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Q1. Which of the following cannot be charged by friction, if held by hand? (a) a plastic scale (b) a copper rod (c) an inflated balloon (d) a woolen cloth. Why?

Solution

Copper rod Except copper, the other three are insulators whereas copper is a conducting object. As soon as it gets charged by rubbing with another material, the electric charge produced on its surface flow through our hand and body into the earth and it remains uncharged.

Q2. Explain the construction of an electroscope.

Solution

An electroscope consists of a large jar. A metal rod is fitted into the mouth of the jar with the help of the cork. At the lower end of the metal rod a pair of thin leaves of gold or aluminium is suspended.

Q3. How would you use an electroscope to find out whether an object is charged or not?

Solution

Touch the body to be tested with the metal disc of an electroscope. If the leaves of an electroscope open up (diverge), the body is charged. If the leaves remain unaffected then the body has no charge.

Q4. What are focus and epicentre of an earthquake?

Solution

The place inside the earth's crust where the earthquake is generated, is called focus of the earthquake. The point on the earth's surface directly above the focus is called epicentre.

Q5. What are fault zones?

Solution

Since earthquakes are caused by the movements of the plates, the boundaries of the plates are the weak zones where earthquakes are most likely to occur. Such areas are called seismic or fault zones.

Q6. Suppose you are outside your home and an earthquake strikes. What precautions would you take to protect yourself?

Solution

The following precautions should be taken :- 1. Find a clear spot, away from buildings, trees, poles and electric poles, signboards and overhead power lines and drop to the ground.  2. Do not use elevators. 3. If you are in a car or a bus, do not come out and drive slowly to a clear spot. Stay inside a car till the tremors stop.

Q7. What happens when we touch the metal cap of a charged electroscope with our finger? What is this process known as?

Solution

The leaves of an electroscope collapse as soon as we touch the metal cap with hand because the leaves of the charged electroscope lose charge to the earth through our body. This process is known as EARTHING.

Q8. Describe an activity to show that charging can be done by rubbing the objects.

Solution

Take a used ballpen refill and rub it vigorously with a piece of polythene. Bring it close to tiny pieces of paper. It will attract the pieces of paper.This suggests that the refill has acquired an electric charge by rubbing.

Q9. Does a charged object get uncharged on touching with hand? Explain. What is this phenomenon called?

Solution

Yes. This is because human body is a conductor of electricity. So, when we touch a charged object, our body conducts its charges to the earth. That is why a charged object loses its charge, if we touch it with our hand. This phenomenon is known as electric discharge.

Q10. What kind of electric charge is acquired? (a) by a glass rod rubbed with silk cloth? (b) by a plastic comb rubbed with dry hair?

Solution

(a) Positive charge.(b) Negative charge.

Q11. What is the cause of lightning?

Solution

Lightning is caused by the accumulation of charges in the clouds.

Q12. What are seismic waves? Name the instrument which records these waves.

Solution

Tremors or vibrations caused by the earthquakes which travel in the form of waves within the earth or along the earth's surface, are called seismic waves. Seismograph is an instrument which records these waves.

Q13. What will you observe when the metal cap of an electroscope is touched with a plastic comb rubbed in dry hair? Give reason for your answer.

Solution

After rubbing, the plastic comb acquires negative charge. When it is touched with the metal cap of an electroscope then both the metal cap and the leaves acquire negative charge due to conduction. Because of negative charge on both the leaves, divergence of leaves takes place.

Q14. By which device the power of an earthquake is expressed?

Solution

The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale.

Q15. What is lightning. Explain the experiment conducted by Benjamin Franklin that showed sparks shared some similarity with lightning ? 

Solution

Lightning is an atmospheric discharge of electricity, which occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In June 1752, Benjamin Franklin raised a kite. On the end of the string he attached a key, and he tied it to a post with a silk thread. After some time he noticed that small pieces of strings were beginning to stand apart like the hair on the back of a scared dog. He then brought his hand close to the key and received a tingle of an electric shock from the key. As the rain came down, the string became soaked and the electricity began to conduct freely through the key.

Q16. What is the function of a lightning conductor running through a tall building? What would happen if it is not installed in the building?

Solution

The function of a lightning conductor is to conduct any lightning strikes safely to the earth without causing any damage to the building. If a tall building is not protected with a lightning conductor, then the tremendous electric energy produced during lightning would pass through the walls of the building, causing damage to the material of the walls and making the walls unsafe. It can even set the building on fire.

Q17. Suppose you are at your home and an earthquake strikes. What precautions would you take to protect yourself?

Solution

The precautions that should be taken are:- 1. Take shelter under a table and stay there only, till the shaking stops. 2. Stay away from the objects which are tall and heavy, that may fall on you. 3. If you are on bed, do not get up and remain there only and protect your head with pillow.

Q18. Why does a plastic comb rubbed with dry hair attract tiny pieces of paper?

Solution

Plastic comb gets electrically charged due to rubbing & therefore it attracts tiny pieces of paper. A charged body can attract an uncharged body.

Q19. Cite any two examples where the objects may get charged when rubbed against any other object.

Solution

(i) When a plastic refill is rubbed with polythene, it acquires a small electric charge. (ii) When a plastic comb is rubbed with dry hair, it acquires a small charge.

Q20. List two places in India which are most threatened by an earthquake.

Solution

Two places in India which are most threatened by earthquake are 1. Kashmir 2. Rann of Kutch.

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