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anode and cathode

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e44-72

Member
Hello

I always thought for any electrical device the anode can be considered positive and cathode can be considered negative. Like anode and cathode on a diode.
However I,ve seen diagrams on the internet showing the positive terminal of a battery to be the cathode not the anode.

On a battery is the positive terminal the anode or the cathode?

Thank you for any help.
 

JimB

Super Moderator
Most Helpful Member
A diode or a good old thermionic valve, conducts when the anode is positive with respect to the cathode.

I cannot ever remember a battery being described as having an anode and a cathode.

However I,ve seen diagrams on the internet showing the positive terminal of a battery to be the cathode not the anode.
This may be some special case, can you give a link to where you saw that?

JimB
 
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Boncuk

New Member
Hi,

you probably mixed up the voltage available at the "output" of the diode.

A diode is a one-way-valve which allows current flow only if the power source is connected with the correct polarity of the diode, meaning current only flows through the diode from anode to cathode if the anode is connected to the positive terminal of the power source.

It might appear that the cathode is positive, but just remember it's a valve opening and closing.

Boncuk
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

The diode might have been drawn backwards, or it could be there for when the battery is to be charged.
 

e44-72

Member
Thank you all for the help. I understand that anode and cathode can mean different things depending on what there being reffered to from the wikepedia link, thank you for the link.

JimB, the image I found a picture of a battery with the anode as negative terminal was here

Thanks all.
 

JimB

Super Moderator
Most Helpful Member
Having just skimmed through the Wikipedia link which John gave, it seems to be that the definition of an anode is "where the current entrers the device".
So in a diode, the (conventional) current does indeed enter through the anode.
And in a battery, the current comes out of the positive end, and goes into the negative end, which makes the negative side of the battery the anode.

Well, you learn something every day!

Having said that, I dont think that I will be going around talking about anodes on batteries any time soon.

JimB
 

jpanhalt

Well-Known Member
Most Helpful Member
Nope - it sems an utterly stupid and deliberately confusing thing to do :D
I agree that in modern use, it is best to refer to the terminals of a battery as the positive and negative electrodes.

However, remember that Faraday studied electrochemistry. From that perspective, defining the anode as the electrode at which oxidation occurs was perfectly reasonable and not confusing, regardless of whether the system being studied was receiving power or generating it.

Even today, we might ask a student at which electrode in an electrolysis cell is oxygen produced from water. Knowing that definition will yield the correct answer. Or more practically, you have probably heard of electrolytic rust removal (i.e, removing rust from a piece of iron in an electrolytic bath). Which electrode goes to the rusted part that you want to recover? Well, rust is oxidized iron, so obviously, you do not want to make it worse by connecting it to the oxidizing electrode. Therefore, it is connected to the negative electrode.

John
 
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jpanhalt

Well-Known Member
Most Helpful Member
John, your link does not seem to do very much.
It just sits there with a blank screen, and telling me that it is downloading from some site.

JimB
It is a custom pdf on ETO and takes some time to load. I will try to get the CWRU link and post and edit here. I was writing when you posted.

John

Edit: Here is the original link to the archives of the electrochemistry department at CWRU (Case Western Reserve University):

http://www.electro-tech-online.com/custompdfs/2012/07/hist-06-Faraday-7-1.pdf

If that doesn't work, go to: http://electrochem.cwru.edu and then click on the historical archives and search for Faraday.

John
 
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