Microbial Fuel CELL

[soccer987]

How much electricity should the microbial fuel cell generate with a 220 ohm resistor?

[deleted-71882]

Hello soccer987,

Quite frankly, I didn't know the answer to your question, but I looked around at various web postings about microbial fuel cells and found that such cells typically produce about 0.15 volts. Ohm's law governs how much current flows through a resistor when a certain voltage is applied to it. The relation is I(current) = V(voltage) / R(resistance. So if you apply 0.15 volts to a 220 Ohm resistor, you should get I = 0.15 / 220 = 0.68 milliamperes. The power of such a current is P = V x I = 0.15 x 0.68x10^-3 = 0.10 milliwatts.
The cell will provide the current only if its internal resistance is much lower than the 220 Ohm load. You can determine the current by using your multimeter to read the current directly. You can also measure the voltage that the cell can produce across a variety of resistors to find out just how much it can put out.

Hope this helps, WW

[soccer987]

Thank you. But for some reason my fuel cell on produces .09V at the maximum. My science teacher thought this was quite low. Do you know why my fuel cell produces such a low voltage? WIthout the resistor it produces about .35V but I understand that I=V/R and no resistance would result in no electrical current.

[soccer987]

I also have another question. How is .15/200 .68? Wouldn't it be .00068? Also, can you explain to my how the power equation works? It would be very helpful if you can. Thanks

[soccer987]

nevermind about the .68 haha i'm blind. I didn't see the milliamperes. Sorry!!!!

[soccer987]

And I understand the power equation! Yikes, I'm slow to catch on. But I was still wondering....how come my voltage is so low?

[deleted-71882]

soccer987,

I think you understand our exchange so far, but just to clarify. The remainder of the 0.35 volts is being dissipated in the cell when 0.68 milliamps (mA) flows. A number of factors can limit the current that the cell can produce. For example, the reaction may be limited by something--say not enough bacteria or not sufficiently concentrated nutrients to produce more current, or there might be resistance to current flow through the salt bridge. In that case, the voltage rises to 0.35 volts when no current is flowing, but is limited to 0.68 mA when the current is allowed to flow. I think you would have to experiment with changing the cell to test each possibility. Use the following observations to focus in on what might be the limit.

Say that the cell produces H+ ions at a fixed rate with little impact from the external current. Then you should find that you get the same 0.68 mA with any resistor of lower value than R = 0.35 / 0.68x10^-3 = 514 Ohms. If the current limit is caused by a restricted salt bridge, and the current through the salt bridge is proportional to the electric field pushing the H+ ions along, then the voltage used up by the salt bridge has to increase to get more current, and the resistor gets only the leftover voltage. In this case, the current and voltage in the resistor will change in inverse proportion.

WW

[soccer987]

Thanks. I don't quite understand but it sort of makes sense.

[soccer987]

Uh oh. Today was the actual science fair and when I shows everyone how much electricity it produced, it produced .3V! Ahhhh! And in my conclusion I put that the resistor limited it to about .09V , but you need a resistor in order to have an electrical current and in turn have power.