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February 2018 RCM&E

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Martin Harris17/01/2018 11:03:45
7696 forum posts
191 photos

I really don't think that it's as easy as simply adopting the convention that an anode is positive - although this is the case for the majority of devices. This extract gives food for thought:

An anode is an electrode through which conventional current flows into a polarized electrical device. A common mnemonic is ACID for "anode current into device". The direction of (positive) electric current is opposite to the direction of electron flow: (negatively charged) electrons flow out the anode to the outside circuit.

The terms anode and cathode do not relate to the voltage polarity of those electrodes but the direction of the current: whether positive charge is flowing into or out of the device. Conventional current quantifies the flow of positive charge. In most cases, positive charge flows into the device via the anode, and positive charge leaves the device via the cathode.

Conventional current depends not only on the direction the charge carriers move, but also the carriers' charge. The currents outside the device are usually carried by electrons in a metal conductor. The flow of electrons is opposite to conventional current because electrons have a negative charge. Consequently, electrons leave the device via the anode, and electrons enter the device through the cathode.

The anode and cathode have slightly different definitions for electrical devices such as diodes and vacuum tubes where the electrode naming is fixed and does not depend on the actual charge flow (current).

In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.

Note that this doesn't imply a change of polarity - and for the record, I always thought that the definition of an anode was that it was the positive terminal until I started looking into this!

I suspect that the conclusion to be drawn is that although Mike was correct (I haven't read the article) the terms anode and cathode should only be used when discussing cell chemistry and for all practical purposes, positive and negative terminals should be quoted.

P.S. I was taught my basic electronics when earth was often regarded as positive and my "Principles A" lecturer taught us to remember that earth falls through a funnel - hence the origin of the diode symbol! (He was a rather eccentric Czech who'd stayed on after coming over during WW2 to work on electronics research...but as my motorbikes were old enough to have positive earths it made sense to me.)

Edited By Martin Harris on 17/01/2018 11:31:09

Nigel R17/01/2018 12:57:10
1413 forum posts
309 photos

"Perhaps it is for best shelf life of the battery or possibly it’s just the safest way to transport and store them."

Shelf life is best around 30% to 50% charged.

As for transport, it is now FAA regulations to have 30% or less, and of course rest of world follows suit. It's all about reducing the stored energy being shifted about - they don't want unexploded incendiary devices filling up aircraft, for some reason...

The Wright Stuff13/02/2018 13:32:56
1264 forum posts
225 photos

Well, the March issue has arrived, and I'm still reading the February one.

I have to say, the article on engine safety, and particularly the use of suitable mounts delivers an important message. But please, if you are going to write "Wankel" on an engine stand to be photographed in a national magazine, please write it out in full... blush

Edited By The Wright Stuff on 13/02/2018 13:33:21

MaL13/02/2018 15:55:55
111 forum posts
12 photos
Posted by Bryan Anderson 1 on 17/01/2018 08:44:48:
Posted by Mike Freeman on 17/01/2018 06:37:33:
Posted by Tom Sharp 2 on 17/01/2018 00:36:54:

How about terminals

But the terminals are always labelled + & -, the cathode and anode..... I mean electrodes attached will have either a +ve or -ve charge depending on whether the cell is being charged or discharged.

An anode is always positive, a cathode negative regardless of whether it is being charged or discharged. Check with a voltmeter.

On charge, the current flow is reversed because the electromotive force ( open circuit voltage) of the charger is greater than that of the battery. On discharge, the emf of the battery drives the current through the load. The currents are reversible but the voltage is not.

Using incorrect terminology is potentially dangerous if it misleads people as to polarity.



potentially,,,,ha ha ha!

Just seen another post that explains what i was going to say in a much better way of post deleted.


Edited By MaL on 13/02/2018 15:59:33

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