into the page (using the orientation shown in the
sketch).
a) When you complete the circuit, does the current in the
bent metal wire flow overall to the left or overall to the
right? Briefly explain your choice.
b) Without constructing the circuit, determine which end of
the magnet is pointing down, the N pole end or the S pole
end, using a Right Hand Rule. Briefly describe your
reasoning.
c) Test out your prediction from part b) by constructing the
circuit. Briefly report your results.
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Problem 41: In the Motors and Generators Lab, you constructed a coil for a simple motor. You were carefully
instructed on how to sand the coil. You place your coil in your stand, place some magnets, and connect up a
battery. After some effort involving careful sanding, rebalancing, etc., your coil spun! However, no matter what,
your friend couldn’t get her coil to spin at all. You examine your friend’s coil, and notice an important difference
between your two coils. A sketch of Your Working Coil is shown below on the left. Your friend wasn’t paying as
careful attention as you, and sanded the coil as shown as shown below on the right.
shiny copper
(sanded side)
shiny copper
(sanded side)
shiny copper
(sanded side)
enamel
varnish
insulation
enamel
varnish
insulation
enamel
varnish
insulation
enamel
varnish
insulation
shiny copper
(sanded side)
Your Working Coil
Your Friend’s Coil
a) Why did you need to sand Your Working Coil as shown?
b) Why doesn’t Your Friend’s Coil work?
Problem 42: In the Motors and Generators Lab, you constructed the World’s Simplest Motor. As shown
in the sketch, the S pole side of the magnet points up.
a) You connect the top (positive) side of the battery to the side of the magnet (using a non-magnetic
wire). As viewed from above, will the foil disk spin clockwise, counterclockwise, or not spin at all?
Explain your choice using a Right Hand Rule. Test out your prediction and briefly report your results.
(This is very similar to Problem 40. However, in Problem 40, you know/figure out the direction of the
force and the current and are asked for the direction of the magnetic field. In this problem, you can figure
out the directions of the current and the magnetic field and are asked for the direction of the force, if any).
b) You replace the foil disk with one made of paper, since that’s what you have in your room. You
connect the top (positive) side of the battery to the side of the magnet (using a non-magnetic wire). As
viewed from above, will the foil disk spin clockwise, counterclockwise, or not spin at all? Explain your
reasoning. Test out your prediction and briefly report your results.
S
Problem 43: A bar magnet is released above a circular loop of wire as
shown, and falls through the loop. The loop is held fixed. A Bird’s
eye view (looking at the situation from directly above) is also
provided. In class, we discussed the case when the magnet was
falling, approaching the loop from above.
a) Now, consider the case where the magnet has fallen through the
loop, and is moving away from the loop from below. From the Bird’s
eye view, is the direction of the induced current in the loop clockwise
or counterclockwise?
b) Briefly explain your answer, using Faraday’s Law (“changing
magnetic flux induces an electric potential difference”), what you
know about circuits (what does a potential difference in a conducting
wire do?), and Lenz’s Rule (“fight the flux change”).
c) In the presence of this conducting loop (or many loops, as when you
drop the magnet down a copper tube), the magnet falls much slower
than it would fall without the loop there. Explain why.
Problem 44: A uniform magnetic field has magnitude 1 T. A loop of wire with area 0.02 m 2 is placed in this field
so that all of the field goes straight through the area of the loop. This loop has total resistance of 4 . The magnetic
field is steadily reduced down to 0 T in 0.01 s. Determine the magnitude of the current induced in the loop, if any.
Problem 45: In Lab 8, you examined voltage vs. time using an oscilloscope. A
sketch from an oscilloscope is provided. The TIME/DIV knob is set to
0.2 msec/div, and the VOLTS/DIV knob is set to 0.10 V/div.
a) What is the period T, in seconds, of this oscillating voltage?
b) What is the frequency f, in Hz, of this oscillating voltage?
c) This is the voltage vs. time measured for some coil of wire. Carefully copy
this sketch. Another coil of wire is placed near this first coil such that their
axes are aligned parallel. On the sketch you copied, also sketch the voltage vs.
time you would measure in this second coil. The vertical scale is arbitrary,
but you should take care in drawing the horizontal part.
Problem 46: In Lab 8, you learned how that a loudspeaker and a microphone are the same, just running in
opposite directions. A loudspeaker takes an electrical signal and turns it into sound. A microphone takes sound
and turns it into an electrical signal.
a) Describe how a loudspeaker works. Your discussion should include the electric signal/current; the magnet; the
magnetic force law connecting current, magnetic field, and magnetic force; and the role of the moving membrane.
b) Describe how a microphone works. Your discussion should include how the membrane moves, the magnet,
and Faraday’s law connecting a changing magnetic flux to an induced electric potential difference.
Problem 47:
a) Chapter 9, Exercise 16 (p. 300).
b) Chapter 9, Problem 2 (p. 301).
c) Chapter 9, Problem 6 (p. 301).