I was watching the history channel and I was surprised to see that batteries are made up of a cathode and an anode and don't just store electricity. So how do you recharge a battery?
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I was watching the history channel and I was surprised to see that batteries are made up of a cathode and an anode and don't just store electricity. So how do you recharge a battery?
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Batteries produce electricity through a chemical reaction. In rechargable batteries this chemical reaction can be made to run backwards by passing electricity through the battery and returning the battery to its original state. (Or almost to its original state. Generally there are only so many times a battery can be recharged before its performance degrades.)
Batteries are different from capacitors which actually do store electricity. People are researching ways to make capacitators that can function like batteries for electric cars and other applications.
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I kind of guessed that the electricitty would recharge the cathode and anode inside I just needed to make sure.
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Nope, the cathode and the anode are just the main highways out of the battery.
Everything is in the chemistry....
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I don't quite understand.Nope, the cathode and the anode are just the main highways out of the battery.
Everything is in the chemistry....
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http://en.wikipedia.org/wiki/Battery_%28electricity%29Originally Posted by 3dknight
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You can make a battery by sticking copper and zinc electrodes into a lemon. This will produce a weak current. The current doesn't come from the electrodes, it comes from the potato. The metal electrodes just allow the current to flow. It's the same with commercially made batteries. The anode and cathode aren't involved in the chemical reaction, they just allow the current to flow.
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What does it mean: the current comes from the potato?
Does that mean you can use anode and cathode from the same metal, say both of copper, since the current comes from the potato?
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"The current ... comes from..." That is not a meaningful concept within the laws of physics as we know them. Current flows around a circuit in a complete cycle and cannot be said to have originated in any particular location.
Perhaps you were trying to say where the chemical energy to power this current came from. I do hope you were not were not asserting that it comes from the potato/lemon, because that is a very basic error. It comes from the difference in electronegativity between the copper and the zinc. That is why two copper electrodes won't work. The zinc will be dissolved by the process. In theory it should continue until the zinc is all dissolved, but in practice it won't work that long because the products of the chemical reaction will eventually "poison" it. The potato or lemon contributes no energy to the process, it merely acts as an electrolyte facilitating the passage of ions from one electrode to the other. Some salty water would have done the job.
A basic cheap battery such as you buy in the shop isn't very different. Known as a zinc/carbon battery, it is basically a zinc electrode and a carbon electrode separated by an electrolyte. Running the battery dissolves the zinc. You can, if you are careful, recharge a basic shop-bought zinc-carbon battery (but they warn you not to because most people are not capable of applying the necessary care). When you run a current back through the electrode, the zinc ions now dissolved in the electrolyte are redeposited as zinc back onto the zinc electrode. You can find websites where people tell you how to recharge zinc carbon batteries, but the care you have to take in managing your batteries means that you are probably better off buying NiMH (nickel metal hydride) batteries which have been specifically designed for ease of home recharging.
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Ronald,
You only posted a few minutes ago, so you may be editing as I type
this, but you switched between lemon and potato. I think either can
be used -- might as well use both!
-- Jeff, in Minneapolis
http://www.FreeMars.org/jeff/
"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"
"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves
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Oops! But yes, either can be used. It all depends on what you feel like eating after you're done.
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The chemical reaction that produces the current comes from the potato.What does it mean: the current comes from the potato?
No because the copper is the positive cathode and the zinc is the negative anode. The copper will attract positive cations and the anode will attract negative anions, creating a current. It is possible to use metals other than zinc and copper, but one must attract cations and one must attract anions.Does that mean you can use anode and cathode from the same metal, say both of copper, since the current comes from the potato?
By the way, you can remember that the anode is the negative electrode by making a memonic of the first two letters: A Negative ode
Last edited by Ronald Brak; 2007-Nov-11 at 03:29 PM.
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Thanks for clearing that up. I have to admit I'm no battery expert.
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phew.... batteries man... crazy.
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What about the cathode? Why doesn't the cathode dissolve to?Running the battery dissolves the zinc.
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A lesson in high school chemistry. I doubt I'm a great teacher, its about 25 years since I did any chemistry.
In a chemical cell, an oxidation reaction takes place at the anode, producing electrons which flow into the electric circuit, and a reduction reaction takes place at the cathode, absorbing electrons out of the circuit. Within the electrolyte, the current is carried by the flow of ions rather than electrons, the current can be carried by the flow of positive ions in the electrolyte, for example. See http://en.wikipedia.org/wiki/Anode In the circuit itself, we say that current flows from the cathode to the anode, because "conventional current" flows in the opposite direction to the flow of the electrons, since the convention for the direction of flow of current in a circuit was devised before we knew about electrons.
The anode is the zinc electrode, because it is less electronegative, and therefore wishes to disburden itself of electrons more strongly than copper, or is more hungry to be oxidised than copper. See http://en.wikipedia.org/wiki/Electronegativity (We might colloquially say that Zinc is more reactive than copper, though fluorine the most electronegative element is certainly very reactive. But you will notice that among metals the ones most likely to occur as native metals are the most electronegative, gold for example is the most electronegative metal. But non-metals are mostly more electronegative than most metals.) The oxidation reaction turns the zinc into Zn++ ions, ie a zinc salt, which dissolves.
Copper doesn't actually want to accept the electrons itself, since negative copper ions are energetically improbable. So what happens at the cathode is that the hydrogen ions H+ in the electrolyte are reduced instead, giving off hydrogen gas, which is more energetically favoured.
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What happens is the following:
1.) You have to have to different metals. These two metals have a different "Austrittsarbeit" (I'll be back for translation) Edit "work function" is the only translation I have found. It sounds somehow wrong. I mean the work/energy needed to get an electron out of the metal )fo the electrons in the metal.
2.) Due to these difference you have an electrical potential between the to metals (in common terms a "Voltage").
3.) This Voltage makes electrons travelling from on metal to the other. Now if an electron comes out of the metal (e.g. zinc), it leaves back an positive Ion. (we come back to the electron later)
4.)Positive ions as any ion can be disolved.
5.)Because of the liquid in the lemon, the ion will be dissolved.
6.)Now back to the electron. The electron which came out of the metal will go trough the electrical device you have attached (e.g. a torch) and travel to the other electrode.
7.)The positive ion travels meanwhile to the other electrode.
Edit to correct: There it forms together with another atom or molecule (already present in the liquid) and an electron (which travelled through the circuit e.g. the torch)
a compound and is deposited for example at the electrode or falls out as a saltBTW: If you use a gold cathode and a copper anode, the copper will be disolved.
Additionally this process can be used (and it is actually used) to produce very pure copper.
I hope I was understandable
Last edited by AndreH; 2007-Nov-13 at 07:14 PM. Reason: Edit to correct and translate
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In a zinc copper battery the zinc is the anode and the copper cathode dissolves and the zinc anode gets covered in copper. In a carbon zinc battery the zinc is the cathode and the carbon is the anode so it is the zinc that gets dissolved. For a battery to work effectively one metal must be more likely to lose an electron than the other. So in a zinc copper battery, zinc is more likely to lose an electron than copper causing copper to dissolve and coat the zinc anode, and in a zinc carbon battery the carbon is more likely to lose an electron than zinc, causing the zince cathode to dissolve.What about the cathode? Why doesn't the cathode dissolve to?
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So now we have confused 3dknight completely. We all described the same process, but confused some stuff. I guess I should have a look in my science book again.
Anyway, one electrode is disolved, whereas on the other material is deposited. And I guess you are right, because as far as I remeber if you use galvanic to coat something the material to be coated has to be "less precious" (another term for more eeger to give electrons) than the one which forms the coating.
As I said, I really have to look this up. Unbelievable.
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You said earlier you don't know very much about batteries. So I am not quite sure why you are posting this as if you know it and it is fact, when actually it is mostly wrong. If you are merely surmising it and hoping for someone to correct you, then perhaps you should indicate that more clearly, because you are confusing people who can't work that out for themselves.
"For a battery to work effectively one metal must be more likely to lose an electron than the other." Correct. One mark.
"So in a zinc copper battery, zinc is more likely to lose an electron..." Correct. Another mark.
"...causing copper to dissolve and coat the zinc anode..." Incorrect. The copper metal has to lose electrons to dissolve, and you just said it is the zinc that is losing electrons. You are confusing this with the situation where there is a copper salt in the electrolye, so that there are copper ions already present. In that case the copper ions will be reduced back to copper metal, but this happens at the cathode where electrons are being delivered, so copper will deposit on the copper cathode, not the zinc anode. Lose two marks.
"...and in a zinc carbon battery the carbon is more likely to lose an electron than zinc,..." Incorrect. Carbon is even more electronegative than copper. Lose a mark.
"...causing the zinc cathode to dissolve." The zinc is the anode. It will dissolve but not for the reason you state, because your reasoning is upside down. Lose two more marks.
Total mark 2/7.
The correct situation is that the order of electronegativities, with most electronegative last is Zn, Cu, C. If you checked the wiki article I gave you, you would see that.
So in both cases the zinc is the anode. In both cases the zinc dissolves.
Yes, you could deposit copper onto the copper cathode, but only if you had a copper salt in the electrolyte. If you want the copper to be the anode and to dissolve a copper anode, you need something like a gold cathode as AndreH suggests: presumably carbon would suffice but may be tricky to achieve in practice.
Perhaps you would like to refer to this on-line course on basic electro-chemistry. http://www.wwnorton.com/college/chem...rials/ch17.htm
For some reason, a lot chemistry students find electro-chemistry rather tricky, so you are in good company.
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I just thought I'd add you can read what goes on in a Zinc-Carbon battery on Wikipedia too: http://en.wikipedia.org/wiki/Zinc-carbon_battery . It makes clear that the zinc is the anode. The cathode is a graphite rod, but is packed with manganese dioxide. The cathode reaction is the reduction of manganese dioxide, alias manganese(IV) oxide, MnO2 to manganese sesquioxide, alias manganese(III) oxide, Mn2O3.
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When I saw I had it wrong I looked it up and wrote down what I found, but it looks like I still haven't got it right. Obviously I should stop now. Thanks for catching my mistakes and hopefully I haven't caused too much damage to people's brains.
Banned
Yeah, we might bring up charges.
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One last bit of pedantry, what you call a
battery is really a cell with a battery
really meaning a collection of cells. But
I am as guilty as anyone in prefering to
say battery. Since I was a kid in fact.
"Where are the bat-ter-wees Daddy?"
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Ivan Viehoff keeping score was just cruel. It's not like somebody else didn't already remark on it.
Bad Post-Doc
I don't think it was cruel. I think it was an attempt at being helpful.
_____________________________________________
Gillian
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"
Banned
It was helpful as a post, but the marks were very distracting. Perhaps next time just correct the faults- there is no need for marks. Or comments about how you think a person doesn't know about the subject so shouldn't post.
I don't have a clue how batteries work either, after reading this thread. What I thought I knew turns out to be wrong. That's how learning happens.
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I thought the marks would add some levity, but apparently I misjudged and I apologise to anyone who was offended.
I didn't actually say he shouldn't post, I said he should make it clear that he was just trying to work it out and might be wrong.
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I'll try the most basic explanation I can give. But you'll need some basic high school chemistry (what a chemical reaction is, atoms and ions and electrons) to understand it.
In many chemical reactions, electrons move. For example, if we take zinc metal and oxidise it, we convert it to a zinc salt, which contains zinc ions, which are positively charged.The normal zinc ion has two charges per atom, which we write Zn++. So each zinc atom has lost two electrons. They have to go somewhere. If the zinc was oxidised by “rusting” it in air, it would be the atmospheric oxygen that would take those two electrons, ie the O2 oxygen molecules are “reduced” to O-- ions. The result would be ZnO, zinc oxide, an ionic solid made up of positive zinc ions and negative oxygen ions. “Reduction” is the counterpart of “oxidation”, and oxidation doesn’t have to be done by oxygen, it can be done by any “oxidizing agent”. In essence, oxidation is giving up electrons, and reduction is accepting them. Whenever an oxidation takes place, giving up electrons, a reduction has to take place, accepting them, so that the total number of electrons remains constant.
The clever trick in an electric cell is to separate the oxidation and reduction reactions into separate locations – the anode and the cathode – so that electrons delivered from the oxidation (at the anode) flow to the reduction (at the cathode) through an electric circuit that connects them. Since we need a complete cycle, to make the total charge balance, we need to connect the electrodes by a second route, which is the electrolyte (a solution containing ions), which allows the charge to balance by the flow of the ions in that electrolyte. The lemon or potato, (or something more sophisticated if you look at the electrochemistry lessons I mentioned) acts as the electrolyte connection.
Having actually set up our two electrodes, the reason that the current actually flows, powering our electric gizmo, is that the oxidation and reduction reactions are, on balance, energetically favoured. The oxidation of zinc to zinc ions generates energy, so it "wants" to happen. (Actually it is a bit more complicated than just energy, because entropy comes into it too, but that is beyond the scope of this lesson.) In the case of Ronald’s lemon/potato cell, the reduction at the cathode is the reduction of hydrogen ions to molecular hydrogen. Now that is not energetically favoured, but the oxidation of the zinc is more energetically favoured than the reduction of the hydrogen. So the zinc oxidation is powering the reduction of the hydrogen ions, and has some energy left over to power the electric circuit. Or alternatively, some of the energy in oxidising the zinc is lost in generating the hydrogen. Of course, we could get some of that back by burning the hydrogen.
(You can probably see here why it was a simplification to say that the lemon/potato cell worked because copper is more electronegative than zinc. What actually matters is the potential of the Cu/H+ electrode in comparison to the potential of the Zn/Zn++ electrode. Likewise in the zinc carbon battery, the cathode is a C/MnO2 electrode, rather than just carbon.)
The original energy to power this potato/lemon cell came from the manufacturing process to make the zinc metal out of zinc ore. There are fraudulent adverts in various places which give the impression that the energy comes from the lemon or potato, so that this is a way of harvesting the sun’s energy in growing the plant to convert it to electricity. Ronald appears to have seen one such fraudulent advert in one of his earlier posts, because he suggests, as they do, that the energy came from the plant matter, not the metal.
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There was a constructional project in an
electronics magszine some years ago for a
device for "rejuvenating" ordinary cells.
It ueed current limited pulses through the
cell. It could get half as much life again
from one charge, something less from a
further charge then the cell should be
disposed of. A manufacturer was asked to
comment and of course they took a dim view
of the idea, could be dangerous etc. Almost
shooting themselves in the foot as cells
must be safe against accidental short
circuits. They should not explode but
safely expire, just perhaps getting a
bit hot.
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