Lorentz force formula F=qvB has been obtained by experiment. It can be derived   
   from third Hertz's equation:   
      
   rot E = -d/dt B - rot(v x B)   
      
   if derived in such way it changes sign, F=-qvB.   
   Another two formulas can be derived from Lorentz force formula:   
      
   U=BLv; induced voltage equals induction times length times speed   
   F=BIL; mechanical force equals induction times current times length   
      
   both change sign, i.e. U=-BLv and F=-BIL.   
      
   --- SoupGate-Win32 v1.05   
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