Re: [Phys-l] induced emf and induced current

[John Denker <jsd@av8n.com>]

On 10/12/09 23:10, Bernard Cleyet wrote:

> Wiki. "thinks"  its definition is
>
> Voltage is commonly used as a short name for electrical potential  
> difference.

Uhhhh .... what if the voltage is not a potential?

The AC power that comes into your home and classroom
is (in virtually all cases) produced by a changing
magnetic field, and is therefore not describable as
a potential -- not even approximately.

Definitions do not repeal the laws of physics.  You
can define voltage, but you cannot define it to be
a potential difference.

> From a fundamental physics point of view, it is not
even necessary to define voltage;  formulating the
basic laws in terms of electric field is convenient
and conventional.

> "In conduction processes occurring in metals and most other solids,  
> electric currents consist almost exclusively of the flow of electrons  
> in the direction B to A. This movement of electrons is controlled by  
> differences in a so-called "total local thermodynamic potential"  
> often denoted by the symbol µ ("mu"). This parameter is often called  
> the "local Fermi level" or sometimes the "(local) electrochemical  
> potential of an electron" or the "total (local) chemical potential of  
> an electron". The modern electron-based definition of voltage (VA −  
> VB) is in terms of differences in µ:"

Hogwash.

The plain old voltage (and even more fundamentally the 
electric field) are defined to apply to test charges.  
The Fermi level and the electrochemical potential µ of 
the electrons apply to _electrons_ not to test charges 
or muons or anything else.  Experiments with muons are
perfectly doable;  the muon responds to the electric 
field, but not to the electron chemical potential or
gradient thereof.  You can choose to measure the electron 
Fermi level in units of volts, but that doesn't make it 
"the" voltage.

Similarly in a neutron star there is a Fermi level and 
a chemical potential µ for the neutrons, but that is not 
"the" voltage either.  In this case it cannot even be 
expressed in units of volts i.e. joules per coulomb; you 
have to use something like joules per mole instead.