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Name: Rahil Status: student Grade: 4-5 Location: MN Country: USA Date: Winter 2011-2012

Question:
Hello, I am a 5th grade student and doing "Fruit Battery" experiment for my science fair. My question is "What produce requires the least amount (in mass) to make a battery that lasts the longest?" I set up trials using potato, apple, orange and lemon. For all these produces when I created the first voltaic cell and measured the open and closed circuit voltages using multimeter, they were same, I am using 2.6V LED to light up which has no resistance. I also did not use any other external resistor. After adding 2nd voltaic cell to the circuit in series, the open and closed voltages differ by some amount. I want to know the reason behind the difference in the open and closed circuit voltages when there is more than one voltaic cell.. Also I tried to measure the current but it always started with one value and kept going down and never got stabilized at one value. May I know the reason for this as well?

Replies:
Rahil,

An open circuit is one that does not have a path in which the electricity can flow. It is like the power going to the lights in your house. If the switch is not on, the circuit is open, and the electricity does not flow through the bulb to produce light. A closed circuit provides a path for the electricity to flow. It is like the switch to your lights being turned on, the circuit is closed, and the electricity flows through the bulb to produce light.

Any battery runs down. So your "fruit battery" will have an initial closed circuit voltage, but the chemical reaction uses up reactants the amount of voltage is reduced.

Ray Tedder, NBCT Chemistry Teacher


Rahil,

It is a common misconception - mostly because when people talk about "produce" batteries they imply that the voltage difference comes from the produce itself - that the produce is the source of the chemical reactions that result in voltage. This is not true. The produce is simply the medium of ion transport, the chemical reaction that results in a voltage is in the fact that we are using two different metals (usually a zinc coated nail and copper wire).

When these electrodes (the metals) are placed into the produce in an open circuit, oxides (for example, the tarnish on a copper wire, or the rust on an iron nail) that are coating the pure metal will dissolve into the acids of the produce resulting in a solution of ions in the produce (there are also ions already in the produce itself). When the circuit is closed, spontaneous chemical reactions happen. Primarily, the metal on the anode will become oxidized (become positively charged), and the ions dissolved in the produce will become reduced and deposit as pure metal onto the cathode (change from positively charged ions to neutral metal). Other reactions will also take place, but they will all be oxidations (loss of electrons) at the anode and reductions (gain of electrons) at the cathode.

This oxidation-reduction chemical reaction results, as already noted in the production of electrons at the anode and the use of electrons at the cathode. These electrons travel from the anode, along the wire connecting the two metals, and when a resistor (such as a light bulb or a multimeter) is placed in between, the energy of the electrons get used in the resistor (to provide a reading on the multimeter or power up the light bulb for example). When the electrons (with reduced energy) reach the cathode, the electrons participate in the reduction reaction there (converting positively charged ions to neutral metals for example).

So the produce does at least two things. It keeps the two electrodes from being in physical contact with each other. If they were in contact the electrons would travel from one electrode to the other without going through the wire - so no potential difference can be measured. The produce also provides a medium for the ions to be dissolved and to be transported. If electrons are going from anode to cathode, then the anode must be steadily becoming positive, and the cathode becoming negative. This charge imbalance will grow unless ions in solution also travel between electrodes. The positive ions go the cathode, the negative ions toward the anode.

To prove that the produce is simply the medium and not the chemical reaction, try using different metals as electrodes (copper, zinc, iron, magnesium strips, lead washers, silver, gold, etc.). With each different pairing of metals, you should see different voltages even when using the same produce.

So, long story short (sorry for the lengthy descriptions), the likely reason you are not seeing much of a difference between produce, is because they are all effective media for ion transport. The voltage is mostly a function of the type of metal pairs you are using (for as long as the produce is an effective transport medium).

The reason you are seeing a drop in current, could be for several reasons: (1) the solution is getting saturated with ions, (2) the metal is being coated and the electric field is diminishing, (3) you are using up the oxidized part of the metal and so fewer ions are available - all contributing to a diminished reaction rate. br>
Greg (Roberto Gregorius) Canisius College


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