Fruity batteries

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Overview:

Create a battery from common foodstuffs, sufficient to light a small lightbulb, LED or LCD display.

See also: batteries and Fruit Clock

A battery creates electricity when two different substances inside the battery react with the contents and cause electrons to flow out of the battery canister and along the outside circuit. It is the outside current that brings power to the electrical devices which are wired along the circuit's path.
If you are using L.E.D.s, ensure you have the correct polarity (flat is negative and goes to the more reactive electrode (Mg).
Resistance is of the cell is very high (~1k Ohm). This means that the current able to be drawn is very low. Do not expect it to light all if any (M.E.S.) bulbs. So you may need to have two cells in parallel to work.

High power LEDs require a lot more current and should be avoided also LEDs to the blue end of the spectrum (higher striking voltage). There is a lot to be said for having cheap red LEDs! LCD displays use less current too. Internal resistance of a fruit cell is high. Only low current ones will light and you may require two cells to work.

Equipment:

  • One large potato or lemon.
  • Zinc electrode - a 3cm x 0.5cm piece of zinc metal will suffice. You can inquire at a local hardware store.
  • Copper electrode - Similarly sized piece of copper metal.
  • Copper wire - Sufficient length of wire to create a circuit from the zinc electrode to a lightbulb (or other device) and copper electrode.
  • Small lightbulb - flashlight or penlight bulbs work best. You can experiment with other devices such as LED displays, or time pieces.

Safety: If no copper electrode is used, hydrogen gas is given off as a byproduct of the reactions taking place. Although the quantity given off is small, be wary of performing the experiment near heat sources or an open flame.

Although the voltages and amperages given off are low, care should be taken in handling the wire and other parts of the circuit.
The electrode metals should be cleaned recently with emery paper or similar.

How to do the experiment:

  • Stick your zinc electrode all the way into the potato or lemon.
  • Place the copper electrode on the opposite side.
  • Connect the small lightbulb to the two electrodes with copper wire.
  • Observe what happens!

Lemon battery.jpg

Electrode (extracted from Kaye and Laby data) potential w.r.t. H (in Volts)  Reaction
(note other reactions may take place)
Magnesium -2.56 Mg2+ +( 2e-) ---> Mg
Aluminium -1.67 Al3+ +(3e-)  ---->Al
Zinc -0.7626 Zn2+ + (2e-)  ---->Zn
Iron -0.44 Fe2+ + (2e-)  ----->Fe
Lead -0.1251 Pb2+ + (2e-)----> Pb
Hydogen (reference) 0.00000 2H+ + (2e-) ----> H2
Copper +0.159 Cu2+ +(2e-)  ---->Cu
Silver +0.7991 Ag+ +(e-)  ---->Ag
Gold +1.52 Au3+ + (3e-)  ---->Au

Reactivity series for various electrodes


To develop this concept more advanced classes may wish to find the internal resistance of a cell. The experiment outline from practicalphysics.org is one method.



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