Science Buddies: "Ask an Expert"

My son has chosen one of the Science Buddies experiments for his 2nd grade science project. A Battery that Makes Cents.

https://www.sciencebuddies.org/science- ... ?from=Home

May you help him with some questions/problems?

1. His data trend doesn't make sense. His stack of 6 pennies/nickels produced a lower voltage than his stack of 4 pennies/nickels. His stack of 12 produced a higher voltage than his stack of 4 and his stack of 6.

2. The voltage readout is not stable. It keeps changing by small degrees and at some point, he just picks an average number for his readout. Is this the way it's supposed to be?

3. He tried to light up a small lightbulb. His stack became too tall and fell down and he still wasn't able to get the lightbulb to light. Is it because he has issues with questions 1 and 2 above that he can't figure out the number of coins needed?

4. Does the paper towel in between the coins need to be just wet or dripping wet? When he builds it, it tends to get very wet and drips to the starting paper that is supposed to be dry. We move the stack to a dry portion of the paper towel, but sometimes it gets wet again.

Do you have any suggestions as to what we are doing wrong? Thank you.

Hello and welcome to the forums!

Each time you add another pair of coins, you should see an increase in voltage. If you're having trouble getting reliable readings, you may want to clean the coins and start over. As the electrolyte corrodes the coins, you may not get a good connection between layers.

I'm afraid that at the low voltages produced by coins you will see some voltage change with time. You might want to record "average" and "maximum" voltages for the piles and compare them.

I doubt that any height of coins will light a light bulb. Resistance lighting takes way too much current to work with a coin battery. You would need a lot of coins in series AND in parallel to be able to light a bulb. A good research extension of this project would be to explain why this is so. Did you try measuring the "current" of the stack as suggested in the experiment? Compare these numbers with the current required by your light bulb. Can you calculate how many coins in series and parallel it would take to light the bulb? This kind of extension of the experiment is what judges are looking for.

The paper towel needs to be wet, but there's a very fine line between wet and dripping wet with paper towels. It's going to be messy

Keith

amymau - In addition to Keith's comments, you asked about whether the paper towel square should be damp or dripping wet. This may result in the observation that the experiment with more cells produced a lower voltage. Each additional pair of coins should produce a higher voltage as the cells are connected in series. However, if the electrolyte drips down the coin, it is conductive, and will have the effect of 'shorting' out a cell. That may account for the lower voltage noted. The spacers should be very damp without leaking over the edge. Just some careful technique. I also wonder what current you may have measured in the experiment. If you can produce something in the range of at least 5 milliamps and a little over 1 volt, you may be able to light a red LED.

Rick Marz

Thank you for the response. I will have my son do the experiment again. Previously, we built separate stacks and measured the voltage separately. I will have him build one stack, measure, and then add on to the stack. Then I should see a trend in the results. I will also have him use a dropper to wet the paper to help us better control the wetness. As for the lightbulb, we bought one requiring the least amount of volts/mA that we could find; it wasn't an LED bulb though. It's listed as requiring 1.5V or 25mA. Our best measurement didn't go over 1V. The best measurement we had was .8V. We will see if our next run gets better results.