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Saltwater voltage source question

  1. Dec 25, 2008 #1
    Hello all,

    I am just a guy playing around. I was trying to see if I understood correctly what electrolytes were, so I set out to make a simple "battery" out of saltwater. It has been done before many times and I thought I could at least make it light an LED.

    Here it is :

    [​IMG]

    [​IMG]
    The problem is that despite the fact that I managed to reach 4.75Volts of voltage I can't make the LED light up.

    To be precise , whenever I make contact and close the circuit it lights up for an instant and then the light dies out. If I repeat .. the same happens each time. That is the reason I am writting this topic too. I would like to ask anyone who might know why it does not function as I thought it would be? I am trying to understand physics just as a hobby so please go easy on me if I said something really stupid in this post. :)

    Thanks in advance for any comments ,
    -Lefteris

    P.S. : Also another question, I read something about amperage but from wikipedia I deduced it is the same as current? Is that correct? Also ... unfortunately my internal fuse has been broken in the multimeter and I can't measure current :(
     
  2. jcsd
  3. Dec 25, 2008 #2

    Integral

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    Staff Emeritus
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    Gold Member

    A few weeks ago we built cu/ lemon/zn cells in the classroom. We had to put more then 10 cells in series to generate enough current to ligth a led. Try more cells.

    As for the missing fuse... do you have any solder or Al foil? Simply short out the fuse holder with a wrap of solder or a strip of foil. You will not be fused, but for this project it should be fine.
     
  4. Dec 26, 2008 #3
    Hello!

    Thanks for the reply. I am using exactly 10 cells. I will try to increase their number and see if it will then light the LED. Thanks. As for the fuse of course I have solder, just need a screwdriver to fit and remove the screws from the multimeter.


    I have another question though. Since this whole thing is something I did to better comprehend what a battery is. I am now wondering. What happens when you totally discharge a battery? Say What happens when I discharge this batery tottaly? The electrodes won't work anymore , or the water-salt combination will need changing? Or both?
     
  5. Dec 26, 2008 #4

    russ_watters

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    Staff: Mentor

  6. Dec 26, 2008 #5
    I suggest the led illuminates briefly then goes out, is because while you are drawing no current from your cell the cell electrolyte slowly builds up a good quantity of ions which discharge quickly and briefly. (a bit like discharging a capacitor). From my own home-made cells I have seen short current spikes of over 1 amp. If you put a sensitive ammeter across the cell, you would see the current spike. The current and voltage then settles down again, but not enough unfortunately. From my experience, you will need to massively increase the area of your electrodes. Sea water is a very weak electrolyte. Unlike commercial cells, the voltage drops extremely quickly as you draw current. Forget about theoretical voltages as per the electromotive series.
    Increasing the number of cells will not necessarily work because it is in the nature of these lashups that one cell will be inefficient quicker, and the others will waste their output trying to equalise/recharge it thus bringing down the whole battery. Thw weak one will gas more vigorously.
     
    Last edited: Dec 26, 2008
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