If your iPod runs out of battery power, will a potato hold up in a pinch?
Electricity requires the presence of negatively and positively charged particles: electrons (-) and protons (+). In order for electricity to flow, there must be movement of electrons along a path.The flow of electrons (current) can be measured using an electrochemical cell.
Did you know? Electrochemical cells are also called “Voltaic cells” after Alessandro Volta, who invented the first electrical battery.
An electrochemical cell uses electrical energy from chemical reactions taking place inside the cell. An electrochemical cell needs two different metals (one as the cathode and one as the anode), a conducting wire to connect them, and an electrolyte. Two commonly used metals are zinc and copper, where zinc is the anode (from which electrons flow away) and copper is the cathode (to which the electrons flow) – although, sometimes platinum can used as an alternate cathode, as in this example. An electrolyte is a solution containing ions (atoms with an electrical charge either negative or positive), such as hydrogen ions (H+). Electrolytes are typically acids, bases, or salts. If an anode and a cathode are placed in an electrolyte solution and properly connected to a voltmeter, the electric current can be measured as it flows from anode to cathode. Fruits and vegetables contain electrolytes. This means that they can be used to make an electrochemical cell. A potato is commonly used to demonstrate this fact as potatoes contain phosphoric acid. The phosphoric acid contains free hydrogen ions (H+), which are reduced to hydrogen gas at the cathode. When the metals are placed in an electrolyte solution, chemical reactions occur at the anode and cathode.
Did you know? Conductors are materials (often metals) containing electrons that can move around freely. Insulators contain electrons that are tightly bound to the nucleus of the atom and not able to move freely.
These are known as redox reactions (reduction/oxidation). In the case of a potato battery, the reactions are Zn(s) -> Zn2+ +2e- at the anode (loss of electrons, or oxidation) and 2H+(aq) + 2e- -> H2 (g) at the cathode (gain of electrons, or reduction).
Different vegetables contain different amounts of acid. Some fruits, such as lemons, are high in citric acid and can also be used to construct electrochemical cells. A fun project you can do is try using different vegetables and fruit in an electrochemical cell to see which create the greatest flow of electrons.
As far as running your iPod using vegetables or fruit, it is possible, but it's very inconvenient. For example it would take more than 5,000 lemons just to power a flashlight bulb (see here for calculation)!
Goodisman, Jerry, “Observations on Lemon Cells”, Journal of Chemical Education, 78 (4), 2001, 516-518.
Article first published June 28, 2011