Re: Induction and moving rod on a U-shaped conductor

["GARY HEMMINGER" <Hemmig@d-e.pvt.k12.nj.us>]

> I like to visualize the electric field in a moving conductor as a smooth
> gradient which will support a current if connected to a stationary return
>  path, but which will exhibit the field regardless (i.e. beyond the 'rails'
> the field in the rod survives...)
> I am uncomfortable with the conceptual model of electrons or charge
>  'piling up'. The force due to a very minor difference in electron 
> density is considerable.
> Brian

Bruce Sherwood and Ruth Chabay in their very interesting 
approach to teaching E&M (Electrical and Magnetic Interactions, 
Wiley),  present a surface charge model - current flows in a wire 
because a gradient of surface charge creates an electric field inside 
the wire.   For ordinary fields they assert that the surface charge 
is the result of a shift by the electron sea of a fraction of an 
atomic diameter.  Therefore, as much as it appeals to us as an image, 
we can't really say that electrons are piling up.   In a resistive 
wire of constant cross section there is a steady gradient of surface 
charge, and the field itself is constant along the length of the wire.

> > When a conducting rod is moving in a uniform magnetic field (rod,
> > magnetic field, velocity mutually perpendicular) charge
> > accumulation establish at the ends of the rod.

Thus I'm not sure that we can say that charge accumulates at the ends 
of the rod.

*****************************************
  Gary Hemminger                   
  Dwight-Engelwood School           
  315 E. Palisade Ave.             
  Englewood, New Jersey            
             07631                   
  e-mail: hemmig@d-e.pvt.k12.nj.us
********************************************