# Demos on Electrostatics

Following simple demos can be performed at home or in the classroom. They will make electrostatics more interesting and enjoyable.

## Electrostatics with plastic scale

The objective of this demo is "to know the reason for tumbling of a plastic scale". You need a 12 inch plastic scale and a comb.

### Procedure

Place a 12 inch plastic scale at the edge of the table in such a way that the 6 inch mark is in line with the edge of the table and the scale just balances. Rub a dry comb with your dry hair. Bring the comb below the end of the scale which is outside the table. You will observe that the comb will attract the scale and it may tumble down.

### Discussion

When you rub your hair, the comb becomes charged. When the charged comb is brought near the scale, the opposite charge is induced in the scale and the scale is attracted by the comb. This attractive force disturbs the equilibrium of the scale and it tumbles down.

## Electrostatics with balloon

The objective of this demo is "to understand why the balloon stick to the wall". When two bodies are rubbed against each other, the charges gets transferred from one body to another. The charges are not created by rubbing.

### Procedure

Rub the balloon on your shirt (except cotton). Take the balloon close to the dry wall. You will observe that the balloon gets stuck to the wall.

### Discussion

When the balloon is rubbed to the shirt, some electron gets transferred from shirt to the balloon. Normally, there are equal number of protons and electrons in a neutral object. But, when the excess of electron accumulates on the balloon, it gets negatively charged. When negatively charged balloon is brought near to the wall, the electrons on the surface of the wall are repelled. Thereby the wall gets positively charged.

Unlike charges attract each other. Hence the balloon get stuck to the wall. If the wall is wet, it will lose the charge very quickly. The same thing happen if the air is humid. Hence, the wall has to be dry.

If the balloon is kept close to the wall for long, then the excess electrons from the balloon will flow back to the wall. The balloon will become neutral and will fall. Moreover, if the balloon is kept for long, it will gradually collapse.

## Electrostatic attraction and repulsion with balloons

When two objects are rubbed against each other, charge gets transferred from one to other. One of the objects becomes positively charged and other becomes negatively charged. The unlike charges attract each other and like charges repel. A charged body brought near an uncharged (neutral) body can cause redistribution of charges on the neutral body and two may attract each other.

### Procedure

Repulsion: Take two balloons and blow air in them. Tie their ends with a string of about 10 cm length. Hold them in two hands and rub them few times against your hair. Hold the two through the strings and bring the rubbed part closer. The repulsion is clearly seen. Now, keep one of the balloon on the table and hold the other in your hand. Bring it close to one on the table and play with repulsion. You can make the balloons to go in circles on the table.

Attraction by induction: Bring a charged balloon close to some pieces of paper. The pieces of paper jump and get stuck to the balloon. This phenomenon is generally described in all science textbooks at the middle level by using a comb. But experience is that the balloon can lift much larger pieces of paper and demonstration becomes much more impressive.

### Discussion

Rubbing produces electrically charged objects. Emphasize that charge has not been created, it is only transferred. When the balloon is brought close to the paper pieces, it attract the pieces even though pieces are uncharged. The attraction is caused by the charge induced in the pieces of paper when they are brought close to the charged balloon.

## Bending of water stream

The water molecule is neutral but has non-zero dipole moment. A stream of water is easily attracted by a charged body due to non-zero dipole moment of the water molecules. We demonstrate this phenomenon in this experiment.

### Procedure

Make a small hole at the bottom of a water bottle. Make sure that a clear stream of water comes out when bottle is filled with water. The hole should not be too small or too big. Rub a plastic scale (or comb) with cloth and bring it close to the stream (near the top portion). You will observe that stream bends towards the scale. This demo shows the concept of induced charges (water is a polar molecule) and electrostatic attraction.

### Discussion

Note: There are claims that polar nature of water is not the right explanation for bending of water stream. The stream is bent because of presence of ions in tap water. We need to do this experiment with distilled water to confirm this.

## Electroscope

When we rub a balloon over our hair, we can make it stick to the walls. The act of rubbing makes the balloon charged. In this demonstration we find out more about this type of charging with the help of a home made electroscope.

### Procedure

1. Hold the two straws together and charge them by rubbing on a silky cloth.
2. Place one of the straw on the bottle and bring the other straw near it. What do you see?
3. Now bring the cloth near to the straw. What do you see?
4. Bring one of the charged straws and touch it to the aluminium ball of the electroscope. Do the leaves show divergence? Why?
5. Bring the charged straw near the electroscope without touching. Do the leaves again show divergence? Why?

### Discussion

Electrons get transferred from one body to the other on rubbing, making the bodies oppositely charged. When the two straws are rubbed together with a silk cloth, both acquire the same charge while the silk cloth acquires an opposite charge.

Charge can be detected using an electroscope. When we touch the charged straw to the aluminium ball of the electroscope, some charges from the straw get transferred to the electroscope. Aluminium being a good conductor, these charges get distributed and the leaves diverge as both acquire the same charge. This is charging by conduction method.

When a charged straw is brought near the electroscope, the leaves again show divergence. By bringing the charged straw near the electroscope the electrons get re-distributed in the aluminium ball and its leaves acquire the same charge as the straw.

## Electrostatics with plastic straw

See electrostatics of rotating straw