Hey there! Sign in to join this conversationNew here? Join for free

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

    • Thread Starter
    Offline

    0
    ReputationRep:
    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!
    • Community Assistant
    • Study Helper
    Offline

    15
    ReputationRep:
    Community Assistant
    Study Helper
    (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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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.
    • Community Assistant
    • Study Helper
    Offline

    15
    ReputationRep:
    Community Assistant
    Study Helper
    (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.
 
 
 
Poll
Do you agree with the PM's proposal to cut tuition fees for some courses?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.