Physics 222 – Modern Physics - Bucknell University



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.

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.

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.

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 m2 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 Ω. 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).

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Your Friend’s Coil

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Your Working Coil

shiny copper (sanded side)

–

+

+

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enamel varnish insulation

shiny copper (sanded side)

enamel varnish insulation

shiny copper (sanded side)

enamel varnish insulation

shiny copper (sanded side)

enamel varnish insulation

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