It consists of inserting two different metallic objects, for example a galvanized nail and a copper coin, into a lemon. These two objects work as electrodes, causing an electrochemical reaction which generates a small potential difference.
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. In order for a more visible effect to be produced, a few lemon cells connected in series can be used to power a standard LED. Flashlight bulbs are generally not used because the lemon battery cannot produce the amount of current required to light such bulbs.
The energy used to power the circuit ultimately comes from the energy used in the electrode factories to reduce metallic ions forming the solid metallic electrodes. The lemon simply provides a salt bridge for the reaction to occur. In a lemon battery, both oxidation and reduction occur. Consider the case of a zinc-copper battery. At the anode, zinc is oxidised:
- Zn → Zn2+ + 2 e-
- 2H++ 2e- → H2
Potatoes, apples, or any other fruit or vegetable containing acid or other electrolyte can be used, but lemons are preferred because of their higher acidity. Other metal combinations (such as magnesium-copper) are more efficient: for example, using a magnesium strip instead of zinc increases the voltage from 1.1 V with zinc to 1.6 V with magnesium (the exact voltage varies depending on the lemons.) However, zinc and copper are usually preferred because they are reasonably safe and easy to obtain.
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