NCERT 8th Class (CBSE) Science: Some Natural Phenomena

Question: If you hold a plastic comb in your hand and rub it in your hair, it will get charged. However, if you do the same with a metal comb it will not get charged. Why? But if the metal comb has a plastic handle and you hold it from the handle, it will also get charged. Why?

Answer: Both plastic and metal comb will get charged when rubbed in the hair. However, since metal is a conductor, the charges will flow to earth through the body and it will lose its charge. If the metal comb has an insulating plastic handle it will retain its charge.

Question: Why do you think charging by rubbing happens best in dry weather?

Answer: In humid conditions, when air holds a lot of moisture, it becomes conducting and the charges leak into the air. This does not happen in dry weather. Therefore charging by rubbing happens best in dry weather.

Question: The rod of a lightning conductor accidentally broke into two from the middle. What danger will this pose if lightning strikes the building? Why?

Answer:  The building will be damaged and may even catch fire as the charges will flow through the building instead of directly to the earth through the conductor.

Question: How can charging take place by rubbing?

Answer: When certain objects are rubbed against another object, electrons may get transferred from one object to another. If an object loses some electron, it is left with surplus number of protons. Thus, this object becomes positively charged. If an object gains electrons, it now has surplus number of electrons. Thus, this object becomes negatively charged. For example:  Take a comb and rub it vigorously in your hairs. Your hairs should be completely dry during this activity. Bring this comb near bits of paper. It is observed that the comb attracts the bits of paper. This activity shows that a charged object attracts uncharged object.

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

Answer: When a sweater is taken off, it gets charged because of rubbing. Due to this, we get to hear a crackling sound.

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

Answer: When a charged body is touched by hand, the human body provides a passage for the charge to the earth. Due to this, the charged body loses its charge.

Question: 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: The destructive energy of an earthquake is measured on Richter scale. An earthquake which measures 3 on this scale would be recorded by a seismograph. An earthquake of this intensity is not likely to cause any damage.