Tuesday, 24 January 2012

CBSE-Class 8 - Science -CH9 - NCERT Solutions: SOME NATURAL PHENOMENA


Select the correct option in Questions 1 and 2.
mature thunderstorm cell
Storm Cloud

Q1. Which of the following cannot be charged easily by friction?
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth.

Answer: (b) A copper rod.

Q2. When a glass rod is rubbed with a piece of silk cloth the rod

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

Answer: (b) becomes positively charged while the cloth has a negative charge.

Q3. Write T against true and F against false in the following statements:

(a) Like charges attract each other (T/F) - False
(b) A charged glass rod attract a charged plastic straw (T/F) - True
(c) Lightning conductor cannot protect a building from lightning (T/F) - False
(d) Earthquakes can be predicted in advance (T/F) - False

Q4. Sometime, a crackling sound is heard while taking off sweater during winters. Explain

Answer: In general, Sweater is made of wool and the other inner clothes like shirts are made up of cotton and synthetic fibres. Due to friction among these, static charge gets accumulated on the sweater. While taking off sweater, an electric discharge occurs between sweater and the body which results in forming of sparks and crackling sound.

Q5. Explain why a charged body loses its charge if we touch it with our hand.

Answer: Human body is conductor of electricity. A charged body loses its charge when we touch it with hand because the charges get transferred from our body to earth. This process of transferring of charges from charged body to earth is known as earthing.

Q6. 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?

Answer: Richter Scale measures the destructive energy of an earthquake. Seismograph can measure an earthquake of magnitude 3 on Richter Scale. It is of low intensity and will not cause any damage.

Q7. Suggest three measures to protect ourselves from lightning.

  • Safety Measures during Lightning:
  • Open space is dangerous. Take cover under a building
  • Thunder is an indication to rush for safer place
  • Once thundering stops, wait for some time before moving out to an open place

Q8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Answer: The charged balloons have similar charges on the surfaces. Since like charges repel each other, that's why the charged balloon is repelled by another charged balloon. When an uncharged balloon is brought near a charged one, the uncharged balloon acquires some opposite charge. Since unlike charges attract each other, therefore a charged balloon attracts an uncharged one.

Q9: What causes lightning?

Answer: The process of massive electric charge between clouds and the earth or between different clouds causes lightning.
As water vapours evaporate in the atmosphere, due to friction they are also charges. Clouds carrying massive  water vapours accumulate electric charge. This accumulation of static charge leads to sparks between clouds of higher potential and clouds/earth of lower potential.