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Unformatted text preview: Solution for Homework 16 Magnetic Force and Field Solution to Homework Problem 16.1(Rotation of Bar Magnet) Problem: The figure to the right shows a very long wire carrying current in the positive x direction. A bar magnet is fixed to a pivot ABOVE the wire and allowed to rotate. If the magnet is allowed to rotate and eventually come to rest, in what direction will the moment of the magnet point when it reaches equilibrium Select One of the Following: (a) + x (b)- x (c) + y (d-Answer)- y (e) The magnet will not rotate x y I N S Magnet is above wire as you look down. Solution (a) Using the Right Hand Rule for a Wire gives the direction of the field into the negative y direction, as drawn above the wire. x y I N S Magnet is above wire as you look down. B (b) The magnet will rotate to try to align its magnetic moment with the magnetic field. The initial direction of rotation is drawn above. The equilibrium orientation is the same as the field direction, in the- y direction. Total Points for Problem: 3 Points Solution to Homework Problem 16.2(Force on Moving Charge) Problem: A pith ball, charged to 10nC , is shot into one of the Pasco lab magnets with speed 20 m s such that the velocity is perpendicular to the magnetic field. The field has a strength of . 25T . Compute the magnitude of the magnetic force on the pith ball. This may be a very small number. 1 Select One of the Following: (a) 5 10 6 N (b) 4 10 4 N (c) 3 10 10 N (d) 3 10 8 N (e-Answer) 5 10 8 N Solution The magnetic force on a moving charge is given by the Lorentz Force, vector F = qvectorv vector B , where q is the charge, vectorv is the velocity, and vector B is the magnetic field. The magnitude of the cross-product can be written as | vector F m | = qvB sin where is the angle between vectorv and vector B . The angle = 90 because the two vectors are perpendicular. So | vector F m | = qvB sin(90 ) = qvB = (10 10 9 C)(20 m s )(0 . 25T) | vector F m | = 5 10 8 N The magnetic force that runs motors is a result of a whole lot of moving charges.The magnetic force that runs motors is a result of a whole lot of moving charges....
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