Phys122 HW 7: Due Thursday, April 6, 2006 Chapter 19 Problems: 2, 10, 26, 30, 60 Magnets and charge A bar magnet is hung from a string through its center as shown in the figure. A charged rod is slowly brought up slowly into the position shown. In what direction will the magnet tend to rotate? Suppose the charged rod were replaced by a bar magnet with the north pole on top. In what direction will the magnet tend to rotate? Is there a difference between what happens to the hanging magnet in the two situations? Explain why you either do or do not think so. Comparing electric and magnetic forces In the picture below are shown a long wire carrying a current I to the right and a long amber rod with a charge density (charge/unit length) of λ. Assume I and λ are both positive. A. The two are separated by a distance d. The point marked x is half-way between them. On the diagram, draw arrows to represent the following. (Be sure to label your arrows clearly so we can tell which one is which.) i. the direction of the magnetic field at the point marked x. ii. the direction of the electric field at the point marked x iii. the direction of the electric force that a charge q placed at x would feel iv. the direction of the magnetic force that a charge q placed at x would feel if it were moving to the right. B. The current, I, is +10 A, the charge density, λ, is -1 nC/m (=10-9 C/m) (note that it is negative), and the distance between the wires is 40 cm. At the instant shown, a proton with charge q = 1.6 x 10-19 C is moving into the page with a speed v = 106 m/s. Ignoring gravity, what is the magnitude and direction of the net force it feels at that time?Magnetic forces In the figure at the left below are shown parts of two long current carrying wires labeled 1 and 2. The wires lie in the same plane and cross at right angles at the point indicated. When it carries a current, each wire carries the same amount of current in the direction shown. At the right is shown a set of coordinate directions for describing the direction of vectors. For each of the vectors labeled (a)-(c) below indicate the direction of the vector by specifying its direction using the coordinate system shown. For example, you might specify "the +x direction" or "the -z direction" or "in the x-y plane at 45º between the +x and +y directions." If the magnitude of the vector requested is zero, write "0". a. The direction of the force on a positively charged ion at the point B moving in the +y direction if only wire 1 carries current. b. The direction of the force on a positively charged ion at the point B moving in the -z direction if both wires carry current. c. The direction of the force on a positively charged ion at the point A moving in the +x direction if only wire 2 carries current. For the next two parts of the problem, select which answer is correct if both wires carry current. d. The magnetic force on wire 1 will i. push it in the -z direction ii. push it in the +z direction iii. tend to rotate it clockwise about the joining point iv. tend to rotate it counter-clockwise about the joining point v. none of the above. e. The magnetic force on wire 2 will i. push it in the -z direction ii. push it in the +z direction iii. tend to rotate it clockwise about the joining point iv. tend to rotate it counter-clockwise about the joining point v. none of the above.Tutorial Problem In lab recently you studied magnets and the forces they put on each other. Below is a statement and drawing from Leah explaining her model of a magnet. “A magnet has two ends, called a north pole and a south pole. The north pole is positively charged, and the south pole is negatively charged. Throughout the magnet there are atoms with positive and negative charge, but in the north pole there are more positives than negatives, and in the south pole there are more negatives than positives. It’s almost like a positive charged piece of tape attached to a negative charged piece of tape except it stays charged a lot longer.” 1. Explain at least three things you could do to test Leah’s model using equipment available in the lab room. Include in your answer all the possible results and whether they agree with her model, disagree with her model, or do neither. 2. What experiences do you have from everyday life that could bear on Leah’s model? Explain if they agree or disagree with this model.3. The figure at the left below shows a cross section of four long parallel wires (labeled A through D ) taken in a plane perpendicular to the wires. One or more of the wires may be carrying a current. If a wire carries a current it is in the direction indicated and has magnitude I0. For each of the four vector quantities (i) through (iv) give the direction of the quantity. To indicate the direction, use one of the letters associated with a directional arrow on the "compass" figure at the right below. If the magnitude of the quantity is zero, write "0". If it is non-zero, but in none of the indicated directions, write "Other". For each answer, give a brief explanation of how you got it. a. Only wires B and D are carrying current. The direction of the force on wire D is ____. b. Only wires B and D are carrying current. The direction of the force felt by an electron traveling in the C direction (on the compass) is ____. c. Only wires B and D are carrying current. The direction of the force felt by an electron traveling in the A direction (on the compass) is ____. d. All four wires are carrying current. The direction of the net force felt by wire A is
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