ICSE Class 8 Physics Chapter 06 Static Electricity

Static Electricity

Learning Outcomes

Static charges, Origin of charges, Charging an object: by friction, conduction, and induction, The gold-leaf electroscope, Flow of electrons, Atmospheric electricity, Lightning conductor

Introduction to Static Electricity

You might have studied about current electricity in detail in class 7. Electricity is a broad term given to a wide range of phenomena from lightning that we see in the sky to lighting a bulb. You know that almost all the gadgets and appliances work by electricity. It lights up bulbs in our houses, makes our fans work, runs our refrigerators, provides us warmth through room heaters, and helps us in many other ways.

This kind of electricity that makes our household appliances work is called current electricity because here electric charges are in motion. But, sometimes charges can be separated from neutral bodies and cause an imbalance of charges in them. It is then called static electricity. In fact, static electricity was the first kind of electricity to be discovered.

In this chapter, you will study about static electricity, how objects can acquire electric charges, and its practical applications. Let us observe the following three figures before studying what static electricity means.

Have you ever tried doing what the girl in the above figures is doing? What do you observe? You will observe that when a plastic ruler is brought close to some bits of paper, the bits don't get attracted to the ruler (a); when that ruler is rubbed against hair (b) and then brought close to the same bits of paper, they get attracted to the ruler (c). Can you explain why this happens?

When the ruler is rubbed against hair, it acquires an electric charge and a force is produced in it due to which it attracts the bits of paper. This force is called electrostatic force and forms the basis of static electricity.

The branch of physics that deals with the study of static electricity which involves electric charges, the force between them, and their behaviour is called electrostatics.

How does the ruler get charged? What is the nature of that charge? What is the force that attracts those bits of paper? Is that force always attractive or sometimes repulsive Does our atmosphere also have electric charges? You will get answers to all these questions as you read this chapter further.

Activity - Forces of Attraction and Repulsion in Static Electricity

Aim: To study the forces of attraction and repulsion in static electricity.

Materials required: Four balloons, four equal-length strings, and a piece of wool.

Procedure: (Note: the activity would best work in cold dry surroundings). 1. Inflate all four balloons and tie their openings tightly to separate strings.

2. Take two balloons and rub them against wool. Release the balloons holding them with the strings. What do you see? You will see that the balloons move away from each other, i.e., they repel.

3. Now take the other set of two balloons. Rub one against wool and the other against your hair, and now bring the balloons close to each other holding them with their strings. What do you notice this time? You will see that this time the two balloons stick to one another, i.e., they attract.

Conclusion: We conclude that when objects are rubbed against one another, forces of attraction as well as repulsion are produced.

Origin of charges

We have just seen that when an object is rubbed against another object, some electric charge develops in it due to which it either attracts or repels the other object. Where do these charges come from? Electric charges are actually present inside atoms. We know that matter and everything around us is made up of atoms. An atom consists of three main particles: protons (the positively-charged particles), electrons (the negatively-charged particles), and neutrons (neutral particles). Figure 6.1 shows the structure of an atom.

Under normal circumstances, the atoms in an object contain the same number of protons and electrons and hence, the body is electrically neutral.

An object is called electrically charged when the number of protons and electrons is not equal. When the number of protons exceeds the number of electrons, the object is said to be positively charged. On the other hand, when the number of electrons exceeds the number of protons, the object is said to be negatively charged.

Thus, we see that an object can have either positive charge or negative charge. When an object is charged, it exerts a force on another object. If two objects of the same type of charge (positive-positive or negative-negative) are brought close together, they repel each other, i.e., push each other away. However, if two objects having opposite charge are brought close (positive-negative or negative-positive), they attract each other, i.e., pull towards each other (Fig. 6.2).

Thus, like charges repel each other and unlike charges attract each other.

Charging an Object

Let us study the methods by which an object can be charged. An object can be charged by the following methods.

1. Charging by friction;

2. Charging by conduction; and

3. Charging by induction.

Charging by friction

Let us go back to the figure on Page 91 where bits of paper were attracted to the charged plastic ruler. We saw that the ruler got charged when it was rubbed against hair. This is called charging a body by friction or rubbing. We must note that whenever a body is charged, it is only the electrons that move in and out of an atom leaving the body negatively or positively charged. Since the protons are tightly bound in the nucleus, they don't move. When one object (material) is rubbed against another suitable object, there is a transfer of charges between them due to friction.

Classic examples of charging by friction are:

1. Rubbing a glass rod with silk; and

2. Rubbing an ebonite rod with flannel or fur.

Teacher's Note

When a glass rod is rubbed with a silk cloth, electrons flow from the glass rod to the silk cloth. Due to the loss of electrons, the glass rod acquires a positive charge; on the other hand, due to excess of electrons, the silk cloth acquires a negative charge. Since they acquire opposite charges, they attract each other. This observation helps students understand why their clothes stick to their dry body during winters and crackle when they are taken off.

When an ebonite rod is rubbed with wool, electrons flow from the wool to the ebonite rod. Due to the excess of electrons, the ebonite rod acquires a negative charge; on the other hand, due to loss of electrons, the wool acquires a positive charge. Here also, since they acquire opposite charges, they attract each other. This helps explain why their clothes stick when charged.

Charging by conduction

In addition to charging an object by friction, it can also be charged by touching it to an electrically charged object. The process of charging an uncharged object by touching it to an electrically charged object is called charging by conduction.

We know that a charged object contains an excess of either protons or electrons. An object with an excess of electrons will be negatively charged, and the one with an excess of protons will be positively charged. What happens when these two are, one by one, brought in contact with an uncharged object?

A positively charged object has a deficiency of electrons. When such an object is brought close to an uncharged (neutral) object, the electrons flow from the uncharged object to the positively charged object (Fig. 6.3 (b)). Due to the loss of electrons, the uncharged object gets positively charged, and the charged object becomes less positive.

On the other hand, if a negatively charged object (having an excess of electrons) is brought in contact with an uncharged object, the excess electrons from the charged object flow to the uncharged object. Due to the gain of electrons, the uncharged object becomes negatively charged, and the charged object becomes less negative.

Remember that if the object that is to be charged is a good conductor of electricity, the charge spreads to all parts of its surface. But if it is an insulator, the charges remain at the place where the contact is made.

Teacher's Note

Understanding charging by conduction helps students grasp why touching a charged object can cause sparks or electric shocks in everyday situations.

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