text: Galvanic cells are extensions of spontaneous redox reactions, but have been merely designed to harness the energy produced from said reaction. For example, when one immerses a strip of zinc metal (Zn) in an aqueous solution of copper sulfate (CuSO), dark-colored solid deposits will collect on the surface of the zinc metal and the blue color characteristic of the Cu ion disappears from the solution. The depositions on the surface of the zinc metal consist of copper metal, and the solution now contains zinc ions. This reaction is represented by
  : Zn (s) + Cu (aq) → Zn (aq) + Cu (s)
  In this redox reaction, Zn is oxidized to Zn and Cu is reduced to Cu. When electrons are transferred directly from Zn to Cu , the enthalpy of reaction is lost to the surroundings as heat. However, the same reaction can be carried out in a galvanic cell, allowing some of the chemical energy released to be converted into electrical energy. In its simplest form, a half-cell consists of a solid metal (called an electrode) that is submerged in a solution; the solution contains cations (+) of the electrode metal and anions (−) to balance the charge of the cations. The full cell consists of two half-cells, usually connected by a semi-permeable membrane or by a salt bridge that prevents the ions of the more noble metal from plating out at the other electrode.
  A specific example is the Daniell cell (see figure), with  a zinc (Zn) half-cell containing a solution of ZnSO (zinc sulfate) and a copper (Cu) half-cell containing a solution of CuSO (copper sulfate). A salt bridge is used here to complete the electric circuit.
  If an external electrical conductor connects the copper and zinc electrodes, zinc from the zinc electrode dissolves into the solution as Zn ions (oxidation), releasing electrons that enter the external conductor. To compensate for the increased zinc ion concentration, via the salt bridge zinc ions leave and anions enter the zinc half-cell. In the copper half-cell, the copper ions plate onto the copper electrode (reduction), taking up electrons that leave the external conductor.  Since the Cu ions (cations) plate onto the copper electrode, the latter is called the cathode.  Correspondingly the zinc electrode is the anode. The electrochemical reaction is
  This is the same reaction as given in the previous example. In addition, electrons flow through the external conductor, which is the primary application of the galvanic cell.
  As discussed under cell voltage, the electromotive force of the cell is the difference of the half-cell potentials, a measure of the relative ease of dissolution of the two electrodes into the electrolyte.  The emf depends on both the electrodes and on the electrolyte, an indication that the emf is chemical in nature.