Re: can I get an electrical current to flow throught an apple pear...ect

The experiment you are performing is easy to conduct but difficult to 
explain so I hope you will be patient with me.  When we attempt to conduct 
a current through any object, we are simply attempting to move electrons 
through the object.  For the electrons to move, they have to have the 
assistance of "charge carriers," or simply put, something that will help 
them move through the object.  The following link gives some brief 
information on the topic of charge carriers: http://www.britan
nica.com/eb/article?idxref=307153

In metal wires, electric current flows by the movement of free electrons 
within the metal.  Gases and liquids normally don't have free electrons; 
the way that current flows through a gas or a liquid is usually by hitching 
a ride on charged particles called "ions."  When some compounds are 
dissolved in a liquid, they tend to break into positive and negative parts 
called ions.  For example, when you dissolve citric acid in water, part of 
the citric acid molecule forms a positive H+ ion, with the remaining part 
of the citrate molecule forming a negative ion.  Table salt is another good 
example of a molecule that forms ions in water.  Many other molecules, such 
as sugar and oil, do not break apart to form ions.  So, sweetness of a 
piece of fruit has nothing to do with electrical conductivity, but tartness 
does.

Normally, the electrical conductivity of a solution depends upon the kinds 
and numbers of ions present.  A really acid fruit, such a lemon, has a lot 
more citric acid than an apple or a pear, so normally, there are more ions 
present, and the lemon will have higher conductivity than the apple or 
pear.  And conductivity within the fruit is determined by the chemistry of 
the liquid; the solid parts of the fruit are insulators.

Now, things get a bit complicated.  If you attempt to pass a low voltage 
direct current through your piece of fruit, when you initially supply 
voltage to the electrodes you have placed in the fruit, the positive ions 
in the fruit are attracted to your negative electrode, and the negative 
ions are attracted to your positive electrode.  Initially, a few electrons 
appear to pass through the solution.  As the ions collect at these 
electrodes, the current stops flowing; each electrode is surrounded by 
oppositely charged ions and so there is no path for an electrode to get all 
of the way from one electrode to the others (kind of like girls surrounding 
the stage of a rock star; he cannot get through the barrier of girls to the 
exit at the other side of the building).  What is formed is called a double 
layer, and is used to make very high energy storage devices called 
"ultracapacitors."  The double layer acts as a very good insulator once it 
builds up.

So, if you want to pass a current through an electrolyte, you really have 
two choices.  First, if you increase the voltage to maybe 5 volts or so, 
there is enough energy for the water in the fruit to break down into 
hydrogen and oxygen.  You chemically blast apart the double layer.  Because 
you are actually performing a chemical reaction at each electrode, 
electrons are consumed at one electrode and produced at the other 
electrode, so the current appears to flow through the fruit.  There will be 
a voltage drop of about 1.2 volts through the piece of fruit, and more acid 
fruits should conduct more current than those with less acidity.

The second choice is not to attempt to pass a direct current through the 
fruit, but rather pass an alternating current through the fruit.  With an 
alternating current, current flows in one direction, then stops and flows 
in the opposite direction.  Current supplied by transformers and by the 
power company is alternating current.  Within the US, the current 
alternates 60 times per second; in some other countries, the current 
alternates 50 times per second.  Using alternating current, the fruit is 
acting like a capacitor in the circuit; each time the current reverses, a 
few electrons are squeezed out of each electrode before the double layer 
builds up.  When companies build meters to measure conductivity in liquids, 
the circuits they use almost always use alternating current to avoid the 
double layer problem.

So, you can try using something like a battery charger which produces a 
high enough direct current to decompose the water within the fruit to 
demonstrate conductivity, or you can use an alternating source of current 
like a toy train transformer to avoid forming a double layer