# _009 - Solution for Practice Test 9 for Test 3 Solution to...

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Solution for Practice Test 9 for Test 3 Solution to Practice Test Problem 9.1(Force On Stationary Electron) Problem: An electron is at rest a point P . A current I fows in the wire in the direction drawn. Calculate the direction oF the magnetic Force on the electron. The last time I checked electrons were still negatively charged. Select One oF the ±ollowing: (a) x (b) y (c) z (d) - ˆ x (e) - ˆ y (F) - ˆ z (g-Answer) Zero. I P y x Solution Zero. I better not have caught anyone on this one. Total Points for Problem: 3 Points Solution to Practice Test Problem 9.2(Newton III for Wires) Problem: The ²gure to the right shows two in²nite parallel wires carrying currents I and 2 I in the direction drawn. Select the ²gure that has the Forces drawn correctly to scale. Select One oF the ±ollowing: (a) ±igure (a) (b) ±igure (b) (c-Answer) ±igure (c) (d) ±igure (d) (e) ±igure (e) (F) ±igure (F) (g) ±igure (g) I 2I (a) I 2I I 2I I 2I I 2I I 2I I 2I (b) (c) (d) (e) (f) (g) Solution (c) Just like the current balance, equal and opposite repulsive Forces. Total Points for Problem: 3 Points 1

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Solution to Practice Test Problem 9.3(Maximum Torque on Dipole) Problem: The frst thing anyone does when we pass out the little chrome magnets used For the speakers is put them in the big lab magnet. Archeological digs have Found skeletons oF primitive humans trying to put little magnets in big magnets, so this is a basic Feature oF human nature. The big lab magnets have a magnetic feld oF 0 . 2T . The little magnets are Neodymium Iron Boron(Nd±eB) and have magnetization density 1 . 02 × 10 6 A / m . The speaker magnets were cylinders with height 1mm and radius 2mm . Compute the maximum torque exerted by the big lab magnet on the speaker magnet. Select One oF the ±ollowing: (a) 2 . 6Nm (b) 5 . 2 × 10 - 3 Nm (c) 1 . 33 × 10 - 6 Nm (d-Answer) 2 . 6 × 10 - 3 Nm (e) 0 Solution (a) Compute the Magnetic Moment: The magnetic moment | vm | is the magnetization density M multiplied by the volume oF the magnet, | vm | = Mπr 2 h = (1 . 02 × 10 6 A / m) π (2mm) 2 (1mm) = 0 . 013Am 2 (b) Compute the Torque: The maximum torque, τ occurs when the moment and feld are at right angles. In this case, τ = mB = (0 . 013Am 2 )(0 . 2T) = 2 . 6 × 10 - 3 Nm Total Points for Problem: 3 Points Solution to Practice Test Problem 9.4(Electron in Static Field) Problem: An electron (still with a negative charge) Feels a magnetic Force in the y direction as it passes through the point P in the uniForm magnetic feld to the right. What direction is it travelling? Select One oF the ±ollowing: (a) x (b) y (c) z (d) - ˆ x (e) - ˆ y (F-Answer) - ˆ z (g) Cannot be done. y x P 2
Solution If vV = | V | ( - ˆ z ) , then v F = qvV × v B = | F | ˆ y . The electron is travelling into the page in the - ˆ z direction. Total Points for Problem: 3 Points Solution to Practice Test Problem 9.5(Maxwell for Static Magnetic Fields) Problem: Which of Maxwell’s equations is most useful in the calculation of the magnetic Felds of static current distributions?

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## This note was uploaded on 05/04/2008 for the course PHYS 2074 taught by Professor Stewart during the Spring '08 term at Arkansas.

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_009 - Solution for Practice Test 9 for Test 3 Solution to...

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