2049_ch28B

# 2049_ch28B - Magnetic Field and Work Magnetic force is...

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PHY2049: Chapter 28 1 Magnetic Field and Work Î Magnetic force is always perpendicular to velocity ± Therefore B field does no work! ± Why? Because Î Consequences ± Kinetic energy does not change ± Speed does not change ± Only direction changes ± Particle moves in a circle (if ) ( ) 0 KF xF v t =⋅ =⋅∆ = r r r r vB r r

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PHY2049: Chapter 28 2 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Trajectory in a Constant Magnetic Field Î A charge q enters B field with velocity v perpendicular to B. What path will q follow? ± Force is always velocity and B ± Path will be a circle. F is the centripetal force needed to keep the charge in its circular orbit. Let’s calculate radius R F F v R v B q F v
PHY2049: Chapter 28 3 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Circular Motion of Positive Particle B q F v 2 mv qvB R = mv R qB =

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PHY2049: Chapter 28 4 Magnetic Force Î Two particles of the same charge enter a magnetic field with the same speed. Which one has the bigger mass? ± A ± B ± Both masses are equal ± Cannot tell without more info x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x A B mv R qB = Bigger mass means larger inertia, less acceleration, thus bigger radius
PHY2049: Chapter 28 5 Work and Energy Î A charged particle enters a uniform magnetic field. What happens to the kinetic energy of the particle?

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## This note was uploaded on 04/12/2008 for the course PHY 2049 taught by Professor Any during the Fall '08 term at University of Florida.

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2049_ch28B - Magnetic Field and Work Magnetic force is...

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