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I made a series of 12 galvanic cells with saltwater as an electrolyte and zinc and copper as electrodes. It produces 8.8 V, but it can't even light a 2.5 V bulb (the bulb can be lit by a 1.5 V source).

Please assist in understanding what could be the problem here.

Update: the galvanic cells almost hit 10 volts (9.99 peak voltage) with 0.4 amps. However, it still can't light the bulb. 

Set of galvanic cells with a multimeter

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    $\begingroup$ Assuming the bulb is working properly, the cell battery does not provide required voltage under the load, i.e. to the closed circuit. Measure the voltage when the bulb is attached. If it is enough then the bulb or the circuit are broken. If it is not enough then the bulb obviously cannot shine and then try changes suggested by Maurice. $\endgroup$
    – Poutnik
    May 20 at 10:33

2 Answers 2

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It is not a problem of voltage. It is a problem of current and power. Whatever its nominal voltage, your bulb has a power to be known in watts. If your $\ce{Zn/Cu}$ cells are not able to produce the required current, the bulb will never be able to emit light. And the current is proportional to the surface of the zinc plate.

So first, determine the power of your bulb in watts. Divide it by 2.5 V. You will obtain the nominal current in Amperes. Measure your current with one of your arrangements. You will probably measure less than the nominal current. Then try to increase the dimension of your zinc and copper plates. Or make up a second arrangement of cells, parallel to the first one.

Also try to put the two plates as near to one another as possible. This will decrease internal resistance, and increase the current.

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  • $\begingroup$ How can I determine the wattage of the bulb? $\endgroup$
    – Vince
    May 23 at 18:00
  • $\begingroup$ @Vince Try reading, what is written on the bulb or on its packaging. Alternative is to measure the current at nominal voltage as P = U.I $\endgroup$
    – Poutnik
    May 23 at 19:47
  • $\begingroup$ The seller said the bulb is 2.5 watts. @Poutnik $\endgroup$
    – Vince
    May 24 at 4:57
  • $\begingroup$ I see it now. 0.3 amps is imprinted on the bulb. @Poutnik $\endgroup$
    – Vince
    May 24 at 5:14
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Maurice points out at the critical problem and answers the old classical problem. Why can't a 12 V ordinary battery be used in a car when a "large" lead-acid batery also generates 12 V? Your current set-up is not able to generate enough current to light the bulb. You can try the following:

(i) Decrease the internal resistance of the battery by adding more salt. You did not mention how much salt you added

(ii) Increase the surface area of the plates

(iii) Add more "cells" or use a bulb / LED that requires less electrical power.

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  • $\begingroup$ Yes, this is another point, decrease the distance between the plates to reduce resistance. $\endgroup$
    – AChem
    May 20 at 20:21
  • $\begingroup$ @AChem Do you mean the decrease of the wires that connect the plates? $\endgroup$
    – Vince
    May 23 at 17:08
  • $\begingroup$ @EdV May you please say how to make saltwater wetted paper in simple steps? Thank you very much. $\endgroup$
    – Vince
    May 23 at 17:11
  • $\begingroup$ @EdV The copper plates are 10 cm long and 2.5 cm wide. The same goes with the zinc plates. The thickness of the copper plates is 0.05 cm and 0.02 cm on the zinc plates. $\endgroup$
    – Vince
    May 23 at 23:37
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    $\begingroup$ @Vince, if those wires are made of copper, then it should not be a proble. Increase the surface area of the plates and decrease the distance between the plates. $\endgroup$
    – AChem
    May 24 at 11:45

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