An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. Why is it so?
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The electric and magnetic forces are defined via the Lorentz force on a charged particle
$$\vec F = q(\vec E + \vec v \times \vec B)$$
The magnetic force comes from the second term which defines it to be perpendicular to the velocity and therefore displacement $\vec {dr}$; so it doesn't do any work.
John McAndrew
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