E field inside of a battery

[jparke]

I searched the threads but was unable to find this question:

The electric field inside of a battery points from the positive to the negative terminal, yet when the battery is connected to a conducting loop charge is transported from the negative to the positive terminal as it flows through the circuit. By what mechanism is charge transported *against* the electric field?

(I am treating the current as the flow of positive charge here)

Thanks!

 

[haiha]

A battery in fact is a chemical reactor where red-ox reaction happens which involve electrons exchange. In the battery there's some medium called electrolyte which can conduct only ions, but not electrons, so the electrons have to go the longer path, which is the external circuit.

 

[jparke]

A battery in fact is a chemical reactor where red-ox reaction happens which involve electrons exchange. In the battery there's some medium called electrolyte which can conduct only ions, but not electrons, so the electrons have to go the longer path, which is the external circuit.

But how is charge transported against the electric field?

 

[haiha]

Ok, the reactants are willing to participate in the process to become products, the willing is something like potentials, chemists call that chemical potential. That is a kind of energy, the chemical energy. You can see a kg of gasoline can have a lot of heat inside. Hope you understand.

 

[jparke]

Ok, the reactants are willing to participate in the process to become products, the willing is something like potentials, chemists call that chemical potential. That is a kind of energy, the chemical energy. You can see a kg of gasoline can have a lot of heat inside. Hope you understand.

No, I don't! :)

It sounds like you are saying that the charges move against the electric field (opposite the direction they would be expected to) because they want to.

 

[haiha]

You are certainly not have a chemistry background. In fact the charges do not move against the electric field inside the battery. In the electrodes, there are layers called double layers which build up a potentials opposite to the outer potential. So inside the battery, the electric field is also from + to -. The chemical potential here helps charges crossing the double layers. Double layer is just several angstroms in thickness.

 

[jparke]

I do not have a chemistry background, this is true. I suspect you do not have a science background! :)

Of course the electric field points from + to -! Static electric fields always point from + to -. The electric field points from + to - outside the battery as well. And electric fields exert forces on positive charges in the direction of the field. So how is it that the cations migrate in the direction opposite the electric field to the cathode?

 

[haiha]

Hope this drawing can help.

 

[jparke]

Hope this drawing can help.

Thank you. I will have to look into the double layer and see what I can learn.

 

[leright]

The electric field produced by a battery is time invariant. We know that time invariant (static) e-fields are are conservative, so the total work a static e-field does on a charge when it is moved around a closed loop is ZERO. So, the e-field due to the charge separation of the battery alone is not sufficient to move a charge around the closed loop of the circuit. outside the battery there is an e-field going around the conducting loop from the positive side of the battery to the negative side. The e-field does not go from positive side back to the positive side, so it is not only the e-field moving the charge around the loop. Inside the battery, there is also an e-field pointing from positive to negative. So, you are absolutely correct that the e-field by itself is unable to move charge around the loop.

What causes the charges to go the extra distance? Well, it's due to the chemical potential inside of the battery. Under normal equilibrium conditions the electrical potential inside the battery exactly equals the chemical potential and no electrons flow. However, if the the battery it placed into a closed circuit, electrons from the negative plate are allowed to flow due to the external electric field formed around the conducting loop. These electrons move around the circuit to the positive plate and effectively reduce the electrical potential inside the battery. This causes the chemical potential to dominate and negative charges are allowed to move from the positive plate to the negative plate until equilibrium between the chemical potential and electrical potential is once again achieved.

This is precisely the mechanism by which a battery causes a current to flow through a circuit.

 

[leright]

You are certainly not have a chemistry background. In fact the charges do not move against the electric field inside the battery. In the electrodes, there are layers called double layers which build up a potentials opposite to the outer potential. So inside the battery, the electric field is also from + to -. The chemical potential here helps charges crossing the double layers. Double layer is just several angstroms in thickness.

technically, jparke is correct. Inside the battery, the negative charges flow IN THE DIRECTION OF THE E-FIELD, which means the negative charges are going AGAINST the electrostatic force set up inside the battery. The electrons are able to flow against the electrostatic force because of an opposing chemical potential.

Normally, a battery which is not shorted out or connected to a load is under equilibrium conditions, meaning the chemical potential inside the battery exactly equals the electrical potential. Under these conditions, no charge carriers flow. If you connect the positive terminal of the battery to the negative terminal through some load the electrons at the negative terminal of the battery flow through the wire to the positive terminal by the electric field set up by the E-field external to the battery. When these electrons reach the positive terminal the E-field inside the battery is momentarily reduced which in turn upsets the equilibrium between the chemical and electrical potential. The chemical potential then dominates and allows the negative charge to continue flowing from positive terminal to negative terminal until equilibrium is once again established. Notice that the electrons flow AGAINST the coulomb force inside the battery. They are able to do this because of the chemical potential, which is slightly greater than the electrical potential when equilibrium is disturbed.

Now, if you wish to understand the meaning of chemical potential and the chemical reaction taking place inside the battery that fights the electrostatic force, go talk to a chemist.

Edit: Haiha, I just noticed your drawing. That is interesting.

 

[cesiumfrog]

Does "chemical potential" here refer simply to the concentration of a particular ion being higher (denser) at one end, and less at the other?

 

[leright]

Does "chemical potential" here refer simply to the concentration of a particular ion being higher (denser) at one end, and less at the other?

Well, in thermodynamics, chemical potential is defined as the time rate of change of internal energy of a system wrt changes in the number of particles in the system (or in the limiting case, the DERIVATIVE of internal energy wrt the number of particles). A difference in chemical potential between two contacting systems with permeable walls will allow for a transfer of particles between the systems until chemical equilibrium is reached, just like a difference in electrostatic potential causes charges particles to flow between two points.

However, in electrochemistry, I think the electrochemical potential is used, which is related chemical potential. The electrochemical potential is the amount of mechanical work needed to assemble charges in such a way so that there is an electrostatic potential set up.

To answer the OP's question: I am not a chemist so I do not understand the exact mechanism that causes the ions to go against the E-field. All I can say is it's because of a 'chemical potential'. A quick wikipedia search will probably answer your question though. I am just giving you the physicist's description.

 

[haiha]

There's no way a charge moves against E field inside a battery, for sure. The key point is the double layer between electrode and the electrolyte.
In a double layer, hetehomogenuous reactions happen and ions and/or electrons cross the layer building up potential which is call electrochemical potential. Electrochemical potential depends on the concentrations of reactants, and on the reactants themselves. This potential in standard conditions can be seen in many physico-chemistry manuals.
Please refer to the drawing I have given above.
Let the reaction H2+1/2O2=H2O for example. This reaction produces a lot of energy. There are two processes happening at the same time : H =>H++e (anode) and O+e=>O--(cathode) then H+ and O-- combine to give H2O.
In hydrogen fuel cell, we just separate the anod and cathode parts and that chemical energy can be exploited in the form of electron current moving from anode to cathode through outer circuit.

 

[vanesch]

There's no way a charge moves against E field inside a battery, for sure. The key point is the double layer between electrode and the electrolyte.
In a double layer, hetehomogenuous reactions happen and ions and/or electrons cross the layer building up potential which is call electrochemical potential. Electrochemical potential depends on the concentrations of reactants, and on the reactants themselves. This potential in standard conditions can be seen in many physico-chemistry manuals.
Please refer to the drawing I have given above.

Really, leright is right on this. Your drawing is not possible, as has been said before: you would otherwise obtain a non-conservative electrostatic E-field.

The whole point is that the flow of electrons (and ions) is not controlled by the electrostatic potential, but by the ELECTROCHEMICAL potential. That electrochemical potential is also function of the concentrations of chemicals and a battery is exactly such a structure, where the gradient in electrochemical potential and the gradient in electrostatic potential are in opposite directions. Hence, it is the electrochemical potential which drives electrons and ions against the electrostatic force. Of course, the electrostatic potential is a PART of the electrochemical potential. So it is true that the electrostatic force tends to diminish the tendency to flow against the E-field, but if the concentration gradients can overcome this, then nevertheless, the charges flow against the electrostatic force. The price to pay is that this flow will change the concentrations of chemicals in exactly the way which is necessary to "drop" the gradient of the electrochemical potential.

The system reaches a static condition when the electrochemical potential is equal everywhere: in that case, charges are not "motivated" to move anymore. This situation can STILL contain both a gradient in electrostatic potential and a gradient in concentrations. This is BTW, what happens in a PN junction in a semiconductor. There, you DO have an E-field, and NO charges flowing (because they are pushed exactly the same amount in the opposite direction by the concentration gradient, and both cancel).

 

[haiha]

OK, i agree with you that in this case the E field is too small to rule the migration of charges, in this case ions. My picture should rubout the E inside because electrolyte is a good conductor, may be.. The whole thing can be imagined as there's a piece of wire which has an electric current, if you put the wire inside an electric field E, not to large, the current is unchanged, no matter how you dirrect it.
Anyway, I should insist that the double layer play an important role here. For example if you dip in an electrolyte 2 identical inert electrodes with a small potential difference, let say 0.5 V, and several cm apart, surely there's a current passing the solution.

 

[vanesch]

OK, i agree with you that in this case the E field is too small to rule the migration of charges, in this case ions. My picture should rubout the E inside because electrolyte is a good conductor, may be.. The whole thing can be imagined as there's a piece of wire which has an electric current, if you put the wire inside an electric field E, not to large, the current is unchanged, no matter how you dirrect it.
Anyway, I should insist that the double layer play an important role here. For example if you dip in an electrolyte 2 identical inert electrodes with a small potential difference, let say 0.5 V, and several cm apart, surely there's a current passing the solution.

Yes, but the point is, that it is a law in electromagnetism that when you integrate the E-field along any closed path, in an electrostatic situation, that the integral is 0. That's the rot E = 0 equation (because B isn't there, and doesn't change). So, IN ANY CASE, a charge going around a closed loop cannot all the time go in the direction of the electrostatic force that works on it. The work done on a charge by the electrostatic force, when it goes around a closed path, is strictly 0. So IF charges go around a closed path, and DO work in some or other way, then it is sure that their motion cannot be solely come from the electrostatic force.
At some point, they have to move AGAINST the electrostatic force. Now, whether this is in the solution, or in the double layer, or... I don't know - probably it is in the double layer.

 

[Scigineer]

I have to say that the picture of Haiha is about a supercapacitor. The electric double layer theory can be found anywhere for supercap. In a capacitor, both electrodes are oppositely charged. There is an electric field between charged items. There is NOT any electric field between two neutral stuffs. Both batteries and fuel cells have neutral electrdes. There is NOT electric field between them because they are not charged during operation (discharging). Both electrons and ions in electrolyte are driven by chemical potential to move. NO wonder they could go for a same direction from both ways external and internal because they simply don't have to go againt a field. This is just about chemical potential, a energy gradiant for charges. Now, this is about discharging. During discharging, it is not a circuit. During charging, it is a little different. You will have a circuit because you need to maintain the outlet neutral. It is an electrolysis because chemical species in a cell need to conduct the current for the circuit. During charging, there might be electric double layer involved. Electrochemistry is uniqe, and electrostatic theory could not explain phenomenons in it.

 

[technician]

Electric field strength is defined as the direction of the force on unit +ve charge.
So it's direction is from + to -
The charge that moves through a battery from the + terminal to the - terminal is -charged electrons and it is electrons that flow through that wires.
If electrons were + charged (!) there would be less misunderstanding

 

[Scigineer]

In a discharing battery, the process is a decoupled redox chemistry reaction. Instead of having reactants contacting each other, a wire is provided for eletron transferring and an eletrolyte is provided for ions tranferrring. The reaction is driven forward by the chemical potential, a energy gradient. There is NO electric field in a chemical reaction. In a discharging battery, neither electrons nor ions are driven to move under electric field. It is confusing that we call electrodes + or -; however, neither elecrodes are charged. They are neautral and there is no E-field between them.

 

[Per Oni]

There is NO electric field in a chemical reaction. In a discharging battery, neither electrons nor ions are driven to move under electric field. It is confusing that we call electrodes + or -; however, neither elecrodes are charged. They are neautral and there is no E-field between them.

This part is completely wrong.

If this battery is not flat there will be a voltage between the electrodes. The electrodes form a capacitor. This capacitor will get charged with +ve and –ve charges. That’s why they are called +ve and –ve electrodes.

Simple really.