CBSE Class 8 Science Some Natural Phenomena Chapter Notes

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Some Natural Phenomena

Electrostatics: Frictional Electricity
Electrostatics is the study of electric charges at rest. Therefore, it is also called ‘static electricity’. The charges at rest are produced due to friction between two insulating bodies which are rubbed against each other. That is why electrostatics is also called ‘frictional electricity’.
We know that when a glass rod is rubbed with a piece of silk, the rod acquires the property of attracting small pieces of paper, leaves, cork or even dust particles towards it. The rod is said to be electrified or charged. Similarly, a comb is electrified on passing through dry hair. When we take off a nylon shirt, we find little sparking. An electric charge also develops on a sheet of paper moving through a printing press and so on.

Electric Charge
Electric charge in an intrinsic characteristic of the fundamental particles making up our own bodies and the familiar objects that surround us i.e., electric charge is a characteristic that accompanies fundamental particles, wherever they exist.

The vast amount of charge in an object is usually hidden as the object contains equal amounts of positive charge and negative charge. With such an equal balance of charge, the object is said to be electrically neutral i.e. it contains no net charge.

If the positive and negative charges are not in balance, then there is a net charge. Thus an object is charged, if it has a charge imbalance or some net charge.

Atmospheric Electricity
The atmosphere of earth extends upto about 400 km above the earth’s surface. On the basis of distinct properties, the earth’s atmosphere is divided into four layers:
1. Troposphere 2. Stratosphere 3. Mesosphere 4. Ionosphere

Following are some of the important electrical properties of earth’s atmosphere.
1. Earth itself is good conductor of electricity.
2. The atmosphere near the surface of earth is a poor conductor of electricity. The conductivity increases, as we move up. Ionosphere is a very good conductor. This is primarily due to ionization of the molecules of air by charged cosmic ray particles having energies of the order of 1014 MeV.

Gold Leaf Electroscope (GLE)

class_8_science_useful_020

It is an instrument used for the detection of electric charge and measurement of potential difference. The essential parts of GLE are shown in figure. D is a metallic disc, R is a metal rod; S is insulating stopper; L, L are gold leaves; G is a glass bell jar; F, F are in foils. M is insulating base on which a metal foil is pasted. When a charged body is held in contact with the disc D, the gold leaves diverge. The divergence of leaves is a measure of potential of the charged body.

Properties of Charge
1. A charged body is that body which has either excess of electrons or deficiency of electrons. A body having excess of electrons is negatively charged and a body having deficiency of electrons is positively charged.
2. Like point charges repel each other, while unlike point charges attract each other. The magnitude of force of attraction or repulsion is given by Coulomb’s law.
3. In charging, the mass of a body changes. If electrons are removed from the body, the mass of the body will decrease and the body will become positively charged.
4. Charge is quantised.
5. A large number of experiments show that in an isolated system, total charge does not change with time.
6. Charge is invariant, charge is independent of frame or reference, charge on a body does not change whatever be its speed.
7. Accelerated charge radiates energy.
8. If charged body is brought near a neutral body, the charged body will attract opposite charge and repel similar charge present in the neutral body. In case of induction it is worth noting that. Induced charge can be lesser or equal to inducing charge (but never greater) and its maximum value is given by;
q´= -q[ 1 -1/K] 
 K is the dielectric constant of the material, for metals, K =¥ and so q´ = –q i.e., in metals induced charge is equal and opposite to inducing charge.

DO you know ?
Can ever the whole charge of a body be transferred to the other? If yes, how and if not, why ?
Answer: Yes; if the charged body is enclosed by a conducting body and connected to it, the whole charge will be transferred to the conducting body, as charge resides on the outer surface of a conductor.

Coulomb’s law
If two point charges q1 and q2 are placed at a certain distance r, then the force of attraction or repulsion (F) between them depends as ,
 

class_8_science_useful_021

CBSE Class 8 Science Some Natural Phenomena Chapter Notes

r, being a pure ratio, has no units.

For example, r for water is 81

3. Coulomb’s law is applicable for point charges only.

4. The forces between two charges are mutual i.e., the forces on the two charges are equal in magnitude but opposite in directions.

5. The force is conservative, i.e, work done in moving a point charge once round a closed path under the action of Coulomb’s force is zero.

Some Natural Phenomena

Induction of Charge: When any charged object is brought close to an uncharged object, the total charge of the uncharged object gets polarised as follows:

CBSE Class 8 Science Some Natural Phenomena Chapter Notes


In both case (a) and (b) without removing the charged object if the far side is touched (earthed) and then the charged object is removed then the uncharged object gain some net excess charge. This is the way we can charge an uncharged object by induction method. In this method the uncharged object gets opposite charge to that of the charged object.


Thunderstorms and Lightning Flashes

The mechanism of thunderstorm and lightning flashes is yet not well understood. However, the experimental observations lead us to the following facts:

1. All the world over, there occur about 4 × 104 thunderstorms per day.

2. In every thunderstorm, charged ions separate on account of some complex process which is yet not understood. The positive charges are carried to a height of about 6 km and negative charges collect at about 2 – 3 km. above the surface of earth. The lower level refers to the bottom of cloud and upper level refers to the top of the cloud.

CBSE Class 8 Science Some Natural Phenomena Chapter Notes
3. The total amount of negative charge at the bottom of the cloud = 20 coulomb. Earth is at a higher potential. The potential difference between earth and cloud bottom is  107 to 198V. Therefore, an electric field of strength (E = V/d)  104V/m is created, directed upwards as shown in figure.

4. Such high electric fields cause an ‘electric breakdown’ of the air i.e. the intervening air gets ionized and conducts charge from the cloud to earth in the form of a lightning flash. Each flash lasts for 2 × 10–3 sec, and deposits about 20 C of charge on earth.

5. After each lightning flash, cloud becomes charged again and is ready for the next flash. With 90 flashes per second all over the world, about 1800 coulomb of negative charge is pumped into earth per second. This balance the +1800 coulomb inflow of charge per second into the earth from the thunderstorm – free regions. Thus the steady state of earth and its atmosphere are maintained.

Lightning Conductor

A lightning conductor is used to save big buildings from the damage by the lightning flashes. A lightning conductor consists of a number of pointed conductors fixed to the top of a building and connected to a thick copper wire. This wire runs down the side of the building and ends on a metal plate buried in the ground. When a negative charged cloud passes over the building, it induces a positive charge on the pointed ends of lightning conductor and an equal negative charge at the metal plate. Due to discharging action of sharp points, a positively charged electric wind is set up, which neutralises the negative charge on the cloud. The negative charge on the metal plate is immediately neutralized in the earth. If the discharge of the cloud occurs, it passes through the conducting copper strip and the building is saved from the damage.

CBSE Class 8 Science Some Natural Phenomena Chapter Notes

Earthquakes

Any sudden disturbance below the earth’s surface may produce vibrations or tremors in the earth’s crust. These vibrations or shaking are known as earthquakes. In other words, the shaking of the ground caused by sudden movements of Earth’s crust is called in earthquake.
The enormous energy released by an earthquake travels through the ground in the form of waves. Thewaves generated by an earthquake are called seismic waves. These are recorded by an instrument, called seismograph or seismometer. The characteristics of an earthquake waves can be recorded on the screen of a seismograph, and is called a seismogram.
The science that deals with the seismic waves is called seismology and earthquake scientists are called seismologists.
The magnitude or intensity of an earthquake depends on the amount of energy released and is based on the direct measurement of the size of seismic waves.

Richter Scale

The magnitude or intensity of an earthquake is measured by the Richter scale, developed by measured by Charles F Richter, a US seismologist, in 1935. The number indicating the magnitude or intensity on Richter

scale ranges between 0 and 9. The Richter scale is not a linear scale. On this scale the vibrations of an earthquake with a magnitude of 2 are 10 times greater in amplitude than those of an earthquake with a magnitude of 1, and the vibrations of an earthquake with a magnitude of 8 are one million times greater in amplitude than those of an earthquake with a magnitude of 2. The severest earthquake ever recorded had magnitude of 8.7.

Earthquakes measuring 0-3 on the Richter scale are often not even noticed. An earthquake of intensity 4.5 is considered moderate and can disturb loose objects. At intensity 5.5, the earthquake is strong and can cause furniture to move and trees to sway. At intensity 6.5 an earthquake can damage weak structures. An earthquake of intensity 8 or more can be very disastrous and can destroy entire cities.

Main causes of Earthquake

The Earth’s lithosphere is divided into about 20 parts called tectonic plates.These plates float over the hot magma below, and are therefore in relative motion to each other. When these plates collide and the rock at the joint is sufficiently slippery, the plates slide over each other, little by little. However, sometimes the rocks at the joints can interlock and get stuck, resisting pressure from within. For years, the forces pushing the plates build up until the strain can rip the two plates apart. The rocks crack and shift, sending shock waves of energy in all directions. When these waves reach the surface, they are felt as earthquakes. The point from which the earthquake waves originate is called the seismic focus. It lies within the crust of the earth. The point on the earth’s surface vertically above the focus is called the epicentre of the earthquake.,The intensity of the earthquake is greatest at the epicentre.
The boundaries of the tectonic plates are the zones where earthquakes are most likely to occur. These are known as seismic or fault zones. In India faulty zone at the boundary of the Indian plate and Eurasian plate passes through Kashmir, western and central Himalayas, the whole of North-East, Rann of Kachchh, Rajasthan and the Indo-Gangetic plain. These areas are therefore most prone to earthquakes.

CBSE Class 8 Science Some Natural Phenomena Chapter Notes
Protection against Earthquake

It is very difficulty to predict the occurrence of an earthquake, as it occurs suddenly. Violent earthquakes are often desructive. They cause serious damage to life and property. We can take the following precautions against earthquakes while building our houses. The following are some steps you should take if you are caught in an earthquake.

• If trapped in your home or a building take shelter under a table and do not move till the shaking stops. Protect your head with your arms. Avoid using lift.

 Do not stay near the windows, bookcase, mirrors, hanging pots, fans during or immediately after an earthquake.

 Leave your home or school building and move to open areas.

 If outdoors, keep away from high-rise buildings, trees, signboards, poles and electric poles and electric wires.

 Do not sit inside a car or a bus.

 Help others according to their needs.

EXERCISES

1. Name the charged particles present inside an atom.

2. An insulator loses some electrons. What type of charge will be developed in the insulator and what will be its potential.

3. How can be charge induced in an uncharged object.

4. Write any three properties of charge.

5. A person coming out of a car touches the door get electric shock. How do you explain this fact.

6. What is a lightning conductor? How does it work.

7. Is there any charge in the atmosphere? If yes then where it is?

8. What are uses of a gold leaf electroscope?

9. A(+) 5 coulomb and (–) 10 coulomb charge are seperated by a distance of 1 m. How much force in Newton will act between them ?

10. What type of protection can be taken to avoid any damage caused by lightning.

11. A positively charged rod is brought closer to the head of an electroscope. What type of charge will be developed at its leaves.

12. Write different possible reasons for which clouds get charged.

Answer the following questions:

1. A body has 80 microcoulomb of charge. Number of additional electrons in it will be
(A) 8 × 10–5 (B) 80 × 10–17
(C) 5 × 10-14 (D) 1.28 × 10–37

2. Two charge placed in air repel each other by a force of 10–4 N. When oil is introduced between the charges, the force becomes 2.5 × 10–5. The dielectric constant of oil is
(A) 2.5 (B) 0.25
(C) 2.0 (D) 4.0

3. Two charges of 2 microcoulomb are 0.5 m apart. If both of them exist inside vacuum, then force between them is
(A) 1.89 N (B) 2.44 N
(C) 0.144 N (D) 3.144 N

4. Two electrons are separated by a distance of 1Å. What is the coulomb force between them
(A) 2.3 × 10–8C (B) 4.6 ×  10–8C
(C) 1.5 ×  10–8C (D) None of these

5. The point at which the earthquake originates is known as
(A) Mentles (B) Seismic focus
(C) Surface wave (D) Body wave

6. Lightning is a process of
(A) Producing positive charges (B) Detecting negative charges
(C) Electric discharge (D) None of the above

 



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