From Bioblast

## Description

**Velocity**, * v* [mΒ·s

^{-1}], is the speed in a defined spatial direction, and as such velocity is a vector. Velocity is the advancement in distance per unit time,

β‘ dvβ dzt^{-1}[mΒ·s^{-1}]

**Abbreviation:** * v* [mΒ·s

^{-1}]

**Reference:** Gnaiger 2018 MiPschool Tromso A2

Communicated by Gnaiger E 2018-10-20

## Velocity in diffusion: from particle format to chemical format

- When a force acts on a particle X in direction
*z*, it causes an acceleration, thus increasing the velocity of the particle up to a stationary state, when the effect of the force is counteracted by the frictional resistance. At this stationary state, the potential for performing work (*erg*; exergy) is not any longer conserved in kinetic exergy (acceleration), but is dissipated such that the exergy driving the process is consumed only to maintain the particle at constant velocity. Then the motive particle has an average velocityin the**v***z*direction, relative to any reference velocity, irrespective of its speed following the pattern of Brownian motion. Sinceis a vector with information on magnitude and spatial direction, it is written in bold face,**v**

- When a force acts on a particle X in direction

β‘ dvβ dzt^{-1}[mΒ·s^{-1}]

- Similarly, the motive force, d
_{m}*F*_{NX}(units: newton per particle X [Nβx^{-1}]), is a vector or gradient, taken here as being oriented in the*z*direction only, interacting with the motive particle X. The velocity at stationary state is proportional to the motive force, with the mobility,*u*_{NX}, as the proportionality constant (in the particle or molecular format,),*N*

- Similarly, the motive force, d

= -vu_{NX}β d_{m}F_{NX}[m/s]

- For a number of uncharged motive particles or molecules,
*N*_{X}, diffusing at velocity, the corresponding flow of diffusion,**v****I**_{dNX}, across a plane perpendicular to the gradient in direction*z*is,

- For a number of uncharged motive particles or molecules,

I_{dNX}=N_{X}β[xβmβsv^{-1}]

- Subscripts
**m**and**d**indicate the partial process of vectoral motion or specifically diffusion. Subscriptindicates the particle format, expressing X as a dimensionless number (unit [x]). In contrast,*N**n*_{X}=*N*_{X}β*N*_{A}^{-1}is the amount of X in motion expressed in the unit mole [mol] (*N*_{A}is the Avogadro constant). Then flow in the molar format,*n*_{A}, is amount times velocity, replacing particle number*N*[x] by amount of substance*n*,

- Subscripts

I_{dnX}=n_{X}β[molβmβsv^{-1}]

- To simplify symbols, we may drop the subscript
,*n***I**_{dX}β‘**I**_{dnX}, which indicates a molar format, expressing X as amount [mol], compared to the particle format.

- To simplify symbols, we may drop the subscript

**MitoPedia concepts:**
Ergodynamics