Sometimes, a crackling sound is heard while taking off sweater during winters. Explain.
A crackling sound is heard while taking off sweater made of synthetic fibre during winters especially when the weather is dry, because, since there is some humidity also present in the atmosphere and synthetic fibre gets charged and due to attraction forces acting between charged particles they oppose there detachment and create a crackling sound.
Explain why a charged body loses its charge if we touch it with our hand.
When we touch a charged body, the total charge is transferred from the charged body into the earth via the human body and it loses its charge.This process is called earthing.
Question 3Name 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 power of an earthquake is expressed in term of a magnitude on a scale called Richter scale.
The tremors produce waves on the surface of the earth. These are called seismic waves. The waves are recorded by an instrument called the seismograph. The instrument is simply a vibrating rod, or a pendulum, which starts vibrating when tremors occur. A pen is attached to the vibrating system. The pen records the seismic waves on a paper which moves under it. By studying these waves, scientists can construct a complete map of the earthquake. They can also estimate its power to cause destruction.
Question 4Suggest three measures to protect ourselves from lightning.
(i) Once you hear thunder without lightning, rush to a safer place.
(ii) After hearing the last thunder, wait for at least 30 minutes before coming out of the safe place.
(iii) A house or a building is the safest place. If you are travelling by can or by bus. You are safe inside with windows and doors of the vehicle shut.
Question 5Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
We have seen that two objects (balloon with similar charges on them, whether positive or negative, repel each other. Similarly, two objects (balloon) with dissimilar charge on them attract each other. So, similar charges repel each other while dissimilar charges attract each other.
The action of one charged body on another or interaction between two charges can help us to determine whether a given body is charged or not. Suppose you are asked to find out whether a given body is charged. You may bring the given body near a charged body and see whether they attract or repel each other. If they show repulsion, you may be sure that the given body is charged. The repulsion also confirms that the nature of charge on the two bodies is the same. A charged body attracts an uncharged as well as an oppositely charged body. Thus, we may conclude that repulsion between two bodies is a definite proof that both of them are charged.
Question 6Describe with the help of a diagram an instrument which can be used to detect a charged body.
Principle: It works on the principle that like charges repel while unlike charges attract each other.
Structure and working: An electroscope has a metal rod with thin metal strip or leaf attached to it at the bottom. At the top, the rod enters in a cup. The bottom part of the rod and leaf are enclosed in a glass box for protection. When the knob of the electroscope is touched with a charged ebonite or glass rod, the leaves open out or diverge. Extent of divergence depends upon the amount of charge on the electroscope.
Question 8Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
(i) Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
(ii) If we are in a car or a bus, do not come out, ask the driver to drive slowly to a clear spot. Do not come out till the earthquake stop.
Question 9The 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.
Open vehicles, like motorbike, tractor, construction machinery, open cars are not safe. Open fields, tall trees in woods or isolated ones, shelters in a park, elevated places also do not protect from the lightning stroke. Carrying umbrella is not a good idea at all during thunderstorm. If in a forest, take shelter under shorter trees. If no shelter is available, you are in an open field; stay as far away from a tree as it is tall. Stay away from poles or other metal objects. Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands. This position will make you the smallest target.