# Basic principles: momenta signs for deriving conductivity

**URL:** https://forum.solidstate.quantumtinkerer.tudelft.nl/t/basic-principles-momenta-signs-for-deriving-conductivity/1888
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**Created:** [April 11, 2022, 12:00pm UTC](https://forum.solidstate.quantumtinkerer.tudelft.nl/t/basic-principles-momenta-signs-for-deriving-conductivity/1888 "2022-04-11T12:00:51Z")
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### Author: ![Roland](https://forum.solidstate.quantumtinkerer.tudelft.nl/user_avatar/forum.solidstate.quantumtinkerer.tudelft.nl/roland/32/489_2.png) [@Roland](https://forum.solidstate.quantumtinkerer.tudelft.nl/u/Roland)
#### Post date: [April 11, 2022, 12:00pm UTC](https://forum.solidstate.quantumtinkerer.tudelft.nl/t/basic-principles-momenta-signs-for-deriving-conductivity/1888/1 "2022-04-11T12:00:51Z")

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In _Semiconductors \> Basic Principles \> Conduction_ the following expressions are given: - m\_e v\_e/\tau\_e = -eE and - m\_h v\_h/\tau\_h = eE . I think these should be: m\_e v\_e/\tau\_e = -eE and m\_h v\_h/\tau\_h = eE (which would be consistent with lecture 3). Otherwise you end up with \sigma=-(n\_ee\mu\_e+n\_he\mu\_h).