Science Buddies: "Ask an Expert"

Hi,
I am posting this for my 6th grader son, Joe. His is doing his own version of "Potato Batteries: How to Turn Produce into Veggie Power!" with potatoes and mandarin oranges. The purpose is to determine whether a potato or an orange has more electrical current. The hypothesis is: if the orange is used to create a better, it will produce more electrical current then the potato because it is juicer then a potato.

I attempted to include a picture of how the battery is connected, but was told JPEG file not accepted. He is using copper wire and galvanized wire as the electrodes. Both wires are 14 gauge. He was not able to make an LED or buzzer light with a single potato or single orange. He has also tried connecting in series and parallel the oranges and the potatoes, respectively. The LED still does not light, nor does the buzzer sound. The electrodes are connected with wire connectors (could not find alligator clips, go figure!). Measurements of 4 and 8 potatoes connected in series is 3.5V, 0.11mA and 7.01V, 0.11, respectively. Measurements of 4 and 8 potatoes connected in parallel is 0.9V, 0.01mA and 0.922 V and 0.07mA, respectively. Measurements of 4 and 8 oranges connected in series is 2.45V, 0.03 mA and 6.4V and 0.03 mA, respectively. Measurements of 4 and 8 oranges connected in parallel is 0.916V, 0.01 mA and 0.923V, 0.01mA, respectively. Second trial measurements are similar, but notably, all current measurements are 0.11m. By the way, the potatoes soaked in water overnight.

The LED is from RadioShack, and indicates on the package FW current 20mA and FW supply 1.9V (typical), 2.4V (max). The buzzer only reads 1.5V DC on the back, no mention of current.

Joe thinks that even though the voltage is high, there is not enough current to light the LED or make the buzzer sound.

He has seen similar experiment on various web sites have success at lighting the LED. RadioShack says there is not an LED with lower FW current. The LED lights when connected to a 9V battery.

Question: Is there something incorrect in the connections that is not allowing for better current? As seen on numerous youtube videos, he was hoping to create enough voltage and current to make an iPod charge, or at least make it look like it is charging.

Thank you in advance. He has been working on this for 2 weeks. I just discovered this site, hence the question is close to the due date. The third trial is due 11/4 and the lab report is due 11/8. Currently, he is trying different electrodes (nails) and connected them with leads.

Please feel free to response to me as mom or Joe, my son, the student.

Cathy9 - Veggie batteries are fun to experiment with but they have some limitations. You should read some basic information about voltage and current. Voltage is analogous to 'pressure' while current is analogous to 'flow'. Not a perfect comparison, but it will help you. Veggie batteries develop voltage because of the electro-chemical differences between the electrodes when operating in an electrolyte (the juice of the orange or potato).

Most veggie batteries develop voltages of 1 volt and below, and currents of just a few milliamperes. The current requirements for an LED and a buzzer can be very high compared to the output of the veggie battery. An electro-mechanical buzzer may require over 100 milliamperes. The LED, while stating a rating of 20 milliamperes may still be visible at only 5 milliamperes or so.

Here are a few suggestions.

1) Red LEDs have a lower voltage than other colors, so should be used. The LED is also polarized, so appropriate connections to the positive and negative electrodes are required. If you don't see light, reverse the connections.

2) Joe is likely right about the battery producing too little current. Current can be increased be increasing the area of the electrodes. More electrodes or use of foil electrodes will help.


Hope this helps, follow up and we can continue to help.

Rick Marz

Hello again,
Just a bit of background. There is a video going around on Vine showing how to charge an iPod with a watermelon. This was how Joseph decided he wanted to work with food batteries for his science fair project. I failed to state that earlier. He found a few reasonably reliable web sites (mostly you tube) demonstrating the ability to light an LED with a potato or lemon. He is frustrated that his project is not working, but he realizes things don't always work. He did not expect to be able to charge an iPod, but was hoping to try.

Joe did write a short research paper on batteries, voltage and current. He liked the water flow analogy for voltage and current. He understands the reason for copper and zinc electrodes is because of the chemical reaction that occurs in the potato or fruit. We had 2 LEDs. The first was a red one, but when testing on a 9 V better, it blew. The LED in use now is a bright light LED. A piece of black paper is in the background, and the lights somewhat dimmed when trying to see if it lights.

His uncle is an EE, so we asked him questions today. Joe got a better understanding of the differences in voltage and current with series and parallel circuits. While I was looking at directions of the multimeter today, I developed a question. It states if the measured current is less than 400uA, then select the uA position. If the measured current is 4mA to 400mA, select the mA position. All along, he has used the mA position. I assume the difference between uA and mA is the same as anything in the decimal system. Might there be a difference in sensitivity when using either position? I believe the procedure of remeasuring voltage is correct. The multimeter was purchased from HomeDepot, item # 730 696, model # MS8232B.

Thanks for your reply.
Cathy (mom) and Joe

Cathy9 - I looked up the specs on the multimeter you are using. It is an 'auto-ranging' model, so you shouldn't have any difficulties reading it properly. The microampere vs milliampere is, as you correctly stated, just a 1000-to-1 range difference and shouldn't be of any consequence. 1 ampere=1,000 milliamperes=1,000,000 microamperes. I spent 5 minutes in the garage with a galvanized nail and a piece of copper wire that I placed into a small citrus fruit. I only measured 0.95 volts (probably enough to meet the threshold of a red LED), but only about 200 microamperes of current. Didn't have time to find a red LED. The issue with generating any power here is likely the small surface area of the electrodes. Perhaps a piece of galvanized steel such as you might find in rain gutter or flashing material can be obtained. That would give you an electrode with many times the area of a small zinc coated nail. Same as for the copper. Some copper foil might be obtained that could also increase the electrode area. I'll hunt some red LED's down tomorrow and see if I can make them light up.

Rick Marz