# Stationary electrons?

## Homework Statement

When considering conservation of energy and momentum in the collision between a photon and an electron (in Compton scattering for example), is it reasonable to worry about 'stationary' electrons?

## The Attempt at a Solution

From what I can recall the derivation of the Compton scattering formula was based on a stationary electron. But how is this justified? Are electrons in stationary atoms always stationary?

Thanks for any help!

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by any chance do you do PHYS2013 @ ANU?

imo, It is not sensible to worry about stationary electrons. Electrons will only be stationary at absolute zero, so they’re not really worth worrying about. An electron can only be stationary if its wavelength is infinity (ie. The electron hasn’t got a definable location).

turin
Homework Helper
When considering conservation of energy and momentum in the collision between a photon and an electron (in Compton scattering for example), is it reasonable to worry about 'stationary' electrons?
I don't understand this question.

From what I can recall the derivation of the Compton scattering formula was based on a stationary electron.
This is true. However, in QM, stationary is not a good concept. A related concept is the Compton wavelength, which is sometimes though of as the effective size of the particle, in the sense that, using photons as a probe, it cannot be resolved to a position more precisely than about a Compton wavelength.

Are electrons in stationary atoms always stationary?
Well, they are bound, which is sort of like having negative kinetic energy.

I don't understand this question.
Well, they are bound, which is sort of like having negative kinetic energy.
Um, no. Bound electrons have negative energy with respect to the vacuum level, but they always have a non-negative kinetic energy.

You need to think about the energy scales involved in Compton scattering. Treating the electron as unbound is equivalent to ignoring the binding energy of the electron. Does it make much difference if we neglect the binding energy of the electron?