NCERT Textbook Questions
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth
Correct Answer: (b) A copper rod
Reason: Copper is a good conductor of electricity. Any charge developed on it flows into our body and then to the earth (electrolytic earthing), so it doesn't retain charge like insulators.
(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.
Correct Answer: (b) becomes positively charged while the cloth has a negative charge.
(a) Like charges attract each other. (F)
(b) A charged glass rod attracts a charged plastic straw. (T)
(c) Lightning conductor cannot protect a building from lightning. (F)
(d) Earthquakes can be predicted in advance. (F)
The crackling sound is due to the electric discharge that takes place between the sweater and our body or inner clothes. When we wear a sweater, friction between the sweater and our body causes them to get charged. When we take it off, the separation of charges causes tiny sparks accompanied by a crackling sound.
Human body is a good conductor of electricity. When we touch a charged body, the excess static charge flows from the body through our hand and body into the earth. This process of transferring charge to the earth is called earthing. As a result, the body loses its charge.
The scale is called the Richter scale.
Yes, an earthquake of magnitude 3 would be recorded by a seismograph.
No, it is not likely to cause much damage. Earthquakes of magnitude 3 are very weak and are often felt only as a slight tremor.
- Stay indoors: If you hear thunder, rush to a safe place like a house or a building.
- Avoid open vehicles: Do not use open cars, motorbikes, or tractors. Closed cars with windows shut are safe.
- Do not use wired phones: Avoid using landline phones during a thunderstorm. Mobile phones are safer.
- Repulsion: Two charged balloons have the same type of charge (like charges). Since like charges repel each other, they repel.
- Attraction: A charged balloon induces an opposite charge on the surface of the uncharged balloon near it (induction). Since unlike charges attract each other, the uncharged balloon is attracted to the charged one.
The instrument is called an electroscope.
Construction: It consists of a glass jar with a metal rod passing through a cork. Two thin strips of gold or aluminium foil are attached to the lower end of the rod inside the jar. The upper end of the rod has a metal disc.
Working: When a charged body touches the metal disc, the charge travels down to the foil strips. Since both strips receive the same charge, they repel each other and open up. The divergence of leaves indicates the presence of charge.
Three states in India prone to earthquakes are:
- Jammu and Kashmir
- Gujarat
- Assam
- Move to an open space away from buildings, trees, and overhead power lines.
- Drop to the ground.
- 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 tremors stop.
Earthquakes are caused by the movement of tectonic plates deep inside the earth's crust. When these plates brush past one another or collide, they cause disturbances in the earth's crust, which manifest as earthquakes.
Detailed Chapter Discussion: Some Natural Phenomena
1. Lightning
You might have seen sparks on an electric pole when wires become loose. You might also have seen sparks when a plug is loose in its socket. Lightning is also an electric spark, but on a huge scale. In ancient times, people did not understand the cause of these sparks. They were afraid of lightning and thought that the wrath of gods was visiting them. Now we understand that lightning is caused by the accumulation of charges in the clouds.
Historical Fact: In 1752, Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomenon.
2. Charging by Rubbing
When a plastic refill is rubbed with polythene, it acquires a small electric charge. Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. These objects are called charged objects. In the process of charging the refill and the plastic comb, polythene and hair also get charged.
Types of Charges and Their Interaction
- There are two types of charges: positive charge and negative charge.
- Like charges repel each other (e.g., two charged balloons repel).
- Unlike charges attract each other (e.g., a charged balloon attracts a charged refill).
- The electrical charges generated by rubbing are static charges. They do not move themselves.
When rubbed with silk, a glass rod acquires a positive charge. By convention, the charge acquired by a glass rod rubbed with silk is called positive. The other kind of charge is said to be negative.
3. Transfer of Charge
When charges move, they constitute an electric current. An electroscope is a simple device used to detect whether an object is charged or not. It works on the principle that like charges repel each other.
Earthing: When a charged body is touched with the hand, the charge flows to the earth through our body. The body loses its charge. The process of transferring of charge from a charged object to the earth is called earthing. Earthing is provided in buildings to protect us from electrical shocks due to leakage of electrical current.
4. The Story of Lightning
During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges.
- Positive charges accumulate near the upper edges of the clouds.
- Negative charges accumulate near the lower edges of the clouds.
- There is accumulation of positive charges near the ground also.
When the magnitude of the accumulated charges becomes very large, the air (which is normally a poor conductor) is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is called an electric discharge.
5. Lightning Safety
Lightning can destroy life and property. Safety measures are imperative.
Do's and Don'ts
- Find Safe Place: No open place is safe. A house or building is a safe place.
- Vehicles: If travelling by car or bus, stay inside with windows and doors shut.
- Open Fields: If in an open field and no shelter is available, stay away from tall trees. Stay away from poles. Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands.
- Home Safety: Lightning can strike telephone cords, electrical wires and metal pipes. During a thunderstorm contact with these should be avoided. Unplug electrical appliances like computers, TVs. Bathing should be avoided to avoid contact with running water.
Lightning Conductors: It is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides an easy route for the transfer of electric charge to the ground.
6. Earthquakes
An earthquake is a sudden shaking or trembling of the earth which lasts for a very short time. It is caused by a disturbance deep inside the earth's crust.
What Causes an Earthquake?
The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision, they cause disturbance in the earth's crust. It is this disturbance that shows up as an earthquake on the surface of the earth.
Seismic Zones: The boundaries of the plates are the weak zones where earthquakes are more likely to occur. These weak zones are also known as seismic or fault zones. In India, Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo-Gangetic Plane are seismic zones.
7. Measuring Earthquakes
The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale.
The tremors produce waves on the surface of the earth. These waves are called seismic waves. The waves are recorded by an instrument called the seismograph. The instrument is simply a vibrating rod or a pendulum, which starts vibrating when tremors occur. A pen attached to the vibrating system records the seismic waves on a paper moving under it.
8. Protection Against Earthquakes
We know earthquakes cannot be predicted. So, it is important to take precautions.
- Construction: Buildings in seismic zones should be designed to withstand tremors ("Quake Safe" structures). Use of mud or timber is better than heavy construction materials.
- Indoors: Take shelter under a table. Stay away from tall and heavy objects that may fall on you. If you are in bed, do not get up. Protect your head with a pillow.
- Outdoors: Find a clear spot away from buildings, trees and overhead power lines. Drop to the ground.
50 Key Facts: Some Natural Phenomena
A massive electric spark caused by the accumulation of charges in clouds.
Electricity generated by rubbing two objects, which produces stationary charges.
American scientist who proved in 1752 that lightning and static sparks are the same phenomenon.
A resin which, when rubbed with fur, attracts light objects like hair. Greek name is 'elektron'.
An object that has acquired an electric charge by rubbing or induction.
When a polythene refill is rubbed with wool, both acquire opposite charges.
There are two types of electric charges: positive (+) and negative (-).
Like charges repel each other (e.g., positive repels positive).
Unlike charges attract each other (e.g., positive attracts negative).
A glass rod rubbed with silk acquires a positive charge, while the silk becomes negative.
A device used to detect whether an object carries an electric charge.
Uses thin gold (or aluminum) leaves to show divergence when charged.
The process of transferring electric charge from a charged object to the earth.
The flow of electric charge through a gas, liquid, or solid.
Caused by electric discharge between clouds or between a cloud and the earth.
The loud sound that follows a flash of lightning, caused by rapid expansion of air.
A metal rod installed on buildings to protect them from lightning strikes by conducting charge to the ground.
A sudden shaking or trembling of the earth caused by disturbances deep inside the crust.
The outermost layer of the earth where earthquakes originate.
Large fragmented pieces of the earth's crust that are in constant motion.
Weak zones along plate boundaries where earthquakes are more likely to occur (Seismic Zones).
A logarithmic scale used to measure the magnitude (destructive energy) of an earthquake.
An instrument used to record seismic waves generated by earthquakes.
Waves of energy that travel through the earth's layers as a result of an earthquake.
The point on the earth's surface directly above the focus of an earthquake.
The point deep inside the earth where the earthquake originates.
Huge sea waves caused by an underwater earthquake.
Earthquakes of magnitude 7 or higher on the Richter scale are considered highly destructive.
A major earthquake in Bhuj, Gujarat occurred on 26 Jan 2001 (Magnitude > 7.5).
A major earthquake in Uri and Tangdhar towns of North Kashmir occurred on 8 Oct 2005.
During an earthquake, take cover under a sturdy table and stay away from glass or heavy objects.
Find a clear spot away from buildings, trees, and power lines. Drop to the ground.
A lightning bolt can carry an electric current of up to 30,000 Amperes.
Lightning can heat the air around it to temperatures 5 times hotter than the sun's surface.
In clouds, positive charges gather at top edges and negative charges at bottom edges.
Copper is a conductor (allows charge flow); Plastic is an insulator (does not allow charge flow).
Process by which a charged body produces opposite charge on a nearby uncharged body without touching.
India sits on the Indian tectonic plate, which is moving northwards into the Eurasian plate.
Modern buildings in seismic zones use isolation techniques to decouple the building from ground shaking.
Caused by static electric discharge between the wool and body/clothes.
Safe to use during lightning (unlike corded phones), but avoid charging them.
Safest position if caught outdoors in lightning: squat low, hands on knees, head between hands.
Repulsion is the surest test of electrification because attraction can occur between charged and uncharged bodies.
Volcanic eruptions can also cause earthquakes in nearby areas.
Earth earthquakes can rarely be caused by meteor impacts.
Underground nuclear tests create artificial seismic waves detected by seismographs.
Combing dry hair charges the comb, allowing it to pick up small pieces of paper.
Charging by rubbing is scientifically known as the triboelectric effect.
In an electroscope, the extent of divergence of leaves depends on the amount of charge.
In highly seismic areas, lightweight mud or timber houses are safer than concrete ones.