ICSE Class 8 Physics Chapter 06 Static Electricity

Static Electricity

In your previous class, you have learnt about the nature of electricity, its sources and its uses. Most of this was about electric currents caused by the flow of charges through conductors. Thus, you know about the effects of charges in motion.

In this chapter, we will discuss static electricity, or the properties and effects of charges which are at rest. You may remember that charges are of two types - positive and negative. You may also remember that a charged body carries an excess of one type of charge and exerts electrostatic force on bodies near it. We will now learn how bodies get charged and the kind of force they exert on other charged bodies.

A comb rubbed against dry hair and a plastic pen rubbed against wool or paper get charged. They exert electrostatic force on bits of paper and a stream of water.

Atoms, Charges and Electricity

You have learnt that all matter is made up of atoms. However, each atom is not a single particle in itself. It is made up of smaller particles. The structure of an atom is somewhat like that of the solar system. Almost the entire mass of the atom is concentrated at the centre, which is called the nucleus. Negatively charged particles called electrons move around the nucleus, the way planets move around the sun. The nucleus itself is made up of two types of particles packed close together. These are positively charged particles called protons and neutral (uncharged) particles called neutrons.

Protons and neutrons have almost the same mass and are much heavier than electrons. The charge on a proton is equal and opposite to the charge on an electron. Normally, an atom contains the same number of (positively charged) protons and (negatively charged) electrons. Hence, an atom is usually electrically neutral, or uncharged.

Free Electrons

The protons and neutrons of an atom are bound very tightly and cannot normally break away. Some of the electrons moving around an atom, however, can break loose. These are called free electrons, and can move about freely inside a material. It is these charged particles, called charge carriers, that are responsible for all electrical processes. A body may gain or lose free electrons and become charged. As long as such charges in a body are at rest, we study their effects under static electricity.

Conductors have a large number of free electrons or charge carriers, while insulators have very few of them. This explains the difference between the electrical properties of conductors and insulators. Normally, the free electrons inside a conductor move about randomly, or not in any particular direction. However, when a conductor is connected to a source of electricity, or a potential difference is created between the two ends of a conductor, the free electrons flow in a particular direction. This is what gives rise to an electric current. As electrons carry negative charge, the direction of conventional current is opposite to the direction of the flow of electrons.

Charge on a Body

All objects around us normally have equal amounts of positive and negative charges. Thus, they are uncharged, or electrically neutral. (This should be obvious since they are made up of atoms, and atoms are electrically neutral.) However, sometimes a body may either lose or gain some charge. Then it becomes charged, or is no longer electrically neutral. Remember that when we speak of the charge on a body we mean the imbalance of charges, and not the actual amount of charges present in it.

1. If a body gains Q amount of positive charge, the charge on it is +Q.

2. If a body gains Q amount of negative charge, the charge on it is -Q.

3. If a body loses Q amount of positive charge, the charge on it is -Q.

4. If a body loses Q amount of negative charge, the charge on it is +Q.

Atoms can also become charged. When an atom loses electrons, its net charge becomes positive, and it is called a positive ion. When an atom gains electrons, its net charge becomes negative, and it is called a negative ion. In your previous class you have learnt that liquids which conduct electricity are called electrolytes. It is the movement of ions within such liquids that helps to conduct electricity.

Unit of Charge

The SI unit of charge is the coulomb, with the symbol C. One coulomb is a very large amount of charge. Thus, the unit micro coulomb, with the symbol μ C, is widely used in static electricity. One micro coulomb is one millionth of a coulomb, or 10⁻⁶ C.

Teacher's Note

Static electricity is why socks stick to your clothes in the dryer and why you sometimes get shocked when touching a metal doorknob on a dry day.

Charging a Body

A body can be charged by three methods - by friction, by conduction and by induction. We will discuss each of these methods in some detail.

Charging by Friction

When two bodies rub against each other, some charge usually moves from one to the other. Both the bodies then acquire equal and opposite charge. For example, suppose a body A rubs against a body B and Q amount of negative charge moves from A to B. Then B will have -Q charge and A will have +Q charge.

This process of transfer of charge occurs all the time, whenever bodies rub against each other, for example, when shoes rub against the floor or a pencil rubs against paper. However, in most cases, the amounts of charge which appear on the bodies are so small that we do not notice their effects. Only some materials exchange large amounts of charge when they are rubbed against each other. Some examples of such materials are given in Table 6.1. The materials in the table are arranged in such a way that a material that is higher up in the series becomes positively charged when it is rubbed against a material that is lower down the series. Such a list or series of materials is called a triboelectric series.

Glass, Human hair, Nylon, Wool, Silk, Aluminium, Paper, Cotton, Amber (a resin), Hard rubber, Polyester, Styrofoam, Polythene

Let us try to charge different sets of materials by friction and observe their behaviour. Remember that simple experiments on static electricity work best in winter. This is because the charges which build up on bodies usually leak away through the moisture in the air, and the air is rather dry in winter.

Rub a piece of Styrofoam with a strip of polythene. (Alternatively, rub a plastic ruler or pen with paper, wool or nylon.) Bring the Styrofoam and polythene close to bits of paper. They will both attract the pieces of paper, showing that both have been charged by friction.

Force between charges

A look at Table 6.1 will tell you that the Styrofoam is positively charged, while the polythene is negatively charged. If you bring them close to each other, you will find that they attract each other. This is always true, and we say that unlike charges attract each other. You can see examples of this all around you, especially in winter. For example, when you brush past a curtain made of synthetic material, it sticks to you. Nylon sweaters and socks cling to you when you try to take them off. Dishes packed in plastic bags stick to the bags and strands of hair get drawn to your comb while you comb your hair.

This activity will help you observe the behaviour of similar or like charges. Blow up two balloons and attach strings to them. Rub the balloons with wool. According to Table 6.1, both the balloons should get negatively charged. Hold the strings together. You will notice that the balloons move apart, showing that like charges repel each other.

We can now come to the following conclusions.

1. Two positive charges repel each other.

2. Two negative charges repel each other.

3. A positive charge and a negative charge attract each other.

The magnitude (strength) of the force between charges depends on the magnitude (amount) of the charges. The greater the charges, the greater is the force between them. The force exerted by charges also depends on the distance between them. The greater the distance between the charges, the smaller is the force between them.

Like charges repel, unlike charges attract.

Charging by Conduction

When an uncharged body comes in contact with a charged body, charge flows from the charged body to the uncharged one. Suppose a charged body X carrying Q charge comes in contact with an identical uncharged body Y. Charge will flow from X to Y until they both have Q/2 charge. This process of charging an uncharged body by bringing it in contact with a charged body is called charging by conduction.

Suspend a small piece of paper by a string. Charge a drinking straw by rubbing it with another piece of paper and bring it close to the suspended piece of paper. This paper will get attracted to the straw and finally stick to it. Move the straw away and bring it near the paper again. This time the paper, which has been charged by conduction, will get repelled.

Charging by Induction

In the activity in which you charged a piece of Styrofoam and a strip of polythene by friction, both the Styrofoam and the polythene attracted uncharged pieces of paper. Let us find out how a charged body attracts an uncharged body.

Suppose you bring a positively charged rod close to an uncharged metallic sphere. The negative charge carriers on the sphere will get attracted to the positive charge on the rod and move to the side of the sphere that is close to the rod. There will, thus, be a temporary rearrangement of charges on the sphere. The side of it close to the rod will have more negative charge and the side away from the rod will have positive charge. We can also say that induced negative and positive charges will appear on the sphere. Now it should be clear to you why an uncharged body gets attracted to a charged body.

The induced charges on the sphere will disappear as soon as you move the rod away. However, if you earth the sphere, or connect it to the earth, without moving the rod away from it, the positive charges will flow to the earth, leaving the sphere with a net negative charge. (You may remember that earthing a body means connecting it to the earth to allow charges on it to flow to the earth). The negative charges remain on the sphere and do not flow to the earth because they are attracted or 'bound' by the positive charges on the rod. If you move the rod away from the sphere after earthing the sphere, the negative charge on the sphere will remain on it. This way of charging a body is called charging by induction.

1. Negative charges are attracted towards the rod and get bound.

2. When the sphere is earthed, the unbound positive charges flow to the earth.

3. The remaining charges make the sphere negatively charged.

Teacher's Note

When you charge a balloon by rubbing and stick it to a wall, the wall becomes oppositely charged through induction, creating the attraction that holds the balloon in place.

This is a preview of the first 3 pages. To get the complete book, click below.