Can anyone provide the answer to this cells in series question?

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Science hopeful
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#1
Report Thread starter 4 years ago
#1
Hi all

This sounds like an easy query but its deceptive and I've seen this question come
up on several other forums and caused quite a debate among people of all science
knowledge levels. To the extent that I have yet to see an answer that truly made sense....

It is commonly known that when cells are in series, the voltages are added to make
the combined total. But why exactly is this?

Lets say you have 3 x 1.5 V dry cells stacked anode to cathode as expected. This as we
know will provide a combined potential difference of 4.5 V.

When dealing with a single cell, the situation is relatively simple - electrons flow from the
anode (eg zinc) to the cathode (via wire) where they pass through the metal (copper/brass) cap and into the carbon electrode inside the cell. The de-energised electrons in the electrode then react with the surrounding electrolytic paste inside the cell in order to avoid the build up of negative charge (which would prevent further flow of current) and to avoid the increase of pressure that would occur due to the formation of gaseous species (typically ammonia and hydrogen).

How does this work when dealing with multiple cells?

For there to be a combined 4.5 V this must mean that each coulomb of charge leaving the
anode now has 3x the energy as it did before (4.5 V = 4.5 J / 1 C).

But how does this voltage build up from the top cell's anode to the bottom cell's anode which is connected to the conducting wire? As the cells are in series each anode is in direct
contact with a cathode (apart from the last one), but how do the energised electrons pass
through each cell in order to build up the voltage? They cant pass through the carbon
electrode because they would react with positively charged species such as NH4+ in the
surrounding electrolytic paste and also the carbon electrode isn't in direct contact with both
the cathode and anode.

I've posted this question in the physics forum as well to double my chances!
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charco
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(Original post by Science hopeful)
Hi all

This sounds like an easy query but its deceptive and I've seen this question come
up on several other forums and caused quite a debate among people of all science
knowledge levels. To the extent that I have yet to see an answer that truly made sense....

It is commonly known that when cells are in series, the voltages are added to make
the combined total. But why exactly is this?

Lets say you have 3 x 1.5 V dry cells stacked anode to cathode as expected. This as we
know will provide a combined potential difference of 4.5 V.

When dealing with a single cell, the situation is relatively simple - electrons flow from the
anode (eg zinc) to the cathode (via wire) where they pass through the metal (copper/brass) cap and into the carbon electrode inside the cell. The de-energised electrons in the electrode then react with the surrounding electrolytic paste inside the cell in order to avoid the build up of negative charge (which would prevent further flow of current) and to avoid the increase of pressure that would occur due to the formation of gaseous species (typically ammonia and hydrogen).

How does this work when dealing with multiple cells?

For there to be a combined 4.5 V this must mean that each coulomb of charge leaving the
anode now has 3x the energy as it did before (4.5 V = 4.5 J / 1 C).

But how does this voltage build up from the top cell's anode to the bottom cell's anode which is connected to the conducting wire? As the cells are in series each anode is in direct
contact with a cathode (apart from the last one), but how do the energised electrons pass
through each cell in order to build up the voltage? They cant pass through the carbon
electrode because they would react with positively charged species such as NH4+ in the
surrounding electrolytic paste and also the carbon electrode isn't in direct contact with both
the cathode and anode.

I've posted this question in the physics forum as well to double my chances!
Don't forget that electrons at no time pass through the cells. Electrons only pass through the external wires.

Each cell provides a "push and pull" on the electrons in the external circuit. If you have two cells you have double the push and pull.
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Science hopeful
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(Original post by charco)
Don't forget that electrons at no time pass through the cells. Electrons only pass through the external wires..
Yes and no

In a single cell, the de-energised electrons do not pass through the entire cell but they do pass into the carbon electrode in the middle of the cell via the cathode metal cap. Here the following reactions (using a typical electrolyte paste) occur:

2NH4+ + 2e = 2NH3 + H2
ZnCl2 + 2NH3 = Zn(NH3)2Cl2
2MnO2 + H2 = Mn203 + H20

The problem with the series cells is that EACH coulomb of charge must now have 3xthe energy as it did in the single cell, therefore the energy from the coulombof charge must be transferred but not the coulomb itself, otherwise you wouldstill have each coulomb with 1.5 V and not 4.5 V.
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charco
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(Original post by Science hopeful)
Yes and no

In a single cell, the de-energised electrons do not pass through the entire cell but they do pass into the carbon electrode in the middle of the cell via the cathode metal cap. Here the following reactions (using a typical electrolyte paste) occur:

2NH4+ + 2e = 2NH3 + H2
ZnCl2 + 2NH3 = Zn(NH3)2Cl2
2MnO2 + H2 = Mn203 + H20

The problem with the series cells is that EACH coulomb of charge must now have 3xthe energy as it did in the single cell, therefore the energy from the coulombof charge must be transferred but not the coulomb itself, otherwise you wouldstill have each coulomb with 1.5 V and not 4.5 V.
The potential difference of any number of cells in series is measured across all of the cells. Hence you are only talking about the p.d. in the external wire.

If you were to measure the p.d across each individual cell then the electrons would experience a push/pull of exactly one cell worth of p.d.
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