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This is chapter 27 problem number 24.
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We are given a beam of protons, protons, and this beam is entering a magnetic field that is initially perpendicular to the direction of the speed.
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And then after some time, it exits the fields in a way that the direction of the velocity is perpendicular to the initial direction.
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So from here we can conclude that actually any proton within this beam has traveled a quarter of the circumference, right? this is the quarter.
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So 2 pi r would mean the circumference over 4.
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Let's call this distance traveled, which is given to us in the problem.
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So isolate these pi r over 2 equals x.
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From here we can actually write the radius as 2x over pi.
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Now going back to the force, the magnetic force acting on the proton, q times v times b, v and b being initially perpendicular with respect to each other.
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I'm dropping the sign term here because it's going to be equal to 1...