Are the terminals of a Battery neutral?

[Alex Hughes]

So I've been learning how batteries work. What I learned is that a battery consists of 2 pieces of metal both with different electronegativities. These metals react with an electrolyte.

One metal (called the anode) is oxidized and has its electrons removed, leaving behind a positive ion which dissolves in the electrolytic solution. Once a conductor (like a wire) connects the two terminals, electrons flow from one metal (the anode) and into the cathode since they differ in electronegativities. A reduction reaction occurs here and the positive ions combine with the electrons.

My question is, shouldn't the terminals of a battery be neutral since even though the negative end of the battery gives up an electron, it also has its positive ion leftover dissolved into the solution. So if the terminals of a battery refer to the regions of a positive and negative potential and NOT a positive and negative charge, how can a positive and negative potential result from a neutral charge?

I've always been taught that potential is defined by bringing a positive test charge from infinity (therefore having an external electric field acting on it of 0) to whatever point your measuring near the charge and measuring the electric potential energy per charge. However, if the terminals of the battery were neutral, wouldn't that mean each would NOT have an associated electric potential?

Sorry for rambling but what I'm asking is: How can batteries have a positive and negative potential if the charges of their terminals are neutral?

 

[Drakkith]

My question is, shouldn't the terminals of a battery be neutral since even though the negative end of the battery gives up an electron, it also has its positive ion leftover dissolved into the solution.

Taken in isolation, yes, each electrode and its immediately surrounding solution would still be neutral. However, the two terminals and the electrolyte aren't isolated from each other, especially once you connect the terminals with a conductor. The extra electrons on the negative terminal can now get to the positive terminal to replace the lost electrons and will flow between the two terminals.

 

[Alex Hughes]

The two terminals have different potential relative to each other. The entire battery may be any arbitrary potential relative to infinity; positive or negative.

Just consider GROUD---12V battery---1.5V battery, versus GROUND---1.5V battery---12V battery. The terminals of the batteries are at different potentials relative to ground in those two cases, but each battery has chemistry that keeps its positive 12V (or 1.5V) higher than the negative terminal.

Think relative, not absolute.

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference. That definition using infinity is a useful abstraction to define the magnitude of one volt, but it has no practical application to real world devices like batteries. (Uh oh; now that I said that someone will prove me wrong with an application I never heard of. )

I understand the relative concept. Voltage is just the potential difference between the terminals, so when you take it relative to Earth ground, the potentials could be different between two different batteries of equal voltage. The only thing that matters is the difference in the potentials are the same. Saying that, I'm still confused on how any ELECTRIC potential exists at all on either terminal if they are both neutral. How can you have electric potential without a net charge? Am I misunderstanding what electric potential is?

 

[Alex Hughes]

When people refer to the potential on batteries (for example saying the positive terminal has a higher potential than the negative), are they possibly referring to the potential due to the chemical reactions inside the battery and not the electric potential

 

[anorlunda]

https://www.physicsforums.com/threads/how-do-batteries-work.938750/#post-5934566

This recent thread discussed that question.

 

[Guineafowl]

I understand the relative concept. Voltage is just the potential difference between the terminals, so when you take it relative to Earth ground, the potentials could be different between two different batteries of equal voltage. The only thing that matters is the difference in the potentials are the same. Saying that, I'm still confused on how any ELECTRIC potential exists at all on either terminal if they are both neutral. How can you have electric potential without a net charge? Am I misunderstanding what electric potential is?

Are you thinking that each terminal of a car battery should have a net charge on it, like two mini van der Graaf generators? They do, but very tiny. When we say a battery is ‘charged’, we are talking energy storage, not establishing static charges on the terminals.

By charging it, we have done work against a charge gradient (electric field), but this gradient doesn’t change much in the process - the energy expended is stored as chemical potential energy. The charge gradient/electric field is a function of the chemical makeup of the battery.

Another way to think of it is: Only a very small amount of excess charge accumulates on each terminal before it reaches redox equilibrium. If you want to push through another coulomb of charge, you’ll need to do 12 joules of work (car battery). This energy is stored in chemical reactions that rapidly take place, and the terminals reach redox equilibrium again.

 

[Alex Hughes]

Are you thinking that each terminal of a car battery should have a net charge on it, like two mini van der Graaf generators? They do, but very tiny. When we say a battery is ‘charged’, we are talking energy storage, not establishing static charges on the terminals.

By charging it, we have done work against a charge gradient (electric field), but this gradient doesn’t change much in the process - the energy expended is stored as chemical potential energy. The charge gradient/electric field is a function of the chemical makeup of the battery.

Another way to think of it is: Only a very small amount of excess charge accumulates on each terminal before it reaches redox equilibrium. If you want to push through another coulomb of charge, you’ll need to do 12 joules of work (car battery). This energy is stored in chemical reactions that rapidly take place, and the terminals reach redox equilibrium again.

So does this mean in terms of a battery, its the chemical potential energy of the reactions that results in electricity through the wire and not the electric potential at the terminals since the charges at the terminals are so small? When somebody says the positive terminal is at a higher potential I've always thought this meant the electric potential is higher in reference to the negative one. Is this not true?

 

[Borek]

So does this mean in terms of a battery, its the chemical potential energy of the reactions that results in electricity through the wire and not the electric potential at the terminals since the charges at the terminals are so small?

It is both. There is no current without the reaction replenishing the charge, there is no current without potential difference which requires separated charges.

 

[Guineafowl]

Consider a capacitor of 100 microfarads, with 100 volts between its plates.

Because Q=CV, this means there’s a charge difference of only 0.01 coulombs. This is a tiny amount - if it were flowing by each second, that would be a current of 10 mA. But that cap would give you a nasty nip if you bridged the leads with your finger - there’s energy there. (0.5 joule).

Now consider a capacitor of 1000 microfarads with 10 volts. Still 0.01 coulomb charge difference, but now the energy stored is 0.05 joule, a tenth of the above.

So whether the device involves chemical or electrical potential energy, the actual net charge difference is a minor determinant if how much energy is there.

 

[Alex Hughes]

Consider a capacitor of 100 microfarads, with 100 volts between its plates.

Because Q=CV, this means there’s a charge difference of only 0.01 coulombs. This is a tiny amount - if it were flowing by each second, that would be a current of 10 mA. But that cap would give you a nasty nip if you bridged the leads with your finger - there’s energy there. (0.5 joule).

Now consider a capacitor of 1000 microfarads with 10 volts. Still 0.01 coulomb charge difference, but now the energy stored is 0.05 joule, a tenth of the above.

So whether the device involves chemical or electrical potential energy, the actual net charge difference is a minor determinant if how much energy is there.

Oh I think I understand. There has to be a slight charge difference in order for there to be an electric field and a potential difference across the terminals, even if the charges are very small. Without a potential difference, the electrons would have no incentive to move from the anode to cathode, correct? But what determines the magnitude of the voltage is the amount of energy stored in whatever it is (in the case of a battery the energy is stored in the chemical reactions). If that's true can you give me a possible formula that would help me see this better. Lastly, if you have a 9v battery. Does this just mean it takes 9J of work per coulomb to go from the negative terminal (the reference) to the positive?

 

[Guineafowl]

Oh I think I understand. There has to be a slight charge difference in order for there to be an electric field and a potential difference across the terminals, even if the charges are very small. Without a potential difference, the electrons would have no incentive to move from the anode to cathode, correct?

Correct, but I dislike ‘anode and cathode’ - needlessly confusing when ‘negative and positive’ are unambiguous when talking of batteries.

But what determines the magnitude of the voltage is the amount of energy stored in whatever

The magnitude of the voltage is determined by the electrochemistry of its components (and how many cells are in series). A lead-acid cell will develop just over 2 volts whether it’s big or small. The bigger one, however, will supply a given current for much longer because it has more energy stored - the amp-hour (Ah) rating.

Lastly, if you have a 9v battery. Does this just mean it takes 9J of work per coulomb to go from the negative terminal (the reference) to the positive?

Assuming positive charge, yes.

 

[Alex Hughes]

Correct, but I dislike ‘anode and cathode’ - needlessly confusing when ‘negative and positive’ are unambiguous when talking of batteries.
The magnitude of the voltage is determined by the electrochemistry of its components (and how many cells are in series). A lead-acid cell will develop just over 2 volts whether it’s big or small. The bigger one, however, will supply a given current for much longer because it has more energy stored - the amp-hour (Ah) rating.
Assuming positive charge, yes.

So if making a battery bigger only results in more energy being stored and supplying a certain current for longer, how would you output more current? Could you achieve this by increasing the difference in electronegativity of the two metals? Also you said the magnitude of the voltage is determined by the electrochemistry of its components. What does this mean exactly. Thanks

 

[Guineafowl]

In a capacitor, the energy stored is given by 1/2CV².

The energy stored is in the form electrical potential energy. As charge accumulates on the plates, it repels further charge, so it becomes harder and harder to charge it - more and more work needs to be done.

 

[Guineafowl]

So if making a battery bigger only results in more energy being stored and supplying a certain current for longer, how would you output more current? Could you achieve this by increasing the difference in electronegativity of the two metals? Also you said the magnitude of the voltage is determined by the electrochemistry of its components. What does this mean exactly. Thanks

Well, I only said “a given current” to demonstrate a fair test of the two cells. The maximum current a cell can supply is limited by its internal resistance, which a function of size and design, surface area and thickness of plates, etc.

On a car battery, this is indicated by the CCA or CA rating ((cold) cranking amps). Generally, a bigger or heavier-duty battery can supply a bigger current, as its internal resistance is lower by design and heft.

Re: electrochemistry - you are dragging me back a long way into my educational past! Redox reactions and half equations are your next step to research. But a particular combination of electrodes and electrolyte will give a particular voltage.

 

[Borek]

But what determines the magnitude of the voltage is the amount of energy

No. The voltage depends on the reaction identity (different chemistries give different voltages). Amount of energy depends a bit on the voltage (after all assuming constant V,i work done is V*i*t), but more on the battery "size" (amount of substances that react) - you can make the battery twice larger and double the stored energy, but the voltage will be the same.

 

[Alex Hughes]

Well, I only said “a given current” to demonstrate a fair test of the two cells. The maximum current a cell can supply is limited by its internal resistance, which a function of size and design, surface area and thickness of plates, etc.

On a car battery, this is indicated by the CCA or CA rating ((cold) cranking amps). Generally, a bigger or heavier-duty battery can supply a bigger current, as its internal resistance is lower by design and heft.

Re: electrochemistry - you are dragging me back a long way into my educational past! Redox reactions and half equations are your next step to research. But a particular combination of electrodes and electrolyte will give a particular voltage.

Thank you, that makes a lot more sense now. I have one last question. We established that the battery terminals are not actually neutral since if they were perfectly neutral, no potential difference would exist between the two metals. Instead, they have very little charge on both ends. I'm just confused on where this little charge comes from. If the chemical reaction in batteries causes electrons to be sent over, but also the leftover positive ions to be dissolved in solution at the same time, to me it seems that the net charge will never actually change and will stay neutral at both ends. Unless maybe to begin with, the charges of both metals aren't perfectly neutral.

 

[anorlunda]

Thank you, that makes a lot more sense now. I have one last question. We established that the battery terminals are not actually neutral since if they were perfectly neutral, no potential difference would exist between the two metals. Instead, they have very little charge on both ends. I'm just confused on where this little charge comes from. If the chemical reaction in batteries causes electrons to be sent over, but also the leftover positive ions to be dissolved in solution at the same time, to me it seems that the net charge will never actually change and will stay neutral at both ends. Unless maybe to begin with, the charges of both metals aren't perfectly neutral.

Welcome to Kirchoff's Current Law. In a closed circuit, the charges leaving one side of the battery exactly balance charges entering from the other end. Otherwise, there would be a buildup of positive or negative charges someplace in the circuit.

 

[Borek]

I'm just confused on where this little charge comes from.

What happens on the molecular level is that when you put an electrode in the solution (say, copper rod in the solution containing Cu²⁺ - while that is for many reasons impractical it is perfectly possible to make a working battery using Cu/Cu²⁺ as one of the half cells) an equilibrium is established - in this case, some of the copper ions will get reduced and deposited on the electrode surface (copper "likes" to be in a metallic form). As each of these cations becomes part of the electrode, the electrode gains a positive charge, until it is charged enough so that no other copper cations will get deposited (actually it is a dynamic equilibrium some of atoms from the surface will get into the solution, some will get deposited, but the net effect is as if there were no changes at all).

Now, when you close the circuit, this small positive charge gets neutralized opening place for new cations. On the other end of the circuit almost the same happens, just in an opposite direction: electrode is in an equilibrium with ions in the solution, but is selected in such a way that it gets a negative charge (or positive one, but smaller than the one on the copper electrode).

 

[Guineafowl]

Thank you, that makes a lot more sense now. I have one last question. We established that the battery terminals are not actually neutral since if they were perfectly neutral, no potential difference would exist between the two metals. Instead, they have very little charge on both ends. I'm just confused on where this little charge comes from. If the chemical reaction in batteries causes electrons to be sent over, but also the leftover positive ions to be dissolved in solution at the same time, to me it seems that the net charge will never actually change and will stay neutral at both ends. Unless maybe to begin with, the charges of both metals aren't perfectly neutral.

The little charge comes from reduction at the cathode, and oxidation at the anode. Reduction and oxidation - Redox. There’s that damn cathode/anode business again.

Remember OILRIG - oxidation is loss, reduction is gain (of electrons).

Metal ions (+ve) leave the anode and go into solution, leaving behind electrons - net charge. This imbalance inhibits further reaction, but produces a potential difference, in an open-circuit cell. If you allow charge to move (bridge the terminals with a bulb), further reaction can then continue until the reagents are exhausted and the battery is flat.

 

[Borek]

Metal ions (+ve) leave the anode and go into solution, leaving behind electrons - net charge. This imbalance inhibits further reaction, but produces a potential difference, in an open-circuit cell. If you allow charge to move (bridge the terminals with a bulb), further reaction can then continue until the reagents are exhausted and the battery is flat.

You need to close the circuit on the other end as well, just connecting half cells through a bulb is not enough.

 

[Guineafowl]

You need to close the circuit on the other end as well, just connecting half cells through a bulb is not enough.

Yes, I meant by sharing the electrolyte.

 

[Alex Hughes]

The electrons are being attracted to the metal on the positive terminal side, correct? So why is the reduction half of the battery needed. As long as the oxidation reaction happens, and the electrons are flowing to the other metal due to their differing electronegativities, why do the electrons need to combine back with the electrolyte and reduce it to complete the circuit. Is this simply just to stop the build up of negative charge that would result on the negative side?

Also, I watched several videos where they talk about a salt bridge used to stop positive charge from building up on the negative terminal and negative charge from building up on the positive terminal. What is the salt bridge in a battery exactly?

 

[Borek]

Is this simply just to stop the build up of negative charge that would result on the negative side?

Yes.

What is the salt bridge in a battery exactly?

It closes the circuit on the other end. You have a bulb (load) on one side, and you need some kind of connection on the other side - this is done using a salt bridge, which is just a solution in which ions can travel to transfer the charge.

Important thing: charge can transfer through the phase boundary only when there is a reaction taking place at the phase boundary, but it can travel trough a solid (or liquid) just by the migration of "free" electrons or ions.

I suggest you read a bit about galvanic cells, questions you are asking now are answered in every discussion of how these cells work.

 

[sophiecentaur]

if they are both neutral.

Can we re-wind back to the use of the word "Neutral"? I think we started off on the wrong foot.
What does the OP actually mean by this? Neither terminal is "Neutral" when not connected externally because there will be a surplus of + charge on one and a surplus of - on the other. It's only if they are connected by a zero resistance wire that they will be "Neutral" - in that situation, there will be a continual flow of charge in the electrolyte and the battery will cook or go flat.

 

[Guineafowl]

Can we re-wind back to the use of the word "Neutral"? I think we started off on the wrong foot.
What does the OP actually mean by this? Neither terminal is "Neutral" when not connected externally because there will be a surplus of + charge on one and a surplus of - on the other. It's only if they are connected by a zero resistance wire that they will be "Neutral" - in that situation, there will be a continual flow of charge in the electrolyte and the battery will cook or go flat.

OP was wondering, I think, about how much net charge was on each terminal and how this related to voltage and energy storage.

Conclusion was, there is a very tiny net charge on each, to establish the electric field. The strength of the latter is related to the energy density difference of that net charge (joules per coulomb, voltage), but the absolute coulomb value is very low. IE, the absolute charge difference of the terminals is a minor determinant of battery/cell characteristics.

Energy storage was explained as the ability of the battery/cell to maintain that potential difference when the charge is allowed to move between the terminals, and for how long.

 

[sophiecentaur]

the absolute coulomb value is very low.

Very low, as we know, but it has to be non-zero for a battery to work. This thread is demonstrating how difficult it is to discuss electric circuit matters in terms of charge and fields. It really is not surprising that the two concepts very seldom come together in 'real life'.
PS The charge accumulated will depend upon the Capacitance between the terminals and that would usually be only a few pF.

 

[anorlunda]

I'm going to split off these Earth potential and grounding questions into a new thread. The new thread will be "The Potential of Earth Ground"

This thread is temporarily locked until that is complete.

 

[anorlunda]

Moderator's note. A number of posts in this thread were moved to a new thread.
https://www.physicsforums.com/threads/the-potential-of-earth-ground.940336/