Quiz _6 F07 - A basic principle of relativity is that the...

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P260 F07 Quiz # 6 NAME___________________________ More on the velocity selector. Suppose we have set up the crossed E and B fields throughout a region of space so that B E v = , where v is the velocity of a + charged particle projected to the right and moving perpendicular to both E and B. 1. Label the E and B field vectors in the diagram. 2. If we “time reverse” the problem and run the particle backwards from right to left (“run the video in reverse”) is this arrangement of fields still a velocity selector for v? NO YES (technical detail: we haven’t truly time-reversed the whole problem, we have just reversed the velocity of the particle—the laws of E&M are indeed invariant under time reversal**…….) 3. Imagine you are in an inertial reference frame moving to the right at velocity v. The particle is at rest in your reference frame and hence cannot have a magnetic force on it . In this frame you see a magnet moving past you to the left along with a pair of electrostatic deflection plates.
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Unformatted text preview: A basic principle of relativity is that the physics is the same regardless of which inertial reference frame you are in. Hence, “the particle cannot not be undeflected.” (Yogi Berra or Casey Stengel?). In other words, the particle must still have zero net force on it and zero acceleration; it is still undeflected. What is the inescapable conclusion forced upon you about the fields in your reference frame that is moving with respect to the magnetic field (how does the magnetic field transform into your reference frame)? You can ask the same question of a charged particle moving with velocity v in a uniform B field—in the rest frame of the particle there is no magnetic force but in the rest frame of the magnet there is a magnetic force…… ** Under the complete time reversal operation the B field vector also flips direction because B fields are generated by currents and time reversal changes the directions of these currents. v +...
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