The Galvanic Cell
The Galvanic cell is commonly used as a battery. Batteries are extremely important because they allow for the portability of electronic devices and in the future they may be powering cars. They power conventional submarines (the ones that are not nuclear) as combustion engines cannot work underwater because they need lots of oxygen (the combustion engines works when afloat just to charge the batteries, which will power electric engines).
The chemistry of batteries
Let's try a substance with a positive reduction potential and another with a negative reduction potential, and put them together. The first wants to give electron and the other wants to receive electron, so that it is a perfect combination! Electrons will pass from one to another, forming an electric current.
Example: Copper and Zinc cell
This is the typical example. Let's look at the redox potentials of these two:
Cu2+ (aq) + 2 e- --> Cu(s) ; Eo = +0.34 V
Zn2+ (aq) + 2 e- --> Zn(s) ; Eo = -0.76 V
We can readily see that aqueous Cu2+ likes to pick up electrons . And that Zn(s) likes to give electrons.
A simple and interesting experiment is to place a piece of Zn in a beaker containing copper sulfate. The copper ions will pick up electrons from the Zn (s) and become Cu (s). It is then observed that the blue colour of the copper sulhfate solution fades and metallic copper forms, in its characteristic reddish/brown colour, coating the piece of iron. Alternatively you can place a nail in the copper sulfate solution to observe the same effect, as Fe is also on the negative side of the redox potential table.
Imagine that now you separate the copper from the zinc, but that have a conducting wire connecting them. The electrons will still flow from on to the other, this time through the wire. That means that you have an electric current!
Voltage of the cell
There is a 0.34 V tendency to give electrons , from copper, and a 0.76 V tendency to receive electrons, from Zn. One wants to give and the other wants to receive. They join forces, and the total voltage of the battery will be the sum of the two: 0.76 +0.34 = 1,10 V