FE +FRiction =0 m -e (9) The group Hem is the electron "mobility"((m/s)/(volt/m)). The current density is the flux of charge due to motion of all charges. If only the electron motion is counted (it dominates in this case from(9), The group is the conductivity of the plasma (si, Let us consider the collision frequency. Suppose a neutral is regarded as a sphere with a cross-section area Qen. Electrons moving at random with (thermal) velocity Ce intercept the area Qen at a rate equal to their flux n c Qen. Since a whole range of speeds ce exists, we use the average value Ce for all electrons but this is for all electrons colliding with one en neutral. We are interested in the reverse (all neutrals, one electron), the part of v due to neutrals uld be n ceQen. Adding the part due to ions, 16.522, Space P pessan Lecture 10 Prof. Manuel martinez Page 3 of 12G G F + F E Friction = 0, or JG G m Veν = -e E e e JG ⎛ e ⎞ G V =- e ⎜ ⎟E (9) ν ⎝mee ⎠ The group µe = e is the electron “mobility” ((m/s)/ (volt/m)). The current density m νee is the flux of charge due to motion of all charges. If only the electron motion is counted (it dominates in this case) G JG j -en V ≡ e e (10) and from (9), G 2 ⎛ en ⎞ G e j= ⎜ ⎜ m ν ⎟ ⎟E (11) ⎝ ee ⎠ The group 2 e νe σ = (12) m νee is the conductivity of the plasma (Si/m). Let us consider the collision frequency. Suppose a neutral is regarded as a sphere with a cross-section area Qen. Electrons moving at random with (thermal) velocity ce intercept the area Qen at a rate equal to their flux n c Qen . Since a whole range of ee speeds ce exists, we use the average value ce for all electrons. But this is for all electrons colliding with one neutral. We are interested in the reverse (all neutrals, one electron), so the part of νe due to neutrals should be n c Qen e . Adding the part due to ions, n ν = n c Qen + n c Qei e (13) e n e i 16.522, Space Propulsion Lecture 10 Prof. Manuel Martinez-Sanchez Page 3 of 12