Science Buddies: "Ask an Expert"

https://www.sciencebuddies.org/science- ... p026.shtml
^^^
I am thinking about doing this project for my science fair. My only question about the procedure is about the 220 ohm resistor. Why do I need to attach this to my external circuit? Will this lower the amount of electricity I am receiving from my fuel cell? Thanks for your help in advance.

EDIT: I can also obtain 100 ohm resistors and 400 ohm resistors. How will this affect my experiment?

If you attached a very large resistor, in the limit an "open" circuit, you would see the largest voltage from the fuel cell, but the current would be very small, zero in the limit. Thus the power delivered to the external load (i.e. the resistor) would be Vmax volts times 0 amps = 0 watts. On the other hand, if you short out the cell with just a wire, the resistance will be very small, close to zero, and the current will be the maximum the cell can produce, but the voltage across the short will approach 0 volts. Thus the power delivered will be 0 volts times Imax amps = 0 watts. In order to get any useful power out of the cell we need to maximize the product of the current and the voltage. The value of 220 ohms was probably recommended based on experience to get good power performance. It would be a very nice experiment to vary the resistor while measuring the voltage and calculate the power for each value of R. Note that the current = (voltage across resistor)/resistance, so that the power being delivered to the resistor (which serves as the "device" being driven by the cell) is V times (V/R) = V^2/R, so you do not need a separate meter for the current. Be aware that the actual resistance of the resistor you use will differ from the value marked (in color code) on the resistor by an amount that depends on the precision class of the resistors you purchase. The cheapest resistors will be "off" by about 10%, while a "precision" resistor will be correct to 1% (or even less). But that's not a problem, since your multimeter can be used to measure the true value of the resistor before you put it into the circuit. The accuracy of the readings of resistance and voltage by the multimeter should be given in the manual for the multimeter. Another factor to watch for is the temperature of the cell and of the resistor; both devices are somewhat temperature sensitive. If the resistor starts to feel warm, it may be a good idea to cool it with a fan so that the various readings are made with the resistor at room temperature (which I am assuming will be fairly constant). You can learn a lot more about resistors by looking them up on Wikipedia.

Best of luck with your experiment!

Thank you very much, this is just what I was looking for!

I'm working on this project and have a question about the supplies
https://www.sciencebuddies.org/science- ... p026.shtml

I'm uncertain if the compression fitting size listed is correct. The instructions include the following:

* Compression fitting, 1/2-inch (in.) ---- Is this Inner Diameter (ID) or Outer Diameter (OD)?
The local hardware store only have 1 1/2 inch PVC compression fittings. The 1/2 OD compression fittings they had were all metal.

* Using the 3/4-inch spade drill bit, drill a hole --- Why is this size larger than the size of the fitting (1/2 inch)?

* The picture of the end cap (Figure 2. Roughening one of the end caps) looks a lot bigger than 1/2 inch

Help!

I hope the following URL will help to answer your question:

http://www.gizmology.net/pipe.htm

Good luck on your project!

Thanks the Pipe size background is helpful. My question is whether the instructions are correct and a 1/2 compression fitting isn't a typo.

I think this is the one I'm looking for
http://www.amazon.com/King-Brothers-Inc ... 60&sr=8-22

The instructions to make the electrode say to strip 6 inches of wire and wrap it around the edge of the 5 cm square piece of carbon cloth. My results don't look like the sketch in the instructions. See attached document.

Is the sketch incorrect or perhaps the dimensions?
Not sure why some measurements are in centimeter and others in inches.

I couldn't see anything when I opened the attached file. However, the sketch of the 6" of wire lying on the 5 x 5 cm carbon paper seems to me to makes sense: 5 cm is 1.97", so running the wire along three 5 cm edges takes close to 6" of wire. I agree that it would be better to specify all dimensions in cm.

The Upload Attachment functionality in the forums doesn't seem to work.
After attaching a basic word document it downloads as some other file thing. I think attachment function is broken.

The file had to be less than 250K also, which isn't very practical. You also cannot upload just an image file. It complains that the following image file types are not supported as attachment, jpeg, gif, tif, bmp.... so I tried an office document.