asmith123183
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Potato Battery

Postby asmith123183 » Wed May 22, 2019 5:45 am

I'm trying to demonstrate electrochemistry with my chem students. I have a minibulb which I tested using a regular battery (reading 1.5 V) and it works fine. I have my potato battery set up which is reading at 1.85 V with two cells. However as soon as I connect the bulb, the voltage reading goes to zero and of course the bulb won't light up. Any suggestions? I'm baffled. Not sure what could be wrong since I am getting a higher voltage reading than the regular battery. I have also tried switching the terminals and that didn't help either (didn't think it would matter, but tried it).

theborg
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Re: Potato Battery

Postby theborg » Wed May 22, 2019 8:23 am

asmith123183,

Thank you for your question. Ohm's law and Watt's law are the driving concepts here, Ohm's law is V = IR, and Watt's law is P = VI where
V = voltage (volts)
I = current (amps)
R = resistance (ohms)
P = power (watts)

The thing that comes to mind is you may be able to measure 1.85V under "no load" conditions, but the load (light bulb) requires a certain amount of power to operate based on the resistance rating (in ohms) of the bulb. If it is more than the current (I), measured in amps (A), produced by the potatoes can handle it will not work.

Think of it this way. Voltage is how much water you are supplying to a water tap, current is how much pressure is behind that water. So, essentially given the same size tap and supplying the same output of water, but at different pressure settings, one you can stop the flow easily, the other you cannot. it's a crude example, but I hope it helps illustrate the point.

Essentially, the resistance (R), measured in ohms, of the light bulb is high enough to completely stop the current of electrons coming out of the potatoes, and since no electronic connection is made, your voltage reading drops to 0V.

Not sure what regular battery type you were using as an analog, but a 1.5V AAA battery is about 1amh, meaning it can supply 1amp of current for 1hour under load. So say the power rating of the light bulb is 0.5W. 0.5W/1.5V = 0.33A, which means you'd be able to operate the light bulb for around 3hrs before the battery died. However, when you change the power supply to the potatoes, my guess is the current able to be generated is much less and just can't generate the power required to operate the bulb.

My suggestion is A) try wiring more potatoes together in series until you can match the current requirement, B) find a load (light bulb) with a much lower resistance (ohm) rating or C) use your volt meter as a means to prove you get voltage across the potato.

You might even keep your original light bulb as an example to prove the relationship between V, I, and R.

I hope it works out well for you and your students.
Hope this helps.

theborg
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asmith123183
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Re: Potato Battery

Postby asmith123183 » Wed May 22, 2019 12:11 pm

Thanks so much! Makes perfect sense. Electricity is not my strength obviously :).

bfinio
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Re: Potato Battery

Postby bfinio » Wed May 22, 2019 4:44 pm

Hi asmith123183,

theborg hit all the key points (the gist of it being that you have to think about voltage AND current, not just voltage), I just wanted to add a couple things. In our version of the "Veggie Power" project, we use LEDs instead of incandescent bulbs. They require much less current, so you can light one up with just a couple potatoes:

https://www.sciencebuddies.org/science- ... to-battery

Even if you aren't using our Veggie Power kit, you could just buy the LEDs separately online. Be careful though, because if you hook an LED directly to a AA battery, it will burn out if you don't include a resistor in series (because in that case the battery provides too much current).

The potential downside there is that LEDs do not follow Ohm's Law (they aren't a resistive load), so they could be confusing if you wanted to teach Ohm's Law.

Also, we have a new video debunking some common potato battery myths that you might find of interest. Unfortunately there are a bunch of hoax videos online claiming that you can charge phones or light up full-sized light bulbs with potato batteries (or other fruits/veggies like lemons, onions, etc). We occasionally get questions from students about this - and part of the explanation is similar to what theborg described (essentially, while you might measure enough voltage to charge a phone by combining a few potatoes in series, it is nowhere near enough current):

https://youtu.be/q9X-ez31oiY

Hope that helps!

-Ben

asmith123183
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Re: Potato Battery

Postby asmith123183 » Thu May 23, 2019 8:27 am

Awesome! I'll check out those resources. Yeah, the internet definitely has some misinformation out there which is why I turned here! I'm really just using it in my chemistry class to demonstrate how redox reactions generate an electric current with two metals of differing electron affinity. We also did electrolysis where we zinc plated a penny, then heated it to create a brass alloy. At least that one worked ;).

I found some LED's rummaging in the back storage closet (since one website suggested that). I wasn't successful there either. I'll take a look at the resources though and see if I can make it work.

Thanks for all your help.

bfinio
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Re: Potato Battery

Postby bfinio » Thu May 23, 2019 8:50 am

No problem! And just for the record, I have the opposite problem - mechanical/electrical engineering background, so the circuit part makes sense to me but the chemistry part is a mystery :-). Please don't hesitate to write back if you have more questions.

As for the LEDs - if you wind up buying them, search for "super bright" or "high efficiency" LEDs. That's what we use in our kit - they're much easier to light up. For example:

https://www.jameco.com/shop/StoreCatalo ... =jamecoall

* additional edit - I should add that there's a chemistry aspect to the phone-charging myth as well, that may be more related to what you're doing. Most of the hoax videos show a regular USB charging cable plugged into the fruit/veggie. However, the contacts on those cables are all one type of metal (usually a copper alloy for the standard ones, maybe gold-plated for expensive ones). But of course...you don't have two dissimilar metals, so there's no reaction and thus no current. I'd be curious if your students could figure that out on their own, without seeing our video first.

theborg
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Re: Potato Battery

Postby theborg » Sat Jun 01, 2019 4:14 pm

One thing to check...LEDs are pole dependent. They are diodes, meaning they are meant to allow current to flow in only one direction. So if you put them in a DC current backwards, they will not work. If you think you have enough current to drive them but are not getting them to light up turn them around. If they still don't work they may be broken. Or the circuit still isnt enough to support them.
Hope this helps.

theborg
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