vegetable electricity

[janeelindsey]

We have a circuit of fruit (grapefruits) that the volt meter says is putting off more than 5 volts but we cannot power any of the loads we have tried (calculator, 1.5 volt bulb, 1.5-3 volt buzzer.) What are we doing wrong?

[rmarz]

Janeelindsey - Although you are able to generate and measure 5 volts (think of voltage being similar to pressure) this experiment is not producing much current (flow of electrons). To power either a light bulb or buzzer you may need many square inches or even feet of electrodes (probably metals like zinc and copper) to produce the 10's or 100's of milliamps (0.001 ampere) required to power these devices. You may have good luck with the LCD calculator. Typically a small LCD calculator may draw no more than microamps (0.000001 ampere) to milliamps (0.001 amperes) to operate. Also, you have to observe correct polarity of the power source for the calculator to work. It sounds like you have a sensitive voltmeter, so that can be used to establish correct polarity. If your meter is capable of measuring current you can attempt to determine the actual current the grapefruit(s) are producing. This experiment is really demonstrating a chemical reaction between the acid in the citrus fruit acting on two dissimilar metals having different electro potentials. Your light bulb and buzzer may require too much current to operate. You can see the effect using your voltmeter. While you may measure 5 volts 'open circuit' as soon as you connect the bulb or buzzer my guess is that the meter will read 'zero' as you have overwhelmed it with load, in essence they appear as short circuits in this experiment. Try the calculator, or perhaps a low power piezo-electric buzzer or sounding device (with internal circuitry to operate from direct current). Good luck.

Rick Marz

[deleted-71588]

Another way to approach this is to attempt to measure the internal resistance of you "battery". For this, you will need several resistors covering the range of 100 Kohms down to 1 Kohms. The process is to hook up your volt meter to the battery and place different combinations of resistors across the battery connections (shunt) and plot the voltage with respect to the load resistance. The internal resistance of your "battery" will be that resistance where the voltage across the load equals the the resistance. You probably won't have an exact resistor combination to match the internal resistance so you can estimate it by interpolating graphically between two adjacent resistance voltage pairs that are on opposite sides of the half open circuit voltage point.

You can typically decrease the internal resistance by using electrodes with larger surface areas placed closer together in the grapefruit which means that you will be able to draw more current from the grapefruit battery and deliver more of the power being generated to the load. If you conduct a collection of experiments with different surface area electrodes placed at different distances, you can investigate how the internal ion behavior is a function of the surface area and separation distance.

5 Volts sounds like a lot of voltage to come out of a single cell battery. What metals are you using to get this high of a voltage or do you actually have several batteries in series. If you are using the same undivided grapefruit for multiple cells, part of your high internal resistance is probably coming from cross-currents between electrodes of different cells. If this is your situation, you need to put the different cells into electrically isolated pieces (different grapefruits or different insulating containers each with a section of grapefruit and its electrodes making up a cell).

[amyC]

I'm posting the link back to the Science Buddies project: Veggie Power, for reference: https://www.sciencebuddies.org/science- ... p029.shtml

Amy
Science Buddies