-> ->> Say if I had a vacuum tube free floating in space would it 
-> ->> exhibit a net acceleration when an electron current flowed 
-> ->> through it? 
 
-> ->>  M*i*k*e 
 
->  DW> Why should it? 
 
-> Well, because the conduction electrons are colliding with the atomic nuclei  
-> the conducting material.  The nuclei recoil from the collison resulting in a 
-> increase in temperature.  Since the charge carrriers are transferring a nonz 
-> net momentum to the conductor, the conductor should recoil with a net nonzer 
-> momentum.  Or rather, it might, if the conductinmg material were shapped 
-> correctly. 
  
The electrons are travelling in a closed path - part of which is 
through the vacuum in the tube, and the rest is through various wires, 
the power supply, etc.. The net momentum of the electrons is zero, 
since the momentum of the electrons in part of the path is balanced by 
that of those on the opposite side of the loop. 
  
If electrons lose some momentum by collisions with nuclei, a 
counterbalancing force must be applied to electrons elsewhere to keep 
the current going. Basically, this is what happens in the power supply. 
Also, of course, electrons that are travelling around curved parts of 
the circuit must experience a centripetal force. Overall, the net force 
on the electrons is zero, so there is no tendency for the vacuum tube 
to accelerate through space. 
  
                         dow 
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