Electrical mobility definition confusion

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Discussion Overview

The discussion revolves around the definition of electrical mobility, particularly the relationship between drift velocity and electric field strength. Participants explore the implications of the definition, especially in the context of charge carriers such as electrons and holes, and the interpretation of signs in vector quantities.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the definition of electrical mobility as always resulting in a positive velocity parallel to the electric field, seeking clarification on how this applies to different charge carriers.
  • Another participant asserts that electrical mobility can be negative in semiconductors, indicating that the sign of mobility depends on the type of charge carrier (holes vs. electrons).
  • A third participant notes that the Wikipedia article does not use vector notation and suggests that the definition refers to magnitudes, which are always positive, aligning with Kittel's reference book.
  • One participant acknowledges the need for specificity regarding the cancellation of the negative sign due to the relative directions of velocity and electric field.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of electrical mobility, particularly regarding the sign and vector nature of the quantities involved. There is no consensus on the implications of these definitions.

Contextual Notes

Some participants highlight limitations in the Wikipedia source, suggesting it lacks rigor. The discussion also reflects varying interpretations of vector versus scalar relationships in the context of mobility.

Wminus
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Hi! According to this http://en.wikipedia.org/wiki/Electrical_mobility, the definition of electrical mobility ##\mu## is:

##\vec{v} = \mu \vec{E}##. But since electrical mobility is always positive, this means that the velocity is always parallel to the E-field regardless of charge. How can this be?

thanks for all replies.
 
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Actually its not necessarily always positive. It can be negative in semiconductors.

Wikipedia is not a very rigorous source. It is good to get the idea of something and should not be taken too seriously. The mobility is defined as
v = +uE for 'holes.' And v = -uE for 'electrons.' Where v and E are vector quantities
 
Wminus said:
Hi! According to this http://en.wikipedia.org/wiki/Electrical_mobility, the definition of electrical mobility ##\mu## is:

##\vec{v} = \mu \vec{E}##. But since electrical mobility is always positive, this means that the velocity is always parallel to the E-field regardless of charge. How can this be?

thanks for all replies.
In the Wiki article they don't use vectors. The text explicitly mention that E is the magnitude of the electric field. So it is a scalar relation between the magnitudes of the drift velocity and of the electric field.
And this is how mobility is defined in Kittel's reference book: as ratio between magnitudes. The values of mobility are given as positive for both electrons and holes.
 
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Thanks guys
 
I apologise I should have been more specific that the minus sign cancels due to the relative direction of [tex]\vec v \ and\ \vec E[/tex]]
 

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