Electricity from fruit (cont'd)

A. Hi, Amber. A graph is a mathematical language we use to illustrate and explain a relationship between two or more things. What exactly is it that you would like the graph to illustrate and explain? Your question is a bit like saying that your daughter has to write up her project in Spanish or French, so she is confused about what to write :-)  Start by expressing her findings in English, and I'll explain how to graph those findings. Good luck.

Hi, Kimberly. We've tried to explain several times why such a variation can exist, but it's probably just random depending on the particular nail, penny, the spacing between them, and particular sample of fruit and vegetable. But frankly you may be focusing your attention on a triviality. Do you think it is an important scientific fact whether a potato or lemon is better? If not, what is the heart of what you are learning -- that's what is important, so that's what you want to focus on. Best of luck.

Regards,

A. Hi, Jeri. Yes, you are complicating things and making it hard on yourself. You said the purpose of your experiment was to find out if different acidity levels effect the voltage. Stop there for a minute -- because your "purpose" has already identified the independent and dependent variable.

The acidity level is the independent variable that you will be varying and the voltage is the dependent variable that you will be measuring. The best way to get a range of acidity levels is probably to use a range of fruits. Good luck!

But while I can understand why 3rd graders are making fruit batteries, I am a bit disappointed that, 7 years deeper into their education, 10th graders are still playing with fruit instead of conducting more controlled experiments with real reagents :-(

Regards,

A. Hi, Candis. You can measure the resistance of a fruit or vegetable by merely setting the meter to resistance mode, poking the leads of the meter into the fruit, and reading the resistance of the fruit in Ohms. Note that the resistance will vary depending on how far apart the probes are; there will be less resistance when they are close together, and more resistance when they are at the far ends of the fruit.

'Charge' is a bad term. Please don't use it as it is both wrong and misleading. You want to measure voltage. To measure this, set the meter to voltage mode, and check a flashlight battery (D cell) or a AA or AAA battery. Your TV and VCR and DVD remote controls probably all have one. Such a battery will read about 1-1/2 volts if you have set the meter correctly.

Now put a galvanized nail, or something else that is zinc into the fruit, and a penny or something that is copper into the fruit. Measure the voltage from the nail to the penny and it should be in the neighborhood of 1 volt. Best of luck.

Regards,

A. Yes, Dan, the lower forty alone should be enough to keep a nite light going. I wouldn't eat them because they'll have copper and zinc dissolved in them, and you might become the nite light :-)

Regards,

A. Hi, Sanskriti. The pH will probably range from about 3.0 to 5.9. I'm sure the teacher will provide fresh pH paper of the right range and show you how to use it rather than have you guess the pH of a "mixed fruit juice". Sorry, but that kind of guessing is not science.

The use of pH paper is widespread in science projects for grades K thru 5. I don't know what "class 12" equates to in our system, but any student pursuing science classes should know how to use it; wanting to take a pass on this at this point is inappropriate. Good luck!

Regards,

A. Hi, Victoria. Many people recommend pennies and galvanized nails as the two electrodes.

Hi, Judy. Sure you can but ... it will be both difficult to conclusively prove that one type of apple is consistently "better" than the other, and fairly inconsequential when you have done so.

Regards,

A. Hi, Lauren. My hypothesis would be that you can use other foods in general and strawberries in particular. Good luck.

Regards,

A. Hi, Ray. I'll try to fully explain things at adult level and let you explain to him the portion he needs at his level --

You can take a small amount of salt, and stir & dissolve it into water, and the salt will stay dissolved. You can stop stirring, then try pouring it through a coffee filter, but the salt will stay dissolved (in ionic form, or as ions in the water).

But you can't keep dissolving salt into the water indefinitely; rather, the water has a solubility limit for salt. After that point, the extra salt will just settle out at the bottom when you stop stirring. Not all of the salt will settle out, only the amount which is beyond the solubility limit.

Similarly, metals will dissolve into water. Generally, if the water is neither acid or alkaline, very little will dissolve. As the liquid is more strongly alkaline or more strongly acidic, it will generally hold more metal dissolved into it. Once again, there will be a solubility limit.

So at a given acid level, you can dissolve so much copper into it or so much zinc or so much iron. So let's say you've put a piece of zinc into the acid and it has dissolved as much as it can and you take the piece of zinc out. Now you put a piece of copper into the acid, what will you see happen? Nothing.

Now take some fresh acid and put the piece of copper into it. After a while, take the copper out and put a piece of zinc in. What will you see happen? The zinc will become coated with copper. The zinc wants to dissolve more strongly than the copper wants to stay dissolved. For zinc to dissolve into this acid which is already saturated with copper requires that copper be driven out of solution.

We can say that zinc is more "active" or copper is more "noble" (iron is in between these two in activity by the way). But we can actually put numbers on how much more active zinc is. Google "electrochemical series". Compared to acid or hydrogen, copper is +.3419 volts, iron is -.037 volts, and zinc is -.7618 volts.

If you put a piece of copper and a piece of zinc into a perfect acid, you will therefore read a voltage of 1.1037 volts between them. If you put a piece of zinc and a piece of iron into a perfect acid, you will read .7248 volts between them. Between a piece of copper and a piece of iron you'll get .3789 volts.

Your son's numbers are lower than the other kids' because the potential between iron and zinc is less than between copper and zinc. They sound reasonable.

As for "asking the right questions", I'm sure you realize there is some "feel goodism" designed into this experiment, with natural fruits "creating electricity", and we don't want to completely torpedo that -- so I think he's doing fine.

Regards,

A. Hi, Dasser. View the youtube video which we link to at the top of this page. I know you can find a penny to use as the copper electrode, and your mom or dad probably has a galvanized nail to use as the zinc electrode. Good luck.

Regards,

Hi, Darren. Yes, the internet can be a terrific learning tool when people like yourself want to learn. But it can also be a tool for those who don't want to learn, enabling them to find and cut & paste answers without having to do the hard work of thinking or trying to understand the question; that's what parents, teachers, and the rest of us try to guard against :-)

Regards,

A. Hi, Ramon. Please try your best to express your question in terms of the information that people have generously taken the time to make available to you on this page already. I think the "lemon battery" video doesn't leave out too much. So maybe you could pose your question in terms of what you didn't understand in that video?

Regards,