Science corner

15th April 2005 at 01:00
Sophie Duncan looks at static electricity's power of attraction

This week's science corner is made up of two challenges for your students.

The first challenge is to separate salt and pepper.

Give each group of students a small paper plate on which you have placed a mixture of freshly ground salt and pepper. Ask them to think of a way that they could use to separate the two. They should come up with a number of solutions, and you may wish to let them experiment to see if any work.

One way is to rub a plastic spoon with a piece of woollen material. Gently hover the spoon over the salt and pepper and the pepper will leap up and stick to the spoon, leaving the salt on the plate.

Objects have a property which determines how easily electrons are removed from them. When two objects are rubbed together the one that more easily loses electrons becomes positively charged. In the case of rubbing a plastic spoon with a wool cloth the spoon beomes negatively charged, as the wool more readily loses electrons.

When a charged object is brought close to another object it can induce an opposite charge. When the negatively charged spoon is brought close to the pepper and salt the effect is to leave an overall positive charge on the surface of the pepper and salt.

The two opposite charges are attracted to each other. The pepper and the salt are attracted to the spoon, but the pepper is lighter and therefore moves first. If you hold the spoon too near the mixture both the pepper and the salt will stick to the spoon.

The next challenge is to bend a flow of water. Place a cup under a running tap, giving a steady stream of water. The water should not be flowing too fast. Now place the cup so that the water just misses it. Ask the students if they can make the water flow into the cup without moving the cup.

Again, they will have some good suggestions, which you might like to try.

However, static electricity can help again. Take a plastic comb and comb it through your hair several times. Now hold the comb near the flowing water.

The water will be attracted to the comb, and with a bit of practice you will be able to get it into the cup.

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