Solved examples for class 8 science : Chapter 15

    Some Natural Phenomena

    I. Multiple Choice Questions (MCQs).
    Q.1. The rapid flow of charge through air between two oppositely charged clouds is called
    a) electric charge
    b) electric discharge
    c) earthing
    d) none of these
    Answer. (b) electric discharge

    Q.2. Which of the following cannot be charged by friction?
    a) steel spoon
    b) inflated balloon
    c) plastic scale
    d) woollen cloth
    Answer. a) steel spoon

    Q.3. The waves generated by an earthquake are called
    a) ultrasonic waves
    b) infrasonic waves
    c) seismic waves
    d) none of these

    Answer. c) seismic waves
    Q.4. When glass rod is rubbed with silk, they acquire
    a) unequal and unlike charges
    b) unequal and like charges
    c) equal and unlike charges
    d) equal and like charges

    Answer. (c) equal and unlike charges

    Q.5. The plate boundaries are the weak zones, where earthquakes are more likely to occur, these zones are known as
    a) fault zones
    b) seismic zones
    c) both (a) and (b)
    d) none of these

    II. Fill in the blanks by selecting appropriate words from the given list:
    List : Earthing, earthquakes, Lightning, charged, electroscope.
    Q.1. ………………………………. is also an electric spark, but on a large scale.
    Q.2. The balloons are said to be electrically ………………………………when they are rubbed against the hair.
    Q.3. Major …………………………………….are responsible for tsunamis, floods and landslide.
    Q.4. The…………………………………. is used to detect and measure charge.
    Q.5. …………………………………………is used to protect us from electrical shocks due to any leakage of electric current from any electrical appliance.

    Answer.
    1- Lightning ,
    2- charged,
    3- earthquakes,
    4- electroscope,
    5- Earthing

    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5-c

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. There are two kinds of charges-positive and negative.
    Q.2. Human body is a bad conductor of electricity.
    Q.3. When a huge amount of negative and positive charges meet in the sky, an intense spark of electricity and sound is produced.
    Q.4. During lightning and thunderstorm, open place is considered as a safe place.
    Q.5. Major earthquakes can cause a widespread damage over a large area.

    Answer. 1-T, 2-F, 3-T, 4-F, 5-T
    V. Very Short Answer Questions.

    Q.1. Write the nature of charges on glass rod and silk cloth, when they are rubbed with each other.

    Answer. When a glass rod is rubbed with a piece of silk, the glass rod acquires a positive charge and the silk acquires a negative charge.

    Q.2. Write the nature of charges on ebonite rod and wool, when they are rubbed with each other.

    Answer. Ebonite rod becomes negatively charged and wool becomes positively charged.

    Q.3. Name a device which is used to detect and measure charge.
    Answer. Electroscope
    Q.4. Name a process which is used to protect us from electrical shocks due to any leakage of electric current from any electrical appliance.
    Answer. Earthing
    Q.5. Write the general nature of charges of the lower and upper portions of the clouds.
    Answer. The lower portion of a cloud generally carries negative charges and the upper portions carry positive charges.

    VI. Short Answer Questions.

    Q.1. Why do two bodies get charged by rubbing against each other?
    Answer. Molecules tend to have the same number of protons (positive charges) and electrons (negative charges). When you rub these bodies together, you are in fact rubbing molecules together. It is possible to remove an electron from the molecules from one body and temporarily give it to molecules in the other body. Thus two bodies get charged by rubbing against each other

    Q.2. How do clouds get charged?
    Answer. According to scientists, during a thunderstorm, thunder clouds carry electric charges. These charges developed due to rubbing of clouds with air and dust particles present in air. These charges separate out within the cloud. The lower portion of a cloud generally carries negative charges and the upper portions carry positive charges. In In this way clouds get charged.

    Q.3. What are causes of an earthquake?

    Answer. Main cause of earthquake is the disturbance deep down inside the uppermost layer of the earth. This layer is called the crust.

    Q.4. Write the methods of charging of a body.

    Answer. Charging of a body can be done by
    a) Rubbing or friction
    b) Contact of conductor, and
    c) Induction

    Q.5. What do you understand by gold leaf electroscope? Explain its construction and working with the help of diagram.

    Answer. Gold leaf electroscope is an instrument for detecting and measuring static electricity. It consists of a metal rod such as brass with a brass disc at its top. The brass rod passes into a bell jar through a tightly-fitted rubber stopper. At the bottom of the brass rod are mounted two gold strips. These can be made of copper, brass or gold.


    Working of an electroscope with help of a diagram:


    Q.6. What is lightning? How is it produced?

    Answer. Lightning is a rapid flow of charge through the air between the two oppositely charged clouds. During a thunderstorm, thunder clouds carry electric charges. These charges developed due to rubbing of clouds with air and dust particles present in air. These charges separate out within the cloud. The lower portion of a cloud generally carries negative charges and the upper portions carry positive charges. Due to negative charges in the lower portion of clouds, there is accumulation of positive charges near the ground also. When a huge amount of charge builds up, air is no longer able to resist their flow. Negative and positive charges meet, producing an intense spark of electricity and sound in the sky. Such rapid flow of charge through the air between the two oppositely charged clouds is called electric discharge or lightning.

    VII. Long Answer Questions.

    Q.1. What is an earthquake? How does it occur? Describe some important safety measures against it.

    Answer. A sudden movement or a fracture in the crust and the upper layer of the earth is known as earthquake.

    The main cause of earthquake is the disturbance deep down inside the uppermost layer of the earth. This layer is called the crust.

    Some important safety measures against earthquake:

    • We must examine the soil and its type before we build up a building on it. We should also consider how much strength landfills and reclaimed areas have to support buildings during the earthquake.
    • Quality of materials, and the structure and design of the building should be thoroughly checked.
    • Very light weight materials should be used in the structure of buildings in earthquake-prone areas, because it can reduce loss of life if the structures of the building collapse during the earthquake.

    Do’s during an Earthquake

    • During earthquake take shelter under a table and stay there till the earthquake stops.
    • Stay away from tall and heavy objects that may fall on you.
    • Find a clear spot, away from buildings, trees and overhead power lines.

    Don’ts during an Earthquake

    • During earthquake if you are in bed, do not get up. Protect your head with a pillow.
    • During earthquake if you 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 stops.

    Q.2. What is seismograph? How is magnitude of an earthquake measured?

    Answer. Seismograph is an instrument which records the seismic waves. Seismograph consists of a pendulum bob suspended with a string. The bob has a little pen attached to it, and the tip of the pen touches a When earthquakes occur, the whole system vibrates. Pendulum bob also vibrates which consists of a pen. The pen records these vibrations on the paper which moves under it. This is the output of the seismograph, called seismogram.

    The magnitude of an earthquake is expressed in terms of a magnitude on a scale, called the Richter scale. An earthquake of 2.0–4.0 on the Richter scale is not too damaging. More than 4 is a moderate to severe earthquake. An earthquake of magnitude, greater than 8-9 would be very severe.
    This scale is Logarithmic, i.e., a change in the value on the scale by one corresponds to 10 times change in the energy associated with the earthquake. For example, an earthquake of 5.0 magnitude is ten times more powerful than the earthquake of 4.0 magnitude.

    Q.3. (a) Describe lightning conductor in your own words.
    (b) Discuss structure of the earth in your own words.
    Answer. a) Lightning conductors is a device used to protect buildings from the effect of lightning. A Lightning conductor is a metallic rod which runs from the top to the bottom, along the outer wall of the building. The lower end of this rod is connected to a metal (copper) to a metal plate, which is buried deep inside the earth.
    The basic idea behind Lightning protection is to provide a direct, easy path for the transfer of electric charge to the ground. Because lightning always follow the easiest path.

    b) These shells or layers of the Earth can be broadly classified as crust, mantle and core (outer core and inner core)

    1. The crust: The crust is the outermost layer of the earth. It is very thin. The thickness of the crust varies from place to place. It is about 30 km under continents and about 11 km under oceans.

    The crust is the most important layer to all living organisms. About 71 % of the earth crust is covered with water. It contains minerals, fossil fuels, soil, etc. These things are necessary to exist life on the earth. Crust is not in one piece. It is fragmented. Each fragment is called a plate. We will discuss about this later.

    2. The Mantle: It is the second layer after the crust, called mantle. It is about 2900 km thick. It is made up of molten solid rocks, commonly called magma. It is mainly made up of iron and magnesium compounds. It is much hotter than the crust.

    3. The outer core: It is the third layer from top of the earth. It is about 2,300 km thick. It is mainly made up of iron and nickel in a molten state. There is also some sulphur present here.

    4. The inner core: It is the fourth layer from top of the earth. It is about 1,200 km thick. In thin layer iron exists as solid because of the extremely high pressure in that part of the earth. The temperature in this layer is about 4000 °C and pressure is about 37 lakh atmospheres.

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