anode, cathode

[Mark Spahn]

According to various dictionaries,

陽極 anode, positive electrode, positive pole
陰極 cathode, negative electrode, negative pole
負極 anode, negative electrode, negative pole
正極 cathode, positive electrode

anode:
(1) in an electrolytic cell, the positively charged
electrode, toward which current flows
(2) in an electron tube, the principal electrode for
collecting electrons, operated at a positive potential
with respect to the cathode
(3) in a battery that is a source of electric current,
the negative electrode

cathode:
(1) in an electrolytic cell, the negative electrode,
from which current flows
(2) in a vacuum tube, the negatively charged electron
emitter
(3) the positive terminal of a battery

cathode ray: stream of electrons projected from
the surface of a cathode

These definitions are confusing.
As I understand it, electrons move from a negative
potential (negatively charged electrode) to a positive
potential (positively charged electrode), and current
moves in the opposite direction from the direction
of electron flow. So in the definition of cathode (1),
how can current flow *from* a negatively charged
electrode? Electrons flow *from* a negatively charge
electrode, and current flows *toward* it, right?

Why is an anode described as both a positive electrode
and as a negative electrode? Is there some logic behind this?
-- Mark Spahn (West Seneca, NY)

[pls]

Mark Spahn wrote:
> As I understand it, electrons move from a negative
> potential (negatively charged electrode) to a positive
> potential (positively charged electrode)

Yes, outside an electrolytic cell or battery (anybody remember that this
word means "battery of cells") - inside a cell or battery the electrons
move (more correctly, are _being moved_) from the positively charged
electrode to the negatively charged electrode - this is how the
potential difference comes about in the first place...

> and current
> moves in the opposite direction from the direction
> of electron flow.

To be precise, not "current" but "conventional current", which is a
definition/ legacy construct (from a time when people still had some
mistaken notions about the particles that carry electric charge) - it
gets even better: in semiconductors current can flow in either direction
(depending on whether you have an n-type or a p-type conductor). ;-)

> Why is an anode described as both a positive electrode
> and as a negative electrode? Is there some logic behind this?

There is. You gotta know the relevant bits of physics/electronics
themselves to understand which definitions are useful in which context. :-)

Regards: Hendrik

--

[jknagai]

The current on the OUTSIDE of a source is said (except in the US
military) to flow from the positive electrode/pole/terminal/anode of the
source (like a battery) to the negative electrode/pole/terminal/cathode
of the source.

Electrons on the OUTSIDE of a source are said to flow from the negative
electrode/pole/terminal/cathode of the source to the positive
electrode/pole/terminal/anode.

In other words, the current OUTSIDE of a source is said to flow opposite
to the flow of electrons.

WITHIN a source, the above statements are completely reversed.

電極 - per Wikipedia
quote
電池の場合. 陽極 = カソード, 陰極 = アノード. となる。 また電池の場合は
電位の高いほう（陽極）を正極（せいきょく、positive electrode）、低いほう
（陰極）を負極（ふきょく、negative electrode）と呼ぶ場合が多い。 ...
unquote

A battery engineer who works "inside" a battery may call the positive
electrode a cathode.

Your (1) contradicts the Wikipedia's positive=cathode, but correctly
says the current flows from negative to positive electrodes.

In your (3), I have never heard of a battery user calling the "positive"
terminal of a battery a cathode. To him, the anode and cathode are those
of a device connected ultimately to the positive and negative terminals
of a battery, respectively.

Your (2) about an electron tube (old vacuum tube) and cathode-ray tube
(the conventional glass TV tube) are correct, that is, electrons fly
from the cathode to anode of a device (not of a battery).


Joseph Kei Nagai





Mark Spahn さんは書きました:

[Ako]

In electricity, conventionally, current flow was assumed to flow from
positive terminal to negative terminal, but modern experimentation showed
otherwise. Current flow consists of negative charges flowing towards
positive terminal. An electric current is an electron flow. However,
conventional usage is still in place and in USA there is a lot of mixture in
the usage so that one needs to check whether the actual current or
conventional current is mentioned in the text.
[extract from "Electrical Principles for the Electrical Trades" by J. R.
Jeneson]