Re: [Phys-l] Which one is positive?

[Brian Whatcott <betwys1@sbcglobal.net>]

Pete Lohstreter wrote:
> Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
>   
> > There are other problems, including abuse of the terms
> > "anode" and "cathode".
> >  http://www.av8n.com/physics/anode-cathode.htm
> >     
>
> The anode of a device is the terminal where current flows in from outside.
> The cathode of a device is the terminal where current flows out.
For people who wish to work with a concept of "conventional current"
this is a perfectly fine assertion.

But you later notice that the usual current carrier in metals (and 
vacuum)  is the electron, which is conventionally marked with a - sign.
It has its mirror emission, it's true.
> How does this set with 'cathode rays'?  Aren't cathode rays negative?
Cathode rays are an emission of the cathode of a CRT. It turns out that 
in a hard tube, these rays are nearly all electrons, though in the 
earlier day of CRTs for television, it was found necessary to bend the 
beam, so that the heavy ions left in the beam are drawn aside rather 
than burning the tube face.
>   If
> cathode rays come from the cathode, something's backwards. 
>
> I'm really confused now.  How can positive charges leave the negative  electrode?  Shouldn't they attract?
> Pete Lohstreter  
I realize that you are playing 'straight'. But the idea of
anodizing metals, specially aluminum is a clue to chemical action.
When an aluminum strip is placed at the positive output of a battery 
charger and dipped in an electrolytic solution, it develops a porous  
oxide film which can be dyed and sealed.

When a metal pipe is made negative with respect to an external electrode 
it is electrolytically protected against rust and corrosion. The voltage 
needed is a mere 1 volt or less, in this case. This is called Cathodic 
protection.

Brian W