## Effective mass

I have got confused about electron effective mass in crystals. Which of the definitions below are correct?

1) When an electron is in a crystal with periodic potential, it behaves such as a free electron with effective mass $m^*$

2) when an external force is applied on the electron in a crystal, the electron behaves such as a free electron with effective mass $m^*$
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 Quote by hokhani I have got confused about electron effective mass in crystals. Which of the definitions below are correct? 1) When an electron is in a crystal with periodic potential, it behaves such as a free electron with effective mass $m^*$ 2) when an external force is applied on the electron in a crystal, the electron behaves such as a free electron with effective mass $m^*$
But these are pretty much identical statements - they are both sloppy statements of the same definition.
The first one fails to state what it means by "behavior" while the second implies F=ma ... the actual definition uses the quantum mechanical analogy to F=ma.
http://en.wikipedia.org/wiki/Effecti...cs)#Definition