Science STD 8 Chapter 12: Some Natural Phenomena - Exercises

Science STD 8 Chapter 12: Some Natural Phenomena - Exercises

1. Which of the following cannot be charged easily by friction?

  1. (a) A plastic scale
  2. (b) A copper rod
  3. (c) An inflated balloon
  4. (d) A woollen cloth.

2. When a glass rod is rubbed with a piece of silk cloth the rod

  1. (a) and the cloth both acquire positive charge.
  2. (b) becomes positively charged while the cloth has a negative charge.
  3. (c) and the cloth both acquire negative charge.
  4. (d) becomes negatively charged while the cloth has a positive charge.

3. Write T against true and F against false in the following statements.

  1. (a) Like charges attract each other (T/F)

    F

  2. (b) A charged glass rod attract a charged plastic straw (T/F)

    T

  3. (c) Lightning conductor cannot protect a building from lightning (T/F)

    F

  4. (d) Earthquakes can be predicted in advance (T/F)

    F

4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.

A crackling sound is heard because of the accumulation and discharge of electric charges. When a woollen or polyester sweater is taken off, it rubs against the body and gets charged due to friction. This charge builds up on the surface of the sweater and, when discharged, produces a spark and a crackling sound.

5. Explain why a charged body loses its charge if we touch it with our hand.

A charged body loses its charge if we touch it with our hand because our body is a conductor of electricity. When we touch a charged body, the excess charge is transferred from the object through our body to the earth. This process of transferring charge to the earth is called earthing, and it neutralises the charged object.

6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

The destructive energy of an earthquake is measured on the **Richter scale**. An earthquake measuring 3 on this scale would be recorded by a seismograph. However, it is not likely to cause much damage, as only earthquakes with a magnitude higher than 7.5 are considered truly destructive.

7. Suggest three measures to protect ourselves from lightning.

Three measures to protect ourselves from lightning are:

  • Avoid open places during a thunderstorm. A house or a building is a safe place.
  • Stay away from tall trees, poles, and metal objects.
  • Inside a house, avoid contact with telephone cords, electrical wires, and metal pipes.

8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

A charged balloon is repelled by another charged balloon because both have the same type of charge (like charges repel each other). An uncharged balloon is attracted to a charged balloon because the charged balloon induces an opposite charge on the surface of the uncharged balloon that is closer to it. This temporary attraction makes them stick together.

9. Describe with the help of a diagram an instrument which can be used to detect a charged body.

An instrument used to detect a charged body is an **electroscope**. A simple electroscope can be made using an empty jam bottle, a piece of cardboard, a metal paper clip, and two aluminium foil strips. When a charged object is brought into contact with the end of the paper clip, the charge is transferred to the foil strips through the conductor. The strips, having received the same charge, repel each other and spread apart, indicating that the body is charged.

10. List three states in India where earthquakes are more likely to strike.

Three states in India where earthquakes are more likely to strike are **Gujarat** (especially the Rann of Kutch), **Rajasthan**, and the states in the **North-East** region (like Tripura and parts of the Himalayas).

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

If an earthquake strikes while I am outdoors, I would take the following precautions:

  • Find a clear spot away from buildings, trees, and overhead power lines.
  • Drop to the ground and cover my head with my arms.
  • If I am in a car or bus, I would stay inside and ask the driver to stop in a clear area. I would not come out until the tremors have completely stopped.

12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

No, I would not carry an umbrella. An umbrella has a metallic rod, which is a conductor of electricity. During a thunderstorm, there is a risk of lightning strikes. The metallic rod could act as a conductor and attract lightning, making it very dangerous. It is safer to seek shelter in a building or a car with the windows shut.

Suggested Activities and Projects

1. Open a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity.

Outline: This activity demonstrates the principle of static electricity. When a charged refill is brought near the thin stream of water, the water stream will bend towards the refill. This happens because the charged refill attracts the water molecules, which are polar. The report would explain this phenomenon and how the charged object can exert a force on a neutral object.

2. Make your own charge detector. Take a paper strip roughly 10 cm × 3 cm. Give it a shape as shown in Fig. 12.15. Balance it on a needle. Bring a charged body near it. Observe what happens. Write a brief report, explaining its working.

Outline: This activity is a simplified version of an electroscope. The paper strip, when balanced on a needle, acts as a detector. When a charged body is brought near it, the paper strip will be attracted to it and start spinning. This happens due to the induction of charge on the paper strip, which then gets attracted to the charged body. The report would explain this principle and the device's function.

3. This activity should be performed at night. Go to a room where there is a fluorescent tube light. Charge a balloon. Switch off the tube light so that the room is completely dark. Bring the charged balloon near the tubelight. You should see a faint glow. Move the balloon along the length of the tube and observe how the glow changes. Caution : Do not touch the metal parts of the tube or the wires connecting the tube with the mains.

Outline: This activity demonstrates the principle of electric discharge. When the charged balloon is brought near the fluorescent tube, the electric charge from the balloon interacts with the gases inside the tube, causing a faint glow. This is similar to how lightning works on a huge scale. Moving the balloon would change the intensity of the glow, showing how a change in the electric field affects the discharge.

4. Find out if there is an organisation in your area which provides relief to those suffering from natural disaster. Enquire about the type of help they render to the victims of earthquakes. Prepare a brief report on the problems of the earthquake victims.

Outline: This project is a research and social awareness activity. You would identify local organisations that provide disaster relief and learn about their work. The report would detail the problems faced by earthquake victims, such as loss of life, injury, homelessness, and a lack of food and water. It would also explain the type of relief provided by the organisation, including medical aid, shelter, food, and emotional support.