MadSci Network: Physics

Re: Can an external electric field create current?

Date: Sun Aug 17 14:39:19 2003
Posted By: Lawrence Skarin, Rochester Museum and Science Center Technical Assistance Group
Area of science: Physics
ID: 1059111859.Ph

We have to keep clear macroscopic stuff (like electric circuits, where 
microscopic stuff works together so you can look at anything that happens 
in a simple way).  For example, the current in a cylinder of copper is the 
result of an electric field acting on electrons free to move in this 
field.  99.99 percent of the time, this field is the result of a single 
externally applied voltage.

The "current" is the result of a "current density" at any point in the 
cross-section added up (in calculus, we call this "integration") over the 
total cross-section area.  The current density is caused at every 
microscopic point by the electric field acting on a mobile electron.  
Because the current density across the cross-section is constant, we just 
multply by the cross-section area to get current in amperes.  Then we say 
the voltage across the whole cylinder times the current through the whole 
cylinder equals the power dissipated in the cylinder.

Introducing another electric field complicates everything.  Here's my 
recommendation.  Try to learn how a solar battery works.  This device uses 
phenomena you suggest might be there.  A solar battery has a PN junction 
which automatically creates an electric field external to all outside 
influences.  Light particles (photons) landing on the junction create 
charge carriers that are moved by the electric field out of the junction.

Try -- the wonderful 
site that 

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