The copper coin serves as the positive electrode or cathode and the galvanized nail as the electron-producing negative electrode or anode.
These two objects work as electrodes, causing an electrochemical reaction which generates a small potential difference. The origin of lemon and potato batteries is unknown.
The aim of this experiment is to show students how batteries work. After the battery is assembled, a multimeter can be used to check the generated voltage.
For a more visible effect to be produced, a few lemon cells connected in series can be used to power a standard red LED.
Flashlight bulbs are generally not used because the lemon battery cannot produce the amount of current required to light such bulbs. Digital clocks can work well, and some toymakers offer small kits with a clock that can be powered by two potatoes or lemons.
The energy for the battery comes from the chemical change in the zinc (or other metal) when exposed to an acid. The energy does not come from the lemon or potato.
The zinc is oxidized inside the lemon, exchanging some of its electrons with the acid in order to reach a lower energy state, and the energy released provides the power.
In current practice, zinc is produced by electro winning of zinc sulfate or pyrometallurgic reduction of zinc with carbon, which requires an energy input.
The energy produced in the lemon battery comes from reversing this reaction, recovering some of the energy input during the zinc production.
Assuming that zinc and copper electrodes are used (such as a copper coin and a zinc plated nail) then a lemon could generate approximately 0.9 Volts.
The current depends on the size of the electrodes used and the strength of the ion bridge.
1 lemon: 0.9 volts x 0.0003 amperes = 0.00027 watts
Flashlight bulb: 2.4 volts x 0.5 amperes = 1.2 watts, or about 5,000 lemons
Halogen bulb: 12 volts x 0.83 amperes = 10 watts, or about 37,000 lemons
Red LED bulb: 1.7 volts x 0.0005 amperes = 0.00085 watts, or three lemons in series
In a lemon battery, both oxidation and reduction occur. Consider the case of a zinc-copper battery; this battery is similar to the original "simple voltaic cells" invented by Alessandro Volta. At the anode, metallic zinc is oxidized, and enters the acidic solution as Zn2+ ions:
At the copper cathode, hydrogen ions (solvated protons from the acidic solution) are reduced to form molecular hydrogen:
Potatoes, apples, sauerkraut, or any other fruit or vegetable containing acid or other electrolyte can be used, but lemons are preferred because of their higher acidity.
In potatoes, for instance, the electrolyte is phosphoric acid, while in lemons it is citric acid. Other non-rusty metal combinations (such as magnesium-copper) are more effective; for example, using a magnesium strip instead of zinc increases the voltage from 0.9 volts to 1.3 volts with magnesium.
However, zinc and copper are usually preferred because they are reasonably safe and easy to obtain.