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As described in the Bohr model, an electron is circularly orbiting a proton in a hydrogen atom with radius r and velocity v. The orbital angular momentum of the electron is in the + z direction.

Now suppose a weak (in the sense that eB « 2mw, where w is the original angular frequency) magnetic field B（+ z direction） is turned on normal to the plane of this orbit. The original angular frequency w becomes w' = w + dw.

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Suppose the orbital radius is unchanged. What is dw?

Ans: dw= eB/2m (m is mass)