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1. The problem statement, all variables and given/known data

Consider J. J. Thomson’s 2nd experiment, the discovery of the electron. Turn on a magnetic field, but turn off the electric field. If the electrons enter a region of uniform magnetic field B and length l, show that the electrons are deflected through an angle θ ≈ (e l B)/(mvl) for small values of θ, where e is the absolute value of the electron’s charge, m is the mass of the electron, and v is the electron’s horizontal speed. (You may need to use the approximation θ ≈ tan θ ≈ sin θ, which applies for small θ in radians.)

2. Relevant equations

3. The attempt at a solution

So I get how Thomson figured out the ratio of q/m with the whole cathode ray tube experiment. I can certainly show the equation for the ratio as q/m=(tan(θ)E)/(l*B^2)

I know by adjusting the magnetic field and the electrical potential, he was able to eliminate the deflection of the electron, and determine the velocity of the electron.

However, I'm clueless at answering the above question. I know that a force due to the magnetic field will act on the electron with F=qvB, , I'm having a hard time figuring out where to start Any Suggestions?

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# Homework Help: Cathode Ray - No Electric Field

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