Exam 2 practice problems solutions

# O i o d cm f d cm cm d cm m d cm when its 300 cm away

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o i o d cm f d cm cm d cm m d cm       When it’s 30.0 cm away: 1 1 1 1 1 1 120 40.0 30.0 66.7 4.00 100 i o i o d cm f d cm cm d cm m d cm       8. High voltage power lines are often seen suspended high above the ground. There are safety concerns regarding these power lines because of the magnetic fields they generate. Imagine that there are two of these power lines suspended 50.0 m above you, and 15.0 m apart from each other. Each carries an electric current of 10.0 A , but in opposite directions. You are standing on the ground beneath the two power lines, directly between them. a. Predict the magnitude of the force that each 100 m of length that the power lines exert on each other. Is it an attractive or repulsive force? b. What is the direction of the magnetic field created by the two wires where you are standing? Draw a diagram illustrating your reasoning. c. Predict the magnitude of the magnetic field where you are standing. (Challenging: Only worth 2 of the 10 total points for this problem.) Since the two wires carry currents in opposite directions they will repel one another. The force per 100 m of their lengths is:  7 4 0 1 2 4 10 10.0 10.0 1.33 10 2 2 B Tm A A A F I I N L r r The magnetic field where you are standing will point upward. See the diagram. The magnetic field lines of each wire are circles, counterclockwise around the left power line, and clockwise around the right power line. Each F F 1 I 2 I 1 B 2 B B y x r 1, y B

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50 15 m 35 m v x r x a i D h of them contribute to the magnetic field where you are standing. When the two vectors are added their horizontal components cancel. To determine the magnitude of the magnetic field where you are standing I have to add the two contributions to the magnetic field together, as vectors. Since their horizontal components cancel, I can just find the y component of one of them, then double it.  0 1 0 1 0 1 0 1 1, 2 2 2 7 8 2 2 2 2 sin 2 4 10 10.0 7.50 1.17 10 7.50 50.0 y I I x I x I x B B r r r r x y Tm A A m B T m m 9. Several people living around Lake Fishstory claimed to have seen a large sea monster swimming near a shallow part of the lake last night. In order to find the monster the local police department sends a helicopter with a bright searchlight to the scene. The searchlight shines a cone of white light down on the surface of the lake as shown (the refractive index of the lake water varies from 1.34 for 400nm light to 1.32 for 700nm light). The lake water is calm and very clear.
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