Electric lemons

I'm sure you've often tried to comb your hair and the only thing you've managed to do is to make it stand up and stiff.

Let's try it another way:

Rub a plastic comb over the surface of your clothes and bring the comb close to the small stream of water coming out of the tap.

What do you observe?

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Why do you think this is?

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In which direction does the water curve with respect to the comb?

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Now, join two balloons with the same thread and hang them together. Then, on the side where they touch, rub them together for a long time with a piece of wool.

What do you observe?

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Why do you think this is?

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What is the position of the balloons?

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The Greeks, specifically Thales, the mathematician, already discovered the existence of static energy by rubbing a piece of amber with a piece of clothing in 600 BC, but it was not until 1570 that the Englishman William Gilbert christened the phenomenon electricity (from the Greek electron, meaning amber). This form of energy is called static electricity, without movement.

A body can be charged with negative or positive static electricity.

From what has happened with the comb, the water and the balloons, what do you think is the behaviour of the loads?

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Let's go a little further, now take a lemon and make two incisions in it: in one, insert a piece of copper and in the other a piece of zinc, without the two metals touching each other. Stick out your tongue and bring it close to the lemon until it touches both metal ends at the same time.

What do you notice, why do you think it happens?

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A chemical reaction takes place between the metals and the acid in the lemon, which in turn produces an electric current. In this case, the current is not static, it moves, it circulates. Faraday was the first to produce an electric current, in 1831, and he achieved this by moving a magnet inside an iron spiral. The first battery, made by Alessandro Volta, dates from around the same time. The idea was the same as that of the lemon: between a silver plate and a zinc plate, a piece of paper impregnated with salt water was placed; by joining the two metals with an iron wire, an electric current was produced and, to make it more intense, a few of these "sandwiches" were stacked together.

Now that you have a battery, you'd like to see how useful it is, of course!

Take the battery, electric wire, a piece of thin cardboard, aluminium foil, tape and a light bulb with a foot. Do the following:

• Take the cardboard rectangle and fold it in half.
• Wrap a piece of aluminium foil around each half.
• Attach an electrical wire to each piece of foil with the adhesive tape.
• Attach one of the two free ends of the electrical wires to the battery and the other to the bulb.
• Finally, place the cardboard next to a door and hide it under the carpet. You now have a silent alarm system!

Why does the light bulb light up when someone steps on the cardboard?

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What kind of electrical behaviour does aluminium have?

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How to make a battery of lemons