Saturday, February 04, 2006

The 55c Battery

Using the Reactions Between Ions and Metals to Create an Electric Current

This experiment with a few coins and lemon juice recreates an experiment first performed by Alessandro Volta in 18th century Italy – creating a battery, using the reactions between ions and metals to create an electric current.

What you need:

    Lemon Juice

    9 2.5cm x 2.5cm squares of paper towel

    5 pennies (or other copper coins)

    5 dimes (or other coins made out of a metal other than copper)

What to do:
    1)Soak the paper towels with lemon juice

    2)Stack the coins, alternating pennies and dimes, placing a piece of paper towel between each coin

    3)Moisten both your index fingers and hold the pile tightly between these fingers

What happens:

If all goes well, you'll feel a small shock or a tingle

Why? The lemon juice contains an acid (ascorbic acid, or vitamin c, to be precise) – which means that there are charged particles (or ions) dissolved in the fluid – specifically positively charged hydrogen ions and the ascorbate ion (which is negatively charged - an anion). The hydrogen ions want to absorb electrons and pair up to form neutrally charged hydrogen gas. Similarly, the negative ions want to give up their extra electron (or find something positively charged to bond with) so that they are also neutrally charged.

Depending on the exact metals used in the coins, one type of coin will be "better" at giving up its electrons than the other (which one depends on the exact composition of the silver coin). The hydrogen will preferentially collect at this coin (or electrode), making it positively charged. The negative ions will collect at the other coin, making it negatively charged. This process continues through the pile of coins, until we end up with a large positive charge at one end and a large negative charge at the other. Once you place the coins between your fingers, the electrons, which are repelled by each other move through your body to get to the positively charged end – giving you a mild shock.

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