[Amps] "Conventional" current flow

[Jim Garland]

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Gene May
> Sent: Wednesday, October 26, 2016 01:39 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Amps Digest, Vol 166, Issue 26, reference direction of
current flow in
> screen grids
> 
> Concur with Bill W6WRT re "positive vs negative" convention in describing
current.  The
> convention we use now is left over from at least a hundred years ago when
a scientist
> guessed wrong about the polarity of the electron.  To me, thinking in
terms of the direction
> of electron movement is far more clear.
> -Gene WB8WKU

Sorry to disagree with you and Bill,, Gene, but the standard convention for
electric current makes a lot of sense, and it's not just that somebody
guessed wrong a century ago about the polarity of electrons. The important
concept is that electric current is a statistical concept. Electric current
is sometimes carried by positive charges, sometimes by negative charges, but
the sign of the charge carrier is an entirely separate issue from the
direction of current flow.  In a vacuum tube, plate current flows into the
plate and out of the cathode, even though the motion of the electrons in the
tube envelope is in the reverse direction.  Similarly, current flows into
the collector of an NPN transistor and out the emitter (hence the words
collector and emitter), even though transistor current may be carried by
holes.  Similarly, current flows out the positive terminal of a battery and
returns into the negative terminal, no matter whether the charge of the ions
in the battery are positively or negative. It would be a nightmare to keep
track of current flow, if the direction of flow depended on the sign of the
underlying charge carriers.  You might have situations where current flows
out of a B+ supply to the plate of a tube, and then is annihilated by
current flowing in the opposite direction from within the tube, coming from
the cathode. 

One further tidbit. In a metal, copper wire for instance, the conduction
electrons move in all directions. The current is caused when an electric
field shifts the motion a tiny bit. The so-called "drift velocity" of
conduction electrons in a current-carrying wire is only about 1 cm/sec. It
is not, as many people think, the speed of light.

73,
Jim W8ZR