The Class 8 is an important year in a student’s life and Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 8 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete NCERT Class 8 Science Book syllabus.
Exercise Questions
Select the correct option in Questions 1 and 2.
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.
Soln:
The answer is (b) A copper rod.
Explanation:
Only non-conducting materials can be easily charged by friction. Copper is a highly conducting material. Therefore, a copper rod cannot be charged easily by friction.
2. When a glass rod is rubbed with a piece of silk cloth, the rod
(a) and the cloth both acquire a positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire a negative charge.
(d) becomes negatively charged while the cloth has a positive charge.
Soln:
The answer is (b) becomes positively charged while the cloth has a negative charge.
Explanation:
When two objects are rubbed against each other, they acquire opposite charges. By the law of convention, it is known that the rod acquires the positive charge, and the cloth acquires the 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 attracts a charged plastic straw. (T/F)
(c) Lightning conductors cannot protect a building from lightning. (T/F)
(d) Earthquakes can be predicted in advance. (T/F)
Soln:
a) False – Unlike charges attract each other, while the like charges repel each other.
b) True – A charged plastic straw has a negative charge on its surface, while the glass rod has a positive charge on its surface. Unlike charges attract each other, so they both attract each other.
c) False – When lightning occurs, the atmospheric charges are transferred to the earth directly by a lightning conductor. Therefore, the building is protected from lightning.
d) False – Even though the earthquake’s causes are known, there are no instruments that are invented to detect them in advance. Therefore, earthquakes cannot be predicted in advance.
4. Sometimes, a crackling sound is heard while taking off a sweater during winter. Explain.
Soln:
When we take out the sweater, the woollen sweater gets charged due to friction between the sweater and the body. This results in a crackling sound.
5. Explain why a charged body loses its charge if we touch it with our hand.
Soln:
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?
Soln:
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.
Soln:
Various ways to protect ourselves from lightning are
(i) Always remain in a closed place, and if you are in a car, stay there until the lightning is over and keep the windows closed.
(ii) Never touch any electrical wires, telephone cables or metal pipes.
(iii) Never bathe 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.
Soln:
The surface charge on the balloons is 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 to 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.
Soln:
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 them 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 the 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.
Soln:
Gujarat, Assam and Jammu &Kashmir are the three states where earthquakes are more likely to strike.
11. Suppose you are outside your home, and an earthquake strikes. What precautions would you take to protect yourself?
Soln:
The following precautions should be taken when an earthquake strikes:
(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.
Soln:
No, one should not carry an umbrella during a thunderstorm. The thunderstorm is accompanied by lightning, 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 lightning.