Earth science

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Video

Materials

• 1 polysterene ("styrofoam") food box
• 1 aluminum pie pan
• 1 wool sock (or something else made of wool)
• 1 pair of scissors
• Tape

Short explanation

When you rub the polystyrene, you charge it. This charge then "pushes" electrons from the pie pan to your finger. When the pie pan no longer has contact with the polystyrene, it takes back the electrons it lost.

Long explanation

When two materials come in contact with each other, there is always the chance that the atoms in one material take electrons from the atoms of the other material. When you rub the wool sock against the polystyrene box, electrons will jump from the wool to the polystyrene. This makes the polystyrene negatively charged, because electrons are negatively charged. The reason why you should not touch the polystyrene with your other hand is that some of these electrons will then migrate over to you and you don't want that - you want the polystyrene charged.

When you place the aluminum pan on top of the polystyrene box, the excess electrons in the polystyrene will repel the electrons in the aluminum (this is because equal charges repel each other). Aluminum is a metal, and these are characterized by having free electrons, which means that they can move relatively freely within the metal. The electrons that are repelled by the polystyrene will therefore accumulate in the outermost parts of the aluminum pan - closest to you.

Then, when you come near the aluminum pan, the electrons have the opportunity to escape even further from the polystyrene, and jump over to you. You only need to get close for them to jump over to you. As the electrons move through the air, they appear as a flash.

Now the aluminum pan has a deficit of electrons. It has given some to you. Despite this, the electrons don't jump back if you touch the aluminum pan again, as the polystyrene is still nearby and repels them. However, when you lift the aluminum pan, the polystyrene loses its effect, and the aluminum pan's attraction to electrons becomes realized. When you now touch the pan, electrons will jump from your finger to it, again through the air in the form of a flash.

A flash in nature occurs in the same way, by a strong difference in charge between two points that are equalized by electrons being transferred between them. In a thundercloud, particles rub against each other and transfer electrons. It's the lighter particles that take electrons from heavier particles. The lighter particles rise upwards to the top of the cloud where there is now a shortage of electrons (the cloud is now positively charged here). The heavier particles sink to the underside of the cloud where there is now an excess of electrons (the cloud is negatively charged there). A flash can then propagate straight through the cloud, or from the negative underside of the cloud to the positively charged surface of the ground. The ground surface is positively charged because the electrons in the underside of the cloud repel electrons in the ground surface (the electrons in the ground are somewhat mobile, although not as much as in a pure metal).

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you replace the aluminum with some other material?
• What happens if you replace the polystyrene with some other material?
• What happens if you replace the wool with some other material?
• How can you make the flash as large as possible?

Variations

An easier way to generate a flash is to rub a balloon against your hair for 30 seconds, and then hold the balloon close to a metal. The balloon takes electrons from the hair and becomes negatively charged. When you then hold the balloon close to a metal, it repels electrons in the surface of the metal, which becomes positively charged. The charge difference between the negative balloon and the positive metal surface then becomes so big that a current of electrons from the balloon to the metal surface is formed in the form of lightning.

Another way is to take off a fleece jacket. Do this in the dark while looking in a mirror.

Yet another way is to tear open an envelope along the glue. This also needs to be done in the dark for the flashes to be seen.

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