In the YouTube video Monster magnet meets computer, the south pole of a 1 T (roughly) neodymium magnet is held in front of a CRT. Assuming the CRT produces electrons of 30 keV, and that the screen is 0.2 m from the end of the electron gun, how much energy does each electron have when it hits the screen when the magnet is present? To simplify the question, assume the screen has no thickness, and electrons from the electron gun are headed straight for the middle of the screen. Simple high school physics isn't enough to answer this due to the velocities involved, so I can't answer this myself.
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$\begingroup$ Hint: The magnetic force on a charged particle is $q\vec{v}\wedge\vec{B}$, and is therefore always orthogonal to both the magnetic induction $\vec{B}$ and, most importantly, the particle's velocity. Ask yourself how much work is done by such a force. If you've not met this formula before, then you simply take as experimental fact that the force is orthogonal to the particle's velocity, and constant. This may remind you of circular motion. $\endgroup$– Selene RoutleyCommented Jan 7, 2014 at 0:55
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Magnetic fields do no work since $$ \left(\frac{d{\bf{r}}}{dt} \times {\bf{B}} \right) \bullet d{\bf{r}} = 0, $$ so they cannot change a charged particle's speed (they can, however, change the direction of its velocity).
