PARTNER'S NAME

EXPERIMENT 15.1

Equipment:
• Balloon
• Water faucet

These experiments are simple exercises to demonstrate electric force and static charge. In this first experiment blow up a rubber balloon and tie it off. Begin rubbing it against your hair for about 20 seconds. You should feel your hair start to stick up. This is because electrons are rubbing off your hair, onto the balloon. Since your hair now has more protons than electrons, each of your hairs will be positively charged and will repel each other, making them stick out. Now that the balloon is negatively charged, place it next to a water faucet. Turn the faucet on so that a small but steady stream of water is falling out. Watch as the stream of water bends towards the balloon!

Water molecules are made up of two hydrogen atoms and a larger oxygen atom. The shared electrons of the molecule tend to stay closer to the oxygen than to the hydrogen, resulting in a slight negative charge on the oxygen side and a slight positive charge on the hydrogen side. Molecules that are slightly charged in this way are called polar molecules. When the negatively charged balloon is brought close, all the water molecules align so that the positive side is closer to the balloon. The positive side of the molecule is now attracted to the balloon while the negative side of the molecule is repelled. Since the positive side of the molecule is closer, however, the attractive force is slightly larger than the repulsive force, causing the water to be pulled towards the balloon.

Water molecules being attracted by a negatively charged balloon

You can also make the balloon stick to a wall by charging it up on your head, then touching it to the wall. The molecules in the wall attract the balloon in the same way the water molecules were attracted to the balloon. This attractive force can overpower the effect of gravity on the balloon and hold it against the wall.

EXPERIMENT 15.2

Equipment:
• Balloon
• Thread
• Piece of cereal

Begin by tying the thread to the piece of cereal, then hanging the cereal off of something so that it is free to swing. Charge the balloon up as before by rubbing it against your hair. Hold the balloon close to the hanging cereal and notice that the cereal is attracted to the balloon, just as in the previous experiment. Now, bring the balloon close enough that the cereal can actually touch the balloon. Once it touches, hold the balloon still until the cereal falls on its own. Recharge the balloon on your hair once more and bring it close to the cereal again. This time the cereal is repelled by the balloon! Before reading further, try to figure out what’s happening.

It’s nothing new that the cereal is first attracted to the balloon, but something interesting occurs when the cereal touches the balloon. Remember that like charges repel, so all the extra electrons on the balloon are trying to get away from each other but have nowhere to go. When the cereal touches the balloon, some of the electrons jump over onto the cereal. This leaves the cereal with a negative charge. When you rub the balloon a second time to put more electrons on it, you then have a negatively charged balloon and a negatively charged piece of cereal. Put them close together and they repel because they have like charges.