class 8 science Some Natural Phenomena

Some Natural Phenomena

AMEND EDUCATION ACADEMY

Some Natural Phenomena

Electrically Neutral State of Matter:

Most of the matters are present in electrically neutral state in nature. To understand this, let us recall the basics of atomic structure. An atom is made up of electrons, protons and neutrons. An electron carries a negative charge, a proton carries a positive charge and there is no charge on a proton. In an electrically neutral atom, the number of electrons is equal to the number of protons. Thus, the equal numbers of negative and positive charges balance each other. Most of the matters remain in electrically neutral state because of this reason.

Transfer of Charge: When certain objects are rubbed against another object, electrons may get transferred from one object to another. If an object loses some electron, it is left with surplus number of protons. Thus, this object becomes positively charged. If an object gains electrons, it now has surplus number of electrons. Thus, this object becomes negatively charged. The transfer of charges is responsible for static electricity in various objects. Static electricity is the main reason of lightning.

Interaction of Charges: When a glass rod is rubbed against a silk cloth; both of them get charged. As per convention; the glass rod is supposed to be positively charged and the silk cloth is supposed to be negatively charged once they are rubbed against each other. Some important observations about interaction of charges are as follows: A charged object attracts an uncharged object: Take a ball pen refill and rub it against a woolen cloth. Bring this refill near bits of paper. It is observed that the refill attracts the bits of paper. This activity shows that a charged object attracts uncharged object.

Take a comb and rub it vigorously in your hairs. Your hairs should be completely dry during this activity. Bring this comb near bits of paper. It is observed that the refill attracts the bits of paper. This activity shows that a charged object attracts uncharged object. Like charges repel each other: Take two plastic balls and suspend them near each other. Rub a glass rod to induce charge in it. Touch both the balls with this glass rod. It is observed that the plastic balls move away from each other. This activity shows that like charges repel each other.

     Unlike charges attract each other: Take two plastic balls and suspend them near each other. Rub a glass rod with a silk cloth. Touch one of the balls with the glass rod and touch another ball with the silk cloth. It is observed that both the balls move towards each other. This activity shows that unlike charges attract each other.

Electroscope: Electroscope is a simple device which is used to test the presence of charge in an object. The gold-leaf electroscope was developed in 1787 by a British scientist named Abraham Bennet. Gold and silver are among the best conductors of electric current and hence leaves of these metals are used in electroscope.

Structure of Electroscope: It is made of a glass jar. A vertical brass rod is inserted into the jar through a cork. The top of the brass rod has a horizontal brass rod or a brass disc. Two gold leaves are suspended from the brass rod; inside the jar.

Working of Electroscope: When the brass disc of the electroscope is touched with a charged object, electric charge gets transferred to the gold leaf through the rod. This results in the gold leaves moving away from each other. This happens because both the leaves have similar charges.

Charging: Transfer of charge from one object to another is called charging. In case of the gold leaves; charge is transferred through the brass rods. Hence, it can be said that charging can happen through metals.

Electrical Discharge:

In the above activity, the gold leaves resume their normal position after some time. This happens because they lose their charge. This process is called electrical discharge. The gold leaves would also be discharged when someone touches the brass rod with bare hands. In that case, the charge is transferred to the earth through the human body.

Earthing:

The process of transfer of charge to the earth is called earthing.

LIGHTNING

Lightning is observed as a bright streak of light during a thunderstorm. Lightning is always accompanied by the sound of thunder. The transfer of charge from clouds to the earth or from cloud to cloud is called lightning. In simple words; lightning is an electric spark which happens on a grand scale in the sky.

Mechanism of Lightning

During a thunderstorm, the air currents move upward and water droplets move downward. These movements take at very fast pace and it results in separation of charges in the clouds.

The positive charges accumulate at the upper edges of the clouds and the negative charges accumulate at the lower edges of the clouds. Scientists are yet to understand the exact reason for this.

At the same time, the ground becomes positively charged.

Air is a bad conductor of electricity under normal circumstances. But when the amount of charge becomes too much in the clouds, the air is not in a position to hold on to this charge. As a result, the electric charge is transferred to the ground. This appears like a bright streak of light across the sky; which lasts for a few seconds.

Dangers of Lightning:

Lightning can damage houses and trees. It can also kill people and cattle. Sometimes, lightning can main a person for life.

Safety During Lightning:

• No open place is safe during lightning and hence a home or any other building is the safest place during lightning.
• The first sound of thunder is like a warning call for lightning. Hence, one should rush to a building soon after hearing a sound of thunder. One should come out of home only a few minutes after the last sound of thunder.
• You should not carry an umbrella (with a metallic handle) during thunderstorm. Metallic handle can be a potential target for lightning.
• Tall trees and other tall structures are more prone to be hit by lightning. If you are caught in the open, try to hide under a short tree.
• If you are in open and have no place to hide, then crouch on the ground and keep your head between your knees and hands.
• Don’t take shower during lightning.
• Use of phone should be avoided during lightning.
• Television should be disconnected from the antenna.
• Avoid operating the electric switches during lightning.

Lightning Conductor:

This is a simple device which protects a building from being hit by lightning. It is composed of a vertical metallic rod which usually has a trident at the top. The base of the metallic rod is attached to thick metallic wire; which goes very deep inside the earth. This metallic wire provides a passage for earthing.

When lightning strikes, electric charge goes to the lightning conductor and is then goes to earth. Thus, a lightning conductor helps in preventing any damage to the building.

EARTHQUAKE Sudden shaking of the earth; which lasts for a very short duration is called earthquake. Tectonic Plates: The crust of the earth is composed of several pieces of landmass. These are called tectonic plates. The tectonic plates are in continuous motion. They keep on brushing against each other or striking each other. When a tectonic plate brushes against another or strikes another tectonic plate; it results in vibrations in the tectonic plates. These vibrations are experienced by us in the form of an earthquake.

Seismic or Fault Zones: The boundaries of the tectonic plates are at high risk of earthquake. These boundaries are called seismic zones or fault zones. In India; Kashmir, western and central Himalayas, the whole of the north-east, Runn of Kutch, Rajasthan and Indo-Gangetic Plain are at high risk of getting hit by an earthquake. Some parts of the Deccan Plateau are under the seismic zone.

Seismograph: Seismograph is a device which records the seismic activities. It is composed of an oscillator, writing device and a roll of paper. The writing device is attached to the oscillator. In case of an earthquake, the oscillator begins to oscillate. This creates oscillation in the writing device; which plots wave-like patterns on the paper. The wave-like pattern is then analysed by seismologist to interpret the various features of an earthquake.

Richter Scale: The Richter Scale was developed in 1935 by Charles Richter and Beno Gutenberg of California Institute of Technology. This is a logarithmic scale which shows the intensity of an earthquake. The intensity of an earthquake is measured on a scale of zero to 10. But earthquakes of more than 9 Richter have never been reported. The magnitude of earthquake increases by 100 times when we move one point on the Richter scale. This means that an earthquake measuring 5 on Richter scale is 100 times more powerful than one measuring 4 on Richter scale.

Most of the earthquakes go unnoticed by us because they are below 4 on Richter Scale. Earthquakes which measure more than 7.5 on the Richter scale can be highly damaging.

Damages Due to Earthquake: An earthquake cannot cause any damage on its own. Most of the damages caused by an earthquake are because of collapse of man-made structures; like buildings, bridges, poles, etc. Sometimes, an earthquake can result in tsunami which causes large scale damage. The infamous tsunami in the Indian Ocean which came on 26th December 2004 was because of a massive earthquake near Java.

Protection Against Earthquake:

It is impossible to predict an earthquake and hence we need to take several precautionary measures to avoid or minimize the damage which may happen because of earthquake. Some of the preventive measures are as follows:

• Buildings should be made quake proof. Engineers and architects are the experts who can give suitable advice in this regard.
• Light-weight materials should be used for making the buildings in earthquake prone areas.
• Cupboards should be fixed to the walls so that they do not fall on someone during an earthquake.
• People should be made to practice drills for earthquake. Such drills should be frequently organized in schools and at public places.
• In case of an earthquake, you should hide under a table or any other similar structure. If you are in bed, then keep a pillow over your head and do not move out of the bed. If you are in an open area, try to move away from buildings and other structures.
• Proper installation of fire-fighting equipments should be ensured in buildings. Fire due to short circuit and gas leakage is quite common during an earthquake.

Question – 4 - Sometime, a crackling sound is heard while taking off sweater during winters. Explain.

Answer: When a sweater is taken off, it gets charged because of rubbing. Due to this, we get to hear a crackling sound.

Question – 5 - Explain why a charged body loses its charge if we touch it with our hand.

Answer: When a charged body is touched by hand, the human body provides a passage for the charge to the earth. Due to this, the charged body loses its charge.

Question – 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?

Answer: The destructive energy of an earthquake is measured on Richter scale. An earthquake which measures 3 on this scale would be recorded by a seismograph. An earthquake of this intensity is not likely to cause any damage.

Question – 7 - Suggest three measures to protect ourselves from lightning.

Answer: Three measures to protect from lightning are as follows:

Try to remain indoors.

Do not carry an umbrella during lightning.

If in open; do not take shelter under a tall tree or any tall structure.

Question – 8 - Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Answer: When both the balloons carry similar charges, they are repelled by each other. But a charged balloon attracts an uncharged balloon because a charge object has a tendency to attract an uncharged object.

Question – 9 - Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer: Structure of Electroscope: It is made of a glass jar. A vertical brass rod is inserted into the jar through a cork. The top of the brass rod has a horizontal brass rod or a brass disc. Two gold leaves are suspended from the brass rod; inside the jar.

Question – 10 - List three states in India where earthquakes are more likely to strike.

Answer: Jammu & Kashmir, Himachal Pradesh and Uttarakhand

Question – 11 - Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answer: If we are out in the open and an earthquake strikes, we can take following precautions:

Try to move away from buildings and other structures.

If you are in a vehicle, ask the driver to take the vehicle far from buildings.

Keep away from bridges.

Question – 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.

Answer: If a thunderstorm is being predicted then we should not carry an umbrella. The metallic handle of an umbrella can be a potential target for lightning and it is unsafe to carry an umbrella.