Q1: Explain an electroscope and earthing.
Ans: An electroscope is a device that detects the type of charge on a body. Like charges repel and unlike charges attract each other. This is used in an electroscope. An electroscope consists of a glass jar fitted with a cork lid and a metallic wire passing through it. There are two metallic strips at the bottom of the wire. The upper end of the wire is connected to a metal disc. A body that is positively charged is touched to the metal disc, so that the charge is transferred to the metal strips through the wire, and they diverge from each other on gaining a like charge. Now, if a negatively charged object is brought into contact with the disc, the strips converge towards each other, indicating the unlike charge on the body. Similarly, if a positively charged body is brought in contact with the metal disc, the divergence of the metal strips increases, indicating the like charge on the body.
The process of transferring of charge from a charged object to the earth is called earthing. For our safety, most of the electrical appliances and the mains of the house are connected to earth, so that we can be prevented from getting an electric shock.
Q2: Explain two natural destructive phenomena.
During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. By a process, not yet completely understood, the positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges. There is an accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is called an electric discharge. Accumulation of charge leads to lightning.
The process of electric discharge can occur between two or more clouds, or between clouds and the earth. A lightning strike could destroy life and property. It is therefore, necessary to take measures to protect ourselves.
An earthquake is a sudden shaking or trembling of the earth lasting for a very short time. It is caused by a disturbance deep inside the earth’s crust.
Earthquakes occur all the time, all over the earth. They are not even noticed. Major earthquakes are much less frequent. They can cause immense damage to buildings, bridges, dams and people. There can be a great loss to life and property.
Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out. However, most earthquakes are caused by the movement of earth’s plates.
Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones.Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out. However, most earthquakes are caused by the movement of earth’s plates. The power of an earthquake is expressed in terms of a magnitude on a scale called Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale.
Q. 3. Explain the construction and working of electroscope.
Ans. It is an instrument used to detect electric charges.A simple electroscope can be constructed as follows.
(i) Take a glass container.
(ii) Insert a metal wire inside it.
(iii) To the ends of the wire, which are inside the jar, attach 2 metal plates(say aluminium plates).
(iv) The other end is outside the glass container is connected or brought in contact with charged body.
WORKING OF AN ELECTROSCOPE :
(i) In an uncharged electroscope, the leaves hang straight down.
(ii) When a charged object touches the metal knob, electric charges travel down the rod and into the leaves.
(iii) The leaves spread apart, indicating the presence of an electric charge.
(iv) Since the charge on both leaves is the same the leaves repel each other and spread out.
Q. 4. Explain the mechanism of thunderstorms.
Ans. During the development of a thunderstorm, the air currents move upward while the water drops move downward. These movements cause separation of charges. The positive charges collect near the upper edges of the clouds and negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also. When the amount of accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. This process is called an electric discharge. The process of electric discharge can occur between two or more clouds or between clouds and the earth. In this way thunderstorm is caused.
Q. 5. How can you save yourself from lightning?
Ans. Some safety measures are:
(i) We should run to take shelter in the house.
(ii) We should remain in the covered area.
(iii) We should not sit in open, on scooters or bike etc.
(iv) We should take shelter under a small tree while in open.
(v) If there is no tree or other shelter we should sit with head folded.
(vi) We should plug out all the electrical appliances during lightning.
(vii) We should not use wired telephones during lightning. Mobiles and cordless phones are safe.
Q. 6. Explain the safety measures during thunderstorm when you are inside your home.
Ans. Some safety measures are as follows:
(i) During a thunderstorm contact with telephone cords, electrical wires and metal pipes should be avoided.
(ii) It is safer to use cordless phones and mobiles.
(iii) Bathing should be avoided during thunderstorm.
(iv) Computer, TV etc. should be unplugged.
Q. 7. Explain the mechanism of earthquakes.
Ans. The tremors are caused by the disturbance deep down in the uppermost layers of earth. The uppermost layer of earth is called crust. The crust of earth is not a one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another or a plate goes under another due to collision, they cause disturbances and show up as an earthquake on the surface of the earth. Although we know the causes of the earthquake, it is not possible to predict when and where the next earthquake might occur.
Q. 8. What is a seismograph? How does it work?
Ans. The instrument used to record the seismic waves is called seismograph. The tremors produce waves on the surface. The instrument is simply a vibrating rod or a pendulum. Which starts vibrating when tremors occur. A pen is attached to vibrating system. The pen records the seismic waves on a paper which moves under it. By studying these waves scientists construct a map of earthquake. They can also estimate its power to cause destruction.
Q. 9. What suggestions will you give the people to make ‘Quake Safe’?
Ans. Some suggestions are:
(i) Consult qualified architects and structure engineers.
(ii) Make the structure symmetrical so that the mass is distributed uniformly.
(iii) Use of mud or timber is better than the heavy construction.
(iv) The cupboards and shelves are to the walls.
(v) Be careful about where to hang wall clocks, photo frames, water heater etc. So that in the event of an earthquake they do not fall on people.
(vi) All building should have fire fighting equipment installed.