Fruit Battery and Current
Date: Winter 2011-2012
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?
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
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.
Greg (Roberto Gregorius)
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Update: June 2012