Naked Science Forum
Non Life Sciences => Technology => Topic started by: scientizscht on 14/07/2018 20:58:47
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Hello!
Unfortunately I still do not understand how batteries work. The web is full of whole stupid 'explanation' webpages that make no sense. You get lemons, metals, electrolytes etc, totally different things that confuse me. I want to know the Principle.
Any idea?
thanks!
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The most important thing to grasp is the electrochemical series: https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)
Batteries (really galvanic cells) and fuel cells are essentially the same thing. Each employs two "half reactions," one at each electrode: one that releases electrons, and one that receives them. The series can be used to determine which pairs of reactions will provide energetically favorable reactions, and what the maximum theoretical energy can be.
For instance, a galvanic cell can employ these two reactions:
Cu2+ + 2e– --> Cu (+0.337 V)
and
Mg --> Mg2+ + 2e– (+2.70 V)
(I have chosen these because they are both 2-electron reactions, which simplifies the considerations).
Together the total reaction is Mg + Cu2+ --> Mg2+ + Cu (3.037 V)
The maximum theoretical voltage this cell can produce is 3.037 V, but it is likely to be less due to internal resistance.
For current to flow, the circuit must be closed. Electrons must be allowed to flow from the Mg to the Cu2+ through wires (and electronic devices) and ions must be able to flow through the battery (either negative ions from Cu2+ to Mg, or positive ions going the other way.)
That's it! The rest is just details...
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I want to know the Principle (of batteries)
I agree that battery chemistry can be confusing.
Batteries have two electrical conductors (or terminals) coming out.
Chemical reactions inside the battery deliver an excess of electrons to one terminal, and steal electrons from the other terminal, so there is a shortage.
If there is an excess of electrons in one place, and a shortage in another, electrical current will flow from the area with an excess of electrons to the area with a shortage of electrons.
Every kind of atom has electrons, and can gain an extra one, or lose one. But some kinds of atoms prefer to lose an electron, while other atoms prefer to gain an electron. That means that there are many kinds of chemical reactions that can be used inside a battery.
Anyway, that is the Principle.
(Oops! overlap with chiralSPO...)
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I want to know the Principle (of batteries)
I agree that battery chemistry can be confusing.
Batteries have two electrical conductors (or terminals) coming out.
Chemical reactions inside the battery deliver an excess of electrons to one terminal, and steal electrons from the other terminal, so there is a shortage.
If there is an excess of electrons in one place, and a shortage in another, electrical current will flow from the area with an excess of electrons to the area with a shortage of electrons.
Every kind of atom has electrons, and can gain an extra one, or lose one. But some kinds of atoms prefer to lose an electron, while other atoms prefer to gain an electron. That means that there are many kinds of chemical reactions that can be used inside a battery.
Anyway, that is the Principle.
(Oops! overlap with chiralSPO...)
That's a better explanation. Because it does not limit in an example. Sometimes examples are better giving you to understand, but in this time it is confusing, it steals focus.
So basically batteries are devices that bring in contact two materials which one has tendency to release electrons and the other to gain. That way a continuous electron flow is maintained.
1) Is this tendency automatic, ie occurs naturally or there are specific conditions to promote it?
2) the material with tendency to lose electrons is positively charged and that's why it has this tendency? Or it is in atomic level so not total charge is needed? Also when it loses electrons due to its natural tendency, it becomes positively charged? So that would increase the voltage of the battery? Or because of the circuit, the electrons return to it so the potential differential just maintains? And if it maintains, why doesn't it last forever?
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Materials that lose electrons become positively charged, but then the charge can be balanced out by the motion of ions in the electrolyte. Either negatively charged ions move from the electrode that receives electrons towards the electrode that puts electrons out, or positive ions move the other way (or some combination thereof).
The battery goes dead when there isn't enough of the material that accepts electrons, or the material that puts them out, or if there are no more mobile ions in the electrolyte.
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I just saw this PBS documentary on batteries on Netflix.
It focuses on lithium-ion batteries, but it does explain the general principles of batteries too, showing the flow of ions within the battery, and flow of electrons outside the battery.
It also shows what happens when there is a flow of electrons within the battery, with lots of spectacular explosions.
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The battery goes dead when there isn't enough of the material that accepts electrons, or the material that puts them out, or if there are no more mobile ions in the electrolyte.
Why would any of the above take place?
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I think what is missing here is batteries achieve their optimum voltage and stop creating more. As the voltage level drops chemical reactions occur to bring the battery back to it's optimum level. Once the battery is unable to recharge itself or be recharged the voltage level is diminished, thus a dead battery.
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The battery goes dead when there isn't enough of the material that accepts electrons, or the material that puts them out, or if there are no more mobile ions in the electrolyte.
Why would any of the above take place?
Let's go back to my example of the copper and magnesium battery:
Cu2+ + 2e– --> Cu (+0.337 V)
and
Mg --> Mg2+ + 2e– (+2.70 V)
Each atom of magnesium can give no more than two electrons, and each copper ion can receive no more than two electrons, so a battery containing 12 grams of CuCl2 (0.089 moles) and 2 grams of Mg (0.083 moles) is limited by Mg, and can only pass a maximum of 0.166 moles of electrons (4.47 Ah, if I have done my conversions correctly).
It is rare for the electrolyte to run out of ions, but I specified mobile ions, so if the electrolyte dries up and the ions become immobile, then this would also cause the battery to cease working.
I haven't even gotten to discussing rechargeable batteries yet...
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Your chemical reaction makes no sense to me. I don't understand how can you have Cu2e-.
Also, Mg will give out its electrons at first but it will receive them again as part of the circuit.
Also how can an electrolyte dry up? This is a solution in a solvent, how can the solvent dry out? And how can the ions of the electrolyte dry out?
The battery goes dead when there isn't enough of the material that accepts electrons, or the material that puts them out, or if there are no more mobile ions in the electrolyte.
Why would any of the above take place?
Let's go back to my example of the copper and magnesium battery:
Cu2+ + 2e– --> Cu (+0.337 V)
and
Mg --> Mg2+ + 2e– (+2.70 V)
Each atom of magnesium can give no more than two electrons, and each copper ion can receive no more than two electrons, so a battery containing 12 grams of CuCl2 (0.089 moles) and 2 grams of Mg (0.083 moles) is limited by Mg, and can only pass a maximum of 0.166 moles of electrons (4.47 Ah, if I have done my conversions correctly).
It is rare for the electrolyte to run out of ions, but I specified mobile ions, so if the electrolyte dries up and the ions become immobile, then this would also cause the battery to cease working.
I haven't even gotten to discussing rechargeable batteries yet...
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how can the solvent dry out? And how can the ions of the electrolyte dry out?
All solvents can ‘dry out’ or evaporate eg water, alcohol; it isn’t the ions ‘drying out’.
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how can the solvent dry out? And how can the ions of the electrolyte dry out?
The old carbon-zonc battery could dry out if you left it on the shelf for too long. They weren't hermetically sealed.
Rechargeable batteries can dry out the electrolyte if you overcharge them. The traditional lead-acid car battery can be overcharged, and it breaks down the electrolyte, producing hydrogen gas (an explosion risk next to a hot exhaust pipe!).
Lithium ion batteries can boil the electrolyte if you charge or discharge them too quickly. That's why they need electronics to prevent overcharging and excessive discharge.
Some of the complexity of battery chemistry is involved in safely collecting some of these unwanted chemical products that can produce gas in the battery, causing it the volatile electrolyte to leak out.
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Your chemical reaction makes no sense to me. I don't understand how can you have Cu2e-.
Also, Mg will give out its electrons at first but it will receive them again as part of the circuit.
I don't believe that I wrote "Cu2e-" anywhere... Cu2+ is a copper atom that is short by two electrons, and the 2e– represents those two electrons that the Cu2+ can accept to turn back into neutral copper, thus:
Cu2+ + 2e– —> Cu
The complete circuit will not return electrons to the magnesium. The electrons are lost from the magnesium at one electrode, travel through the wire to the other electrode, where they combine with the copper ions, and then either positively charged magnesium ions have to migrate to the second electrode to balance charge, or negatively charged ions (chloride, for instance) must move to the first electrode.
If one is using a rechargeable battery, then electrons will be forcibly removed from the copper and added bacl to the magnesium ions, restoring copper ions and magnesium metal. But this can only happen by virtue of the applied energy. As I mentioned, the Mg-Cu galvanic cell can theoretically produce up to 3.0 V on discharge--similarly, it will require at least 3.0 V to recharge (and because of inefficiencies in both the charging and discharging, it is likely that one could expect something more like 2.7 V on discharge, and 3.5 V to recharge--representing a round-trip efficiency of 77%)
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Your chemical reaction makes no sense to me. I don't understand how can you have Cu2e-.
Also, Mg will give out its electrons at first but it will receive them again as part of the circuit.
I don't believe that I wrote "Cu2e-" anywhere... Cu2+ is a copper atom that is short by two electrons, and the 2e– represents those two electrons that the Cu2+ can accept to turn back into neutral copper, thus:
Cu2+ + 2e– —> Cu
Cu2+ is not able to exist in nature. How do you use this as a starting of your chemical equation? Cu2+ must be an intermediary of another equation preceding.
Also, you mean that the energy of a battery is purely depending on the amount of metals, and thus their amount of electrons available in them?
Also, to understand so far: the power for the electrons transfer is the electronegativity difference of two metals. What is the role of the electrolyte?
Also, as the electrons get transferred from the most electronegative to the most electropositive metal, does that reduce the potential difference? But the battery still works, how can that be?
Also, another thing I want to clear, there is the macroscopic charge of a piece of metal that can create potential difference and there is the microscopic electronegativity. Which are we talking about in batteries? And how are these affected in a battery operating?
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Cu2+ is not able to exist in nature. How do you use this as a starting of your chemical equation? Cu2+ must be an intermediary of another equation preceding.
Also, you mean that the energy of a battery is purely depending on the amount of metals, and thus their amount of electrons available in them?
Also, to understand so far: the power for the electrons transfer is the electronegativity difference of two metals. What is the role of the electrolyte?
Also, as the electrons get transferred from the most electronegative to the most electropositive metal, does that reduce the potential difference? But the battery still works, how can that be?
Also, another thing I want to clear, there is the macroscopic charge of a piece of metal that can create potential difference and there is the microscopic electronegativity. Which are we talking about in batteries? And how are these affected in a battery operating?
I believe that all of these questions can be answered by considering the "spectator ions."
Cu2+ cannot exist as a pure substance in nature (as in, you will never find a chunk of it). However, CuCl2 certainly can (I have chunks of it in my lab), and when dissolved in a liquid with a high dielectric constant (like water or dmf) it will form Cu2+ ions and Cl– ions (in a 1:2 ratio).
Cl– ions do not engage in any redox chemistry in the Cu/Mg cell that I used in my example, and so it doesn't show up in any of the net reactions (it is excluded by convention). But far the sake of clarity I can include them here:
Cu2+ + 2 Cl– + 2 e– —> Cu + 2 Cl–
Mg + 2 Cl– —> Mg2+ + 2 Cl– + 2 e–
And it is the movement of these Cl– through the electrolyte that prevents charge from building up on the two electrodes.
Does this satisfy your questions?
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Also, you mean that the energy of a battery is purely depending on the amount of metals, and thus their amount of electrons available in them?
Yes, the amount of metal is one limiting factor in the energy of a battery.
In the Cu/Mg battery described by ChiralSPO, whichever metal is dissolved first will terminate the energy generation of the battery.
But the electrodes don't have to be metals - carbon makes an effective battery with Zinc as the other electrode. The electrodes do have to be conductive, and most elemental conductors are metals.
The electrolyte also limits the energy output of the battery - if the electrolyte has too high a resistance, energy will be dissipated inside the battery as heat, instead of being delivered to the external circuit as electrical power. Producing gas at the electrode reduces the area of contact with the metal, and reduces the peak power available.
the power for the electrons transfer is the electronegativity difference of two metals. What is the role of the electrolyte?
To carry ions through the battery, balancing the flow of negative electrons through copper wires outside the battery.
as the electrons get transferred from the most electronegative to the most electropositive metal, does that reduce the potential difference?
Yes.
But the battery still works, how can that be?
If you have a battery with no external circuit drawing current, the terminals will develop a potential difference (depending on the metals inside). This potential difference prevents charged ions from migrating within the battery, so the battery just sits there with that potential difference across its terminals.
If you now connect an external circuit, electrons will move from the negative terminal to the positive terminal, through the external circuit. This would reduce the potential difference, except that the ions inside the battery are not prevented from migrating between the terminals, and they restore the potential difference.
When one piece of metal runs out, there is no supply of ions, and the potential difference drops to zero.
Also, another thing I want to clear, there is the macroscopic charge of a piece of metal that can create potential difference and there is the microscopic electronegativity. Which are we talking about in batteries?
The potential difference at the battery terminals comes about as the difference between electronegativity of the metals used in the electrodes. The microscopic generates the macroscopic.
And how are these affected in a battery operating?
As long as there is a pair of electrodes, and a conductive electrolyte, the battery will generate a potential difference.
When these are used up, there is no difference in electronegativity, and the battery goes "flat".
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I still don't understand some things.
So you have two metals that one has tendency to release its electrons and the other to grab electrons, but both are neutrally charged, right?
How do we take advantage of this to make batteries work? We use an electrolyte solution? And this solution reacts with both metals and makes the two electrons generate potential difference? How exactly?
Still no one has explained what happens exactly.
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Still no one has explained what happens exactly.
I think Wikipedia has done a pretty good job of explaining it.
But you find Wikipedia too difficult.
See: https://en.wikipedia.org/wiki/Electric_battery
So you have two metals that one has tendency to release its electrons and the other to grab electrons, but both are neutrally charged, right? How do we take advantage of this to make batteries work?
Maybe we should go back to the original observation of battery action with Luigi Galvani...
According to the traditional story, Galvani was presenting a dissection of a frog to an audience...
- The frog was resting on a zinc table, and a metal scalpel was being used.
- At some point in the dissection, the scalpel was put down on the zinc table, with the blade resting on the frog's leg, on the sciatic nerve.
- This caused the frog leg to twitch, even though the frog was dead.
- This led to the discovery of "animal electricity", and later, the battery (when they discovered that the animals were not necessary)
As you say, here we have have two pieces of metal that by themselves are electrically neutral.
- of these two metals, one has tendency to release its electrons and the other to grab electrons
- As soon as the two pieces of metal are put in electrical contact (in a wet environment), electrons rush from one metal to the other, making them electrically charged.
- But by itself, no electrical current will flow, even though there is a potential difference between them
- If you now complete the circuit (by touching both pieces of metal on a frog's leg, for example), an electric current will flow through the external circuit (passing through the sciatic nerve and causing the frog leg to twitch, in this case).
See: https://en.wikipedia.org/wiki/Luigi_Galvani#Early_life
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Wikipedia mentions another confusing thing, a salt bridge.
This has never been mentioned in this thread. And is it me that I am idiot and don't understand?
If you connect with a cable two metals of different electronegativity, what will happen?
What connections must exist between those two metals for a battery to work?
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You can make a battery with two metals, but it still involves metal ions being turned into neutral metal on one side, and neutral metal being turned into ions on the other. The reason we can use a piece of copper foil and a piece of aluminum foil is that the copper foil has copper ions on its surface (there are ways to remove all of the copper ions, and actually have only copper metal--and this will not make a functional a battery).
A salt bridge is a path for the ions to flow though. Not all batteries have two chambers separated by a salt bridge--some have chambers separated by a membrane, or no chambers and a solid electrolyte, or other configurations...
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is it me that I am idiot and don't understand?
No, it is that batteries (and science in general) has to deal with some complicated things.
So you have to work at it.
I find that Wikipedia has a fairly accessible starting point for teenagers and adults.
- If you don't understand the terminology, they have links to articles on those terms
- Once you understand the Wikipedia topic itself, start to read the references down the bottom
Teenagers and younger children seem to do most of their learning through Youtube - sort of a modern-day apprenticeship, only you can learn from masters all over the world.
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I still don't understand fully.
So both metals are neutral, but have tendency to release or gain electrons.
When connected together, their pairing tendency moves electrons from one to the other.
For the electrons to move, we need to provide a path, which we do via? What are the necessary characteristics of that path? Doesn't simple water do?
When electrons move, they generate positive and negative charged metals that create a potential difference.
What happens as the electrons move apart from charging the metals? So that the battery capacity drops yet the voltage stays the same?
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I still don't understand fully.
So both metals are neutral, but have tendency to release or gain electrons.
While it is possible to use two neutral metals with wildly different electronegativities (like aluminum and palladium), this is not always the case (see my previous post in this thread).
When connected together, their pairing tendency moves electrons from one to the other.
For the electrons to move, we need to provide a path, which we do via? What are the necessary characteristics of that path? Doesn't simple water do?
A wire provides the path for electrons to move. Water (or other highly polar liquids, or ionomers etc.) allow ions to move.
When electrons move, they generate positive and negative charged metals that create a potential difference.
What happens as the electrons move apart from charging the metals? So that the battery capacity drops yet the voltage stays the same?
If ions were not allowed to flow then yes, the there might be a very brief spike in current when the terminals are connected by a wire, but the voltage would drop to zero when the batteries internal electric field cancels out the inherent electron affinities of the different metals (and/or ions). But, when ions are able to move within the battery, their motion will keep the internal electric field very small (zero internal field, for perfect ion mobility), allowing the voltage of the battery to remain constant until one end runs out of electrons to give, or the other end runs out of space to accept electrons, or there are no more mobile ions. (to first approximation--the voltage will actually decrease slightly as one or more of the limits is approached, rather than suddenly dropping from 100% to 0%--but this is a complication you can ignore until you feel more comfortable with the "ideal" case. When ready for this complication, you can read about it here: https://en.wikipedia.org/wiki/Nernst_equation)
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I still don't understand fully.
So both metals are neutral, but have tendency to release or gain electrons.
While it is possible to use two neutral metals with wildly different electronegativities (like aluminum and palladium), this is not always the case (see my previous post in this thread).
When connected together, their pairing tendency moves electrons from one to the other.
For the electrons to move, we need to provide a path, which we do via? What are the necessary characteristics of that path? Doesn't simple water do?
A wire provides the path for electrons to move. Water (or other highly polar liquids, or ionomers etc.) allow ions to move.
When electrons move, they generate positive and negative charged metals that create a potential difference.
What happens as the electrons move apart from charging the metals? So that the battery capacity drops yet the voltage stays the same?
If ions were not allowed to flow then yes, the there might be a very brief spike in current when the terminals are connected by a wire, but the voltage would drop to zero when the batteries internal electric field cancels out the inherent electron affinities of the different metals (and/or ions). But, when ions are able to move within the battery, their motion will keep the internal electric field very small (zero internal field, for perfect ion mobility), allowing the voltage of the battery to remain constant until one end runs out of electrons to give, or the other end runs out of space to accept electrons, or there are no more mobile ions. (to first approximation--the voltage will actually decrease slightly as one or more of the limits is approached, rather than suddenly dropping from 100% to 0%--but this is a complication you can ignore until you feel more comfortable with the "ideal" case. When ready for this complication, you can read about it here: https://en.wikipedia.org/wiki/Nernst_equation)
Very interesting.
So to sum up the principles:
1) the electromotive force for a battery to work is the electronegativity difference of two materials, which we can call them primary.
2) it is also needed to have a wire to allow electrons to transfer and produce work
3) it is also needed to have a SECONDARY connection between the materials AND a pool of positive and negative ions. This is because we need to prevent charging of the primary materials as their electrons move. If this charging is not prevented through the use of a third material's ions in a solution that connects the primary materials, the battery simply would stop to work.
4) the capacity of the battery drops as the battery is in use and the electrons flow from one primary material to the other. The capacity depends on the amount of materials that are contained in the electron donator material and the positive holes/gaps in the electron acceptor material.
5) the flow of the third material's ions in the solution which bridges both the primary materials, does not affect the electronegativity differential of the primary materials. This is only affected by the electrons available in the donator and the gaps available in the acceptor. The solution bridge's ipns only prevent build up of macroscopic charge of the primary materials that would oppose the electrons flow.
6) I suppose we need to supply the adequate amount of ionic solution so that there's protection from static charge build up throughout the life of the battery.
Is the above a comprehensive summary of battery operation?
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I still don't understand fully.
So both metals are neutral, but have tendency to release or gain electrons.
While it is possible to use two neutral metals with wildly different electronegativities (like aluminum and palladium), this is not always the case (see my previous post in this thread).
When connected together, their pairing tendency moves electrons from one to the other.
For the electrons to move, we need to provide a path, which we do via? What are the necessary characteristics of that path? Doesn't simple water do?
A wire provides the path for electrons to move. Water (or other highly polar liquids, or ionomers etc.) allow ions to move.
When electrons move, they generate positive and negative charged metals that create a potential difference.
What happens as the electrons move apart from charging the metals? So that the battery capacity drops yet the voltage stays the same?
If ions were not allowed to flow then yes, the there might be a very brief spike in current when the terminals are connected by a wire, but the voltage would drop to zero when the batteries internal electric field cancels out the inherent electron affinities of the different metals (and/or ions). But, when ions are able to move within the battery, their motion will keep the internal electric field very small (zero internal field, for perfect ion mobility), allowing the voltage of the battery to remain constant until one end runs out of electrons to give, or the other end runs out of space to accept electrons, or there are no more mobile ions. (to first approximation--the voltage will actually decrease slightly as one or more of the limits is approached, rather than suddenly dropping from 100% to 0%--but this is a complication you can ignore until you feel more comfortable with the "ideal" case. When ready for this complication, you can read about it here: https://en.wikipedia.org/wiki/Nernst_equation)
Very interesting.
So to sum up the principles:
1) the electromotive force for a battery to work is the electronegativity difference of two materials, which we can call them primary.
2) it is also needed to have a wire to allow electrons to transfer and produce work
3) it is also needed to have a SECONDARY connection between the materials AND a pool of positive and negative ions. This is because we need to prevent charging of the primary materials as their electrons move. If this charging is not prevented through the use of a third material's ions in a solution that connects the primary materials, the battery simply would stop to work.
4) the capacity of the battery drops as the battery is in use and the electrons flow from one primary material to the other. The capacity depends on the amount of materials that are contained in the electron donator material and the positive holes/gaps in the electron acceptor material.
5) the flow of the third material's ions in the solution which bridges both the primary materials, does not affect the electronegativity differential of the primary materials. This is only affected by the electrons available in the donator and the gaps available in the acceptor. The solution bridge's ipns only prevent build up of macroscopic charge of the primary materials that would oppose the electrons flow.
6) I suppose we need to supply the adequate amount of ionic solution so that there's protection from static charge build up throughout the life of the battery.
Is the above a comprehensive summary of battery operation?
Yes, that's a reasonable summary.
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I know the electronegativity of different metals and I can appreciate how it would make a battery work. But they make batteries from bamboo? sugar? silk? How do these batteries work? I cannot imagine sugar or bamboo being electronegative.
Any idea?
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I know the electronegativity of different metals and I can appreciate how it would make a battery work. But they make batteries from bamboo? sugar? silk? How do these batteries work? I cannot imagine sugar or bamboo being electronegative.
Any idea?
Anyone?
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Essentially any substance can be oxidized or reduced, provided a sufficiently strong oxidizing agent or reducing agent.
Sugar doesn't conduct electricity, so it would be difficult to make an electrode out of sugar, but solutions of sugar can be either oxidized or reduced when exposed to a conductive electrode. The reaction at the other electrode could be reduction of oxygen (an oxygen-sugar "battery" is more properly called a fuel cell), or reduction of silver(I) ions. I know that silver(I) can oxidize sugar because that's how the Tollens test/Tollens reaction works: