Physics

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Materials

• 1 cotton towel or other piece of cotton cloth
• 1 thin plastic bag
• 1 balloon
• 1 pair of scissors

Short explanation

The balloon picks up electrons from the towel and thus becomes negatively charged. The ring also picks up electrons from the towel and it too becomes negatively charged. Because both are negatively charged, they repel each other. Therefore, the ring cannot fall onto the balloon but is kept floating in the air.

Long explanation

When two materials come in contact with each other, there is always a chance that the atoms in one material take electrons from the atoms of the other material. When you rub cotton against plastic, electrons will jump from the cotton to the plastic.

Why this happens is not as difficult to understand as one might think. Different atoms have a different grip on their electrons. Some atoms have few protons in the nucleus (it is the electrical attraction from the positively charged protons that holds the electrons) and/or electrons that are far from the nucleus. These atoms easily lose electrons to other atoms. Other atoms have many protons in the nucleus and/or electrons near the nucleus. These atoms seldom lose any electrons and the positive charge of their atomic nuclei "shines through" their electron clouds and is felt outside the atom. And it is these atoms that can take electrons from other atoms.

When the balloon and the ring have taken electrons from the towel, they both become negatively charged. This is because electrons are negatively charged and these materials now contain an excess of electrons compared to protons.

As the ring approaches the balloon, electrons in the ring will be repelled by the electrons in the balloon. The electric force that arises, which in this case is repulsive, overcomes the gravitational force and keeps the ring floating in the air.

This is an example of static electricity, i.e. charged particles that, without moving anywhere in a current, still affect each other with forces.

Over time, the "magical power" of the balloon and ring will diminish. This is because electrons leave the plastic and move in small currents to the air.

You may notice that the balloon or ring tends to stick to things. For example, the ring tends to stick to your fingers when you are working on it. Why? Your fingers are not positively charged. They are neutral. Well, when the ring is near your fingers, the electrons in your fingers will be repelled by the electrons in the ring. They don't leave your fingers, but end up further into the skin. The surface of your fingers facing the ring will now be positively charged, because of the protons that are still there. Because the ring is negatively charged and your outermost parts of your fingers are now positively charged, they will attract each other.

This is why a negatively (or positively) charged object is attracted to a neutral object. But if the neutral object is a good conductor, electrons will flow to it from the negative object and soon both objects are neutral (and the plastic no longer sticks to your fingers).

You can also rub the balloon against your hair, and make electrons jump from the hair to the balloon. We can see that the hair then lacks electrons and is thus positively charged, by the hair standing up. Because all hairs are positively charged, they repel each other.

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 use a smaller plastic ring?
• What happens if you use a smaller balloon?
• What happens if you make an octopus/spider-like shape out of the plastic bag instead?
• What happens if you use an elongated balloon instead of a round one?
• What happens if you make a thin paper ring instead?
• What happens if you rub the balloon and ring against your shirt instead?

Gilla:		Dela: