Maharashtra Board Class 7 Science Chapter 8 Static Electricity PDF Download

8. Static Electricity

Let's Recall

Do you have experience of the instances given below? What is the cause of these effects?

1. A plastic comb or ruler rubbed on dry hair attracts pieces of paper.

2. If we pass near a polyester curtain again and again, it gets attracted towards us.

3. If we rub a blanket with our hands and take it near a metal object, a spark is seen in the dark.

Do you know of other such instances?

Electric Charge

What do the above observations tell us? These examples are just a glimpse of the 'electric charge' that all objects in our surroundings hold in abundance. Electric charge is stored even in our own bodies. All substances are made of very tiny particles. Electric charge is an intrinsic property of these particles. Though, in this way, electric charge is abundantly present, it is always in a hidden state. This is because two opposite types of charges are present in equal numbers in all these substances. When the positive charge (+) and negative charge (-) on an object are balanced, the object is neutral, i.e. there is no net charge on the object. If these charges are not balanced, the object is said to be 'charged'.

Try This

How would two charged objects interact with each other?

Rub one end of a glass rod against a silk cloth. Due to the rubbing a small charge will get transferred from one object to the other. As a result both the objects will become somewhat charged. Suspend this rod freely in air with the help of a thread. Now charge another glass rod in the same manner and bring it near the suspended rod. What do you see? The two rods push each other away. Now take a plastic rod. Rub one of its ends against a woollen cloth. Bring that end near the suspended glass rod. Now what do you see? The two rods are pulled towards each other.

What did you find in the first experiment? Two rods carrying similar charges push each other away. This is called repulsion. We learn from the second experiment that rods carrying opposite types of charges get pulled towards each other. This is called attraction.

The scientist Benjamin Franklin named the electric charges positive charge (+) and negative charge (-).

Teacher's Note

You can show students how a comb rubbed on your hair attracts small pieces of paper. This is static electricity that we see every day in India.

Exam Trick

Remember: Like charges repel. Unlike charges attract. Think of two friends - same interest means they stay together, opposite interests means they move away from each other.

Points to Remember

Electric charge is an intrinsic property of matter.
There are two types of charges - positive and negative.
Like charges repel each other and unlike charges attract each other.
A neutral object has equal positive and negative charges.
Electric charge is stored in all objects around us.

What Is The Origin Of An Electric Charge?

All substances are made up of particles, and these particles are ultimately made up of very tiny atoms. We shall be looking at the details of atomic structure later. At this stage, it is sufficient to know that each atom contains a stationary positive charge and moving negative charges. These two charges being perfectly balanced, an atom is electrically neutral.

All objects are made up of atoms, which means that they are electrically neutral. Then, how do objects become electrically charged?

For some reason, the balance of electrically neutral atoms gets disturbed. For example, when certain objects are rubbed against each other, the negatively charged particles on one object go to the other object. The object to which they go, becomes negatively charged due to an excess of negatively charged particles. Similarly, the object from which the negatively charged particles go away becomes positively charged due to a deficiency of negatively charged particles. It means that, of the two objects being rubbed, one becomes positively charged and the other, negatively charged.

Try This

Materials: Paper, polythene, nylon cloth, cotton cloth, silk cloth, etc.

Procedure: First take the objects mentioned in the chart near some small pieces of paper and observe what happens. Then rub each of these objects in turn against one of the given materials and take it near the pieces of paper. Record your observations in the chart.

Repeat this procedure with each of the given materials.

Always remember: Each atom is electrically neutral. It has equal amounts of positive and negative charge. If for some reason, the negative charge decreases, the atom becomes positively charged.

Use your brain power! Do all objects get charged by rubbing?

Teacher's Note

Students can rub different objects with cloth and check which ones get charged. In India, we often see this in winter when clothes stick to our body due to static charge.

Exam Trick

Remember: Not all objects get charged by rubbing. Metals like copper and steel do not charge easily. This is because they conduct electricity.

Points to Remember

Atoms are electrically neutral naturally.
Rubbing can disturb the balance of charges in atoms.
Some materials get charged easily, others do not.
Metals are poor at getting charged by rubbing.
Non-metals like plastic and rubber get charged easily.

Do You Know?

About 2500 years ago a Greek scientist named Thales found that feathers are attracted towards a rod of yellow coloured amber which had been rubbed against a woollen cloth. Amber is called 'elektron' in the Greek language. Therefore, this property of amber to attract things was named 'electricity' by Thomas Browne in 1646 A.D.

Frictional Electricity

The electric charge generated by friction is called frictional electricity. This charge is produced only at the place of friction. Hence, it is called static electricity. It remains on the object for a short duration. The charges of static electricity are absorbed in moist air. That is why these experiments should be performed in dry weather, particularly in winter.

Try This

Apparatus: A few straws, woollen cloth (socks or gloves), glass bottle.

Procedure: Place a straw on a bottle. Take another straw near it. Observe what happens. Leave the straw on the bottle as it is. Rub the other straw against a woollen cloth and take it near the straw on the bottle. Observe what happens. Now take two straws and rub them against woollen cloth at the same time. Keep one of the straws on the bottle and take the other near it. See what happens. Keep the rubbed straw on the bottle as it is. Take the woollen cloth on which it was rubbed, close to it.

Record your observations in each of the above procedures in the chart.

Electrically charged objects attract uncharged objects. There is repulsion between like electric charges. There is attraction between unlike electric charges. Hence, repulsion is used as a test for identifying an electrically charged object.

Teacher's Note

This simple experiment with straws helps students understand attraction and repulsion. You can do this in class on a dry winter day in India.

Exam Trick

Remember: Repulsion is the real test for charging. If two objects repel each other, they must both be charged. Attraction can happen even if only one object is charged.

Points to Remember

Frictional electricity is generated only at the place of friction.
Static electricity remains for a short time.
These experiments work best in dry weather.
Repulsion proves that an object is charged.
A charged object always attracts an uncharged object.

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