1. Static Electricity-
ü When charges move they constitute an electric current.
ü The chemical charge generated by rubbing is called static electricity because these charges do not transmit.
ü There are two types of charges-positive charge and negative charge.
ü When we rub two objects, made of different substances, together the charge they acquire are opposite to each other.
2. Earthing-
ü The process of transfer of charges from a charged object to the Earth is called earthing.
ü Earthing is provided in electrical wiring in building to protect us from electrical shocks, in case of any leakage of electrical current.
3. Lightning-
ü When negative charges from the clouds and positive charges on the ground meet, a huge amount of energy is produced as bright light and sound, which we see as lightning.
ü The process is called electric discharge.
ü The process of electric discharge between clouds and the earth or between different clouds causes lightning.
ü Lightning strike could destroy life and property.
ü Lightning conductors cam protect buildings from the effects of lightning.
ü The loud noise which accompanies lightning is called as thunder.
ü A storm accompanied by thunder and lightning is called as thunder storm.
Note: Electroscope is a device used to test whether an object is carrying charge or not. When the charge of an object is due to the excess of electrons, it is called a negative charge. When the charge of an object is due to the loss of electrons, it is called a positive charge. Lightning rod is a device used to secure tall buildings from the effect of lightning conductor. A metallic rod taller than the height of the building is installed in the walls of the building during its construction to protect it from the effect of lightning.
4. Earthquake-
ü An earthquake is a sudden shaking or trembling of the Earth. It is caused by a disturbance deep inside the earth’s crust.
ü An earthquake is a sudden shaking or trembling of the Earth.
ü Earthquake is caused by a disturbance deep inside the Earth’s crust.
ü It is not possible to predict the occurrence of an earthquake.
ü Earthquakes tend to occur at the boundaries of Earth’s plates. These boundaries are known as fault zones.
ü Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property.
ü The power of an earthquake is expressed in terms of magnitudes on a scale called the Richter Scale.
Note: Crust is the uppermost layer of Earth’s surface (8 km – 32 km). The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate.
5. Tsunami-
ü Earthquakes may cause tsunamis in oceans, resulting in huge damage in coastal areas.
ü The seismic waves are recorded by an instrument in the form of a graph called the seismograph.
Note: Tremor is the trembling or shaking of the Earth.
NCERT PRACTICE QUESTIONS
1. 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.
Ans.- (b) A copper rod
2. 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.
Ans.- (b) becomes positively charged while the cloth has a negative charge.
3. Write T against true and F against false in the following statements.
(a) Like charges attract each other (T/F)
(b) A charged glass rod attract a charged plastic straw (T/F)
(c) Lightning conductor cannot protect a building from lightning (T/F)
(d) Earthquakes can be predicted in advance (T/F)
Ans.- a) False b) True c) False d) False
4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Ans.- When we take out sweater, Woolen sweater gets charged due to friction between the sweater and the body. This results in crackling sound.
5. Explain why a charged body loses its charge if we touch it with our hand.
Ans.- The charges get conducted to the earth through our body when we touch it and the conductor loses its charge. This phenomenon is known as electric discharge.
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?
Ans.- Richter scale is used to measure the destructive energy of an earthquake. The scale has a reading from 1 to 10. An earthquake measuring 3 would be recorded by a seismograph. The magnitude of scale 3 would not cause much damage. An Earthquake of magnitude 5 is considered destructive in nature.
7. Suggest three measures to protect ourselves from lightning.
Ans.- (i) Always remain in a closed place and if you are in a car stay there until the lighting is over and keep the windows closed. (ii) Never touch any electrical wires, telephone cables, metal pipes. (iii) Never bath in running water, this may cause electric shock.
8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
Ans.- The surface charge on the balloons are of the same nature hence they get repelled. When a charged balloon is brought near an uncharged balloon due to the induction of charges, it acquires charges which are opposite in nature with that of a charged balloon. As unlike charges attract each other, the uncharged balloon gets attracted by the charged balloon.
9. Describe with the help of a diagram an instrument which can be used to detect a charged body.
Ans.- Instrument to detect charged body. It consists of a metal rod on which two leaves of aluminium foil are fixed to one end and a metal disc at the other end. The leaves are kept inside a conical flask and it is corked to isolate it from the atmospheric air. When a charged body comes in contact with the metal disc, the aluminium leaves move away from each other because some charges get transferred to aluminium leaves through the metal rod. This process is called charging by conduction. The charges on the leaves and the charged body are of same in nature and thus the leaves of aluminium repel each other. If the body is not charged then they would attract each other.
10. List three states in India where earthquakes are more likely to strike.
Ans.- Gujarat, Assam and Jammu & Kashmir are the three states where an earthquake is more likely to strike.
11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
Ans.- (a) Find and go to an open field and stay away from buildings, trees, electric wire and poles. (b) If you are in a car, then drive to an open field and do not come out of your car.
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.
Ans.- No, one should not carry an umbrella during a thunderstorm. The thunderstorm is accompanied by lighting and the charges might travel from the cloud to the metal rod on the umbrella and might cause an electric shock to the person carrying it. So, it is not safe to carry an umbrella during lighting.
Junior Science Topic - Some Important Phenomena. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 15)
By - EduPorm India - The Educational Platform
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