CourseNotes.6 - Part b We have already argued that the...

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Unformatted text preview: Part b) We have already argued that the field must point in the y direction, we must now decide whether it points in the +ˆ or the −y direction. Recalling that field lines originate on y ˆ positive charges and terminate on negative charges, it should be apparent that the field points in the −y direction. ˆ Problem 22.51 Two large parallel copper plates are 5.0 cm apart and have a uniform electric field between them as depicted in figure 3. An electron is released from the negative plate at the same time that a proton is released from the positive plate. Neglect the force of the particles on each other and find their distance from the positive plate when they pass each other. (Does it surprise you that you need not know the electric field to solve this problem?) Figure 3: Problem 22.51 Solution This is really more of a mechanics problem than an electrodynamics one. All we really need to recall from this chapter is that the force on a particle in the presence of an electric field is given by: F = qE Clearly, both particles are going to experience the same force, but it will be in opposite directions. The acceleration that each will experience is: q q E & ap = E ae = − me mp Since the accelerations are constant and both particles are released from rest, we can express the positions of both as functions of time. q q xe = d − E t2 & xp = E t2 me mp The question that we are asking is thus, when does xe = xp = x0 . Setting both equal to x0 and solving the system of equations gives: x0 = d − q E me m p x0 qE ⇒ x0 = me me + mp d = 27.3 µm This result is independent of the strength of the electric field because the electron and the proton have equal and opposite charge. 4 4.1 Chapter 23: Gauss’s Law Flux Gauss’s law is a very powerful tool for solving highly symmetric problems. Before we can discuss Gauss’s law though, we need to discuss the concept of flux. 6 ...
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This note was uploaded on 12/05/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.

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