WBBSE Notes For Class 8 School Science Chapter 5 Analysis Of Natural Phenomena

Chapter 5 Analysis of Natural Phenomena


Natural Phenomenon

A natural phenomenon is an event which occurs physically without human involvement. Like a sunrise, storms, earthquakes, and volcanic eruptions; lightning is also a natural phenomenon.

Electric charge

We are all accustomed to lightning during thunderstorms. We all have seen the spark of light with thunderous sound during northwestern. Let us know in some detail the reasons behind this phenomenon. The spark is actually related to electrical phenomena occurring naturally.

Occurrence of spark in daily life

Earlier, we got a preliminary idea about static electricity. In the winter season when we take off our woollen sweaters or shawl in dark, we have very often seen the spark of light.

This is due to static electricity where opposite charges are formed on the two different surfaces due to rubbing (or friction).

If two pieces of conducting wire are connected to the two terminals of a battery (one on each terminal) and then if the open ends of two electrical wires are quickly touched and then separated, we see that sparks are formed.

Similarly, sparks come out when two live wires are touched together. We have noticed that when an electric train moves, sometimes the pantograph gets disconnected from the overhead electrical lines. The electric sparks occur. This becomes clearly visible at night.

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Reason for generation of electric charge

In 1752, Benjamin Frankl proved that the sparks coming from woollen clothes during the winter season (which is due to the generation of static charges) and lightning is basically the same phenomenon.

Most of the matters are electrically neutral. That means their atoms have the same number of electrons as protons. If an atom of a material obtains extra electrons at its outer shell, due to rubbing with other materials or so, then the material will be a negatively (-) charged body.

Likewise, if an atom of a material loses electrons, it will have an excess of positive (+) charges and will be a positively charged body.

The electric field created due to excess of charges causes static electric effects of attraction or repulsion or a spark as the case may be.

In the atmosphere also, if an atom releases electron(s), it becomes positively charged and the electron combines with another atom or group of atoms or water droplets or dust particles floating in the air and they become negatively charged. These kinds of charged particles are present in the atmosphere in great numbers.

The passage of electric current in the air due to the movement of electric charges is called electric discharge. During electric discharge, the positive and negative electric charges cancel out each other and an electric spark and a crackling sound are produced.

Electrostatic induction

In the winter season, when a plastic comb is rubbed against dry hair, it becomes negatively charged. If this negatively charged comb is brought near some small pieces of paper, the pieces of paper are attracted towards the comb. This occurs due to a phenomenon known as electric induction.

But after rubbing with dry hair, if the plastic comb (which acquires negative charges) is kept on a table for some time, then it loses its power of attracting small pieces of paper.

This occurs because the plastic comb is discharged. Discharging occurs because the charges have left the comb or because the charges are neutralized by opposite charges.

Electric current and potential difference

Electric current or charge flows through a conductor when there is a potential difference between two ends of the conductor.

Electricity in the atmosphere

A potential difference of approximately 4,00,000 volts exists between the surface of the earth and the sky (troposphere). Since air is not a perfect insulator, a very small amount of current (about 10 microamperes per square metre) flows through it.

The presence of charged particles in the air makes it a conductor of electricity. Small charged particles are created in the upper atmosphere directly by cosmic rays coming from outside the earth.

Besides small charged particles other charged particles such as tiny pieces of charged dust particles, charged water droplets etc. also remain floating in the air.

Smaller ions (charged atoms)are most mobile while the larger and heavier charged particles move through the air slowly. The conductivity of air is due to the movement of ions.

The magnitude of electric current in the air is very very small. But if we consider the total surface area of the earth, the total electric current reaching the earth’s surface at any time is approximately 1800 ampere.

The potential difference between the earth and the sky is approximately 4 lakh volts. This means that with a current of 1800 ampere, it is equal to the power of 700 megawatts.

With such a large current coming down from the sky, the potential difference between the earth and the sky would have reduced gradually just within half an hour.

The entire negative charge on the earth would have been completely discharged. But this does not happen actually. The potential difference between the earth and the atmosphere is maintained at a nearly constant value.

So, there must be some mechanism due to which this potential difference is maintained. It is the lightning which inducts the earth with negative charges and the air with positive charges and thereby maintains the potential difference between the earth and the atmosphere.

The thunderstorm and its lightning act like a “battery” which keeps charging the earth’s surface with a negative charge.
Generally, lightning hardly occurs in a particular place.

But if we consider the entire earth, there are about 40,000 thunderstorms per day all over the earth and these thunderstorms occurring throughout the world are charging the earth with an average of 1800 amperes.

Thunderstorm and lightning

A thunderstorm is a type of storm characterized by the presence of lightning and thunder accompanied by strong winds and heavy rain.

Thunderstorms result from the rapid upward movement of warm moist air. As the warm, moist air moves upward (called updraft of air), it cools, condenses, and forms cumulonimbus clouds that can gain heights of 12 km or more.

The upward movement of air in a storm cloud has been measured as more than 80 kilometres per hour. ‘ As the moist air reaches its dew point at the upper atmosphere, it converts to water droplets and tiny ice particles.

These particles begin to fall a long way through clouds and collide with other particles and thus become larger.

Downdrafts of air are created by the falling water droplets because they don’t just drag other droplets down with them as they fall, they drag cooler air with them as well. The combined warm updraft and cool downdraft create a storm cell.


WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena Tunder strom


Lightning is a flow of electric current. It is an -electric discharge in the atmosphere between oppositely charged clouds or between charged clouds and the earth.

To make this electric current, all you need is a cloud having substantial height. A cloud that produces lightning will tend to also have rain falling out of it.

The generation of charges within a cloud is a reason for electrical charges. Lighter, positively charged particles are formed at the top of the cloud. Heavier, negatively charged particles sink to the bottom of the cloud.


WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena Tunder strom Lighting


When the ground is hot, the air above it also becomes hot. This warm air becomes denser and lighter. So it rises up in the atmosphere.

The water vapours in the air, mix with the warm air and make it more lighter. This hot air expands in the upper atmosphere and its temperature falls down considerably.

As a result, water vapour in the air condenses to water droplets and thus forms clouds. During this condensation of water vapour, latent heat is also released into the air (change of gaseous state to liquid state).

So, air mixed with water droplets becomes warmer and lighter than the surrounding air. Hence the upward movement of the warm air with water droplets (cloud) continues and at higher altitudes it becomes supercooled (not frozen at 0°C).

When air continues to rise, the cloud gets bigger and bigger. At the top of the clouds, the temperature is below freezing and the water droplets turn into ice.

Now, the cloud becomes a thundercloud. Lots of small bits of ice bump into each other as they move downward. All these collisions cause a buildup of electrical charge.

Eventually, the whole cloud is filled up with electrical charges. Lighter, positively charged particles are formed at the top of the cloud. Heavier, negatively charged particles sink to the bottom of the cloud.

When the positive and negative charges grow large enough, a giant spark-lightning occurs between the two charges within the cloud.

This is like static electricity sparks you see, but much bigger in magnitude. Most lightning happens inside a cloud or between the cloud and the cloud. This is called sheet lightning.


WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena Tunder strom Lighting


Sometimes lightning happens between the cloud and the ground (or tall objects of the earth). This is called fork lightning.

Usually, storm clouds carry electric charges. If a storm cloud having negative charges at its bottom passes over a tall structure, it induces positive charges on the top of the structure.

When the electric charges on the bottom of the cloud become extremely large, then these tremendous electric charges present on the bottom of the charged cloud suddenly flow to the top of the structure (say, the roof of a building) and we see a flash of lightning coming towards the structure or building.

Thus lightning strikes the earth or its tall structures when electric charges flow between the cloud and the earth through a tall structure, a tree or any other object.

Lightning strikes are more frequent in hilly areas because in such areas clouds are. comparatively closer to the ground than in the plains.

In the plains, lightning usually strikes tall structures, high-rise buildings, tall trees, TV towers, chimneys etc since these tall objects are close to the charged clouds than the ground.

Cause Of Thunder

If the potential difference between the cloud and the earth is very large then the air “breaks down” and a giant “arc discharge” is created. The negative charge is induced into the earth from the cloud like a spark. The lightning strikes the earth.

Consequently, a temporary flow of large amounts of current occurs. The air becomes very hot. The heat also creates a temporary vacuum and as a result, the air vibrates violently producing a thunderous sound.

Each time lightning strikes the earth, a significant amount of charges are brought down and this maintains the potential difference between the earth and the sky.

Safety measures during lightning

When you are in an outdoor environment:

1. Every year a number of people die due to lightning. Hence it is not at all safe to remain outdoors during the thunderstorm.

One must take shelter inside a house and should not remain in the open verandah. The doors and windows of the house should be properly shut and one must not stand or sit near them.

2. If one is travelling by a motor car and if it is not possible to take shelter inside a house, the next best thing would be to remain inside the car with its windows and doors properly shut.

Open vehicles like motorbikes, scooters and tractors are not safe. Going out with an umbrella is not at all recommended during a thunderstorm.

It is not at all safe to remain in an open field or stand beneath a tall tree or stand in an elevated place. If one is forced to remain inside a forest,

it is better to take shelter under short trees. If one is forced to remain in an open field, one should squat low on the ground in position.


WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena Safety measures during lighting


This person should place his hands on his knees with his head between the hands. The position will make the person the shortest object around and there is a minimum chance to be hit by lightning.

When you are inside your house:

During thunderstorms and lightning, it is better to stay away from metal pipes and electrical wires. Land phones should not be used. Household electrical appliances such as television, refrigerator, toasters, microwave oven etc.

Should be switched off and unplugged. The use of running water coming through a pipe should be avoided; instead, a bucket full of water and a mug may be used.

Lightning Conductors

To protect the house and the electrical appliances inside the house from lightning, a lightning conductor is used. This is a metallic rod (usually copper, a conductor of electricity) mounted vertically at the top of a building.

The top of the rod has some small skewers. A wide-based metal plate is buried at least 5 to 6 feet below the ground. A thick copper strip (or wire made up of any good conductor material) is connected between the rod at the top of the building and the metal plate buried deep inside the ground.

If the lighting strip is, the current easily passes through the strip or wire to the ground. Consequently, no substantial damage occurs to the building as the electrical energy associated with the lightning does not pass through the building.

If a building (or any tall structure) is not protected by the lightning conductor, then the tremendous electrical energy produced by lightning would pass through the walls of the building causing damage to the materials of the wall and thus making the wall unsafe. It can even set the building on fire.

A lightning conductor protects a building against lightning by providing its electric energy with an easy path to be discharged to the ground.


WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena lighting conductors


An epidemic is the rapid spread of infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less.

Epidemics of infectious disease are generally caused when host immunity to either an established pathogen (disease-producing agent, like virus, bacterium or another microorganism) or a newly emerging novel pathogen is suddenly reduced and the transmission limit is exceeded.

An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic.

WBBSE Solutions For class 8 Chapter 4 Analysis of natural phenomena epidemic

  1. Some features of the pandemic are-
  2. Affects a wider geographical area, often global
  3. Infects a very large number of people
  4. Often caused by a new virus or a new strain of virus that has been dormant for many years
  5. Spreads quickly in humans due to a lack of immunity
  6. Can cause a high number of deaths
  7. This is associated with social disruption, unrest and economical loss

The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline.

A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not.

Transmission of epidemic diseases

1. Airborne transmission: Airborne transmission is the spread of infection by droplet nuclei or dust in the air. Without the intervention of winds or drafts, the distance over which airborne infection takes place is short, say 10 to 20 feet.

2. Arthropod transmission: Arthropod transmission takes place by an insect, either mechanically through contaminated body parts or biologically when there is growth or replication of an organism in the arthropod.

3. Biological transmission: Involving a biological process, e.g. passing a stage of development of the infecting agent in an intermediate host. Opposite to mechanical transmission.

4. Contact transmission: The disease agent is transferred directly by biting, sucking, chewing or indirectly by inhalation of droplets, drinking contaminated water, and travelling in contaminated vehicles.

5. Faecal-oral transmission: The infectious agent is shed by the infected host in faeces and acquired by the susceptible host through the ingestion of contaminated material.

Characteristics Of Epidemic Diseases

  1. All epidemics have the following common characteristics:
  2. An unexpected number of cases of a particular disease occur at a particular point in time affecting a large segment of the population.
  3. Generally confined to a definite population or geographical area.
  4. Usually have a common source of infection,
  5. Epidemics generally tend to follow a pattern and repeat periodically when the conditions are favourable again.

Types Of Epidemic Diseases

Epidemics generally follow a pattern depending on the geographical and environmental conditions, the distribution and characteristics of the host population, and their cultural behaviour.

If there is no intervention or change in these conditions, those epidemics tend to repeat themselves. Therefore, knowledge about various types of epidemics and the conditions under which they occur can help in managing them.

The various types of epidemics that normally occur are described below:

1. Common source epidemics: A common-source outbreak is one in which a group of persons are all exposed to an infectious agent or a toxin from the same source.

1. Point source epidemic:

If a group of people becomes ilTfor a relatively brief and within the same incubation period (the period between infection and the appearance of the first symptom), then the common-source outbreak is further classified as a point-source outbreak.

The epidemic of leukaemia cases in Hiroshima following the atomic bomb blast and the epidemic of hepatitis-A due to poising from a particular food-each had a point source of exposure.

Continuous Or Multiple Exposure Epidemics

In this type of epidemic, the source of infection is continuous and such epidemics will not cease to exist unless the source is removed.

A well with contaminated water becomes a regular source of infection for the people using it and the epidemic may continue until the water is treated and made safe.

Propagated epidemics: A propagated outbreak results from the transmission of disease-bearing agents from one person to another. Usually, transmission is by direct person-to-person contact, (e.g. Syphilis, the transmission of Hepatitis B or HIV by sharing needles or transmission of Malaria by mosquitoes), fn propagated outbreaks, cases occur in more than one incubation period.

Seasonal epidemics: Certain diseases such as influenza and pneumonia are more common during the winter season where as diarrhoea diseases are more during the summer and rainy seasons. The epidemics, which occur ^particular season, are known as seasonal epidemics.

Cyclical epidemics: Some epidemics tend to occur in cycles, which may repeat over a period of time, which may be days, weeks, months or years. An example of this type of epidemic is measles V which tends to occur in a cycle of 2-3 years.

Some Common Infectious Diseases Induced Epidemics

1. Cholera

Cholera is an acute intestinal infection caused by the ingestion of food or water contaminated with the bacterium Vibrio cholera.

It has a short incubation period and produces an enterotoxin that causes a copious, painless, watery diarrhoea that can quickly lead to severe dehydration and death if treatment is not promptly given.

Vomiting also occurs in most patients. Most persons infected with V. Cholerae do not become ill, although the bacterium is present in their faeces for 7-14 days.

When illness does occur, about 80-90% of episodes are of mild or moderate severity and are difficult to distinguish clinically from other types of acute diarrhoea.

Less than 20% of ill persons develop typical cholera with signs of moderate or severe dehydration.

2. Malaria

Malaria is caused by a protozoan parasite called Plasmodium, which is transmitted via the bites of infected female Anopheles mosquitoes.

In the human body, the parasites multiply in the liver and then infect red blood cells. Symptoms of malaria include fever, headache, and vomiting, and usually appear between 10 and 15 days after the mosquito bite.

  1. If not treated, malaria can quickly become life-threatening by disrupting the blood supply to vital organs.
  2. Key interventions to control malaria include:
  3. prompt and effective treatment with artemisinin-based combination therapies,
  4. use of insecticidal nets and
  5. indoor spraying with insecticide.

3. Dengue

Dengue is transmitted by the bite of a mosquito. A type of mosquito called Aedes Egypt carries the germ of this disease. It is a febrile illness that affects infants, young children and adults with symptoms appearing 3-14 days after the infective bite.

Dengue is not transmitted directly from person to person and symptoms range from mild fever to incapacitating high fever, with severe headache, pain behind the eyes, muscle and joint pain, rash and lowering of platelet count to an alarming level.

It causes the oozing of blood in the skin due to the lowering of WBC. There is no vaccine or any specific medicine to treat dengue. People who have dengue fever should take rest, drink plenty of fluids and reduce the fever using paracetamol or see a doctor. It reduces the immunity power.

4. Plague

Plague is a bacterial disease, caused by Yersinia pestis, which primarily affects wild rodents such as rats. It is spread from one rodent to another by fleas (a type of insect).

Humans bitten by an infected flea usually develop a bubonic form of plague, which is characterized by a bubo, i.e. a swelling of the lymph node draining the flea bite site.

A type of fly called Xenopsylla cheopis carries the bacteria from the body of a rat afflicted with plague. If the bacteria reach the lungs, the patient develops pneumonia (pneumonic plague), which is then transmissible from person to person through infected droplets spread by coughing.

Initial symptoms of bubonic plague appear 7-10 days after infection. In 1987, Waldemar Haffkine invented the vaccine for the plague in Mumbai.

5. Smallpox

Smallpox is a disease caused by the Variola major virus. Some experts say that over the centuries it has killed more people than all other infectious diseases combined. Worldwide immunization stopped the spread of smallpox three decades ago The last case was reported in 1977.

Smallpox spreads very easily from person to person. Symptoms are flu-like. They include high fever, fatigue, headache, backache, and a rash with flat red sores.

There is no treatment. Fluids and medicines for pain or fever can help control symptoms. Most people recover, but some can die. Those who do recover may have severe scars. Edward Jenner, in 1976 used the cow-Pox virus in the human body to develop resistance against smallpox.

6. Kala-azar

Kala-azar is the second largest parasitic killer in the world – only Malaria is more deadly. It is also known as Dumdum fever.
Kala-azar is caused by bites from sandflies – which carry the Leishmania donovani parasite, a type of protozoa, responsible for the disease.

If blood containing Leishmania donovani parasites is drawn from an animal or human, the next person to receive a bite will then become infected.

Initially, Leishmania parasites cause skin sores or ulcers at the site of sand fly bites. If the disease progresses, it attacks the immune system.

Kala-azar presents after two to eight months, with more generalised symptoms including prolonged fever and weakness. Indian scientist Upendranath Brahmachari invented the medicine of Kala-azar.

7. Diarrhoea

Diarrhoea is the passage of loose or liquid stools 3 or more times per day, or more frequently than is normal for the individual.

Diarrhoea means ‘to flow/ It is usually a symptom of gastrointestinal infection, which can be caused by a variety of bacterial, viral and parasitic organisms.

Infection is spread through contaminated food or drinking water, or from person to person as a result of poor hygiene.

Severe diarrhoea leads to fluid loss, and may be life-threatening, particularly in young children and people who are malnourished or have impaired immunity. A quick intake of ORS (Oral Rehydration Solution) is an effective remedy for diarrhoea.

8. Tuberculosis

Tuberculosis, or TB, is an infectious bacterial disease caused by Mycobacterium tuberculosis, which most commonly affects the lungs.

It is transmitted from person to person via droplets from the throat and lungs of people with active respiratory disease.
In healthy people, infection with Mycobacterium tuberculosis often causes no symptoms, since the person’s immune system acts to “wall off” the bacteria.

The symptoms of active TB of the lung are coughing, sometimes with sputum or blood, chest pains, weakness, weight loss, fever and night sweats. It is possible to control the disease by proper treatment through DOTS or Directly Observed Treatment, short course.

9. Hepatitis

Hepatitis is an inflammation of the liver, most commonly caused by a viral infection. There are five main hepatitis viruses, referred to as types A, B, C, D and E.

These five types are of greatest concern because of the burden of illness and death they cause and the potential for outbreaks and epidemic spread.

In particular, types B and C lead to chronic disease in hundreds of millions of people and together, are the most common cause of liver cirrhosis and cancer.

Hepatitis A and E are typically caused by the ingestion of contaminated food or water. Hepatitis B, C and D usually occur as a result of contact with infected body fluids.

Common modes of transmission for these viruses include receipt of contaminated blood or blood products, invasive medical procedures using contaminated equipment and hepatitis B transmission from mother to baby at birth, from family member to child, and also by sexual contact.

Acute infection may occur with limited or no symptoms or may include symptoms such as jaundice (yellowing of the skin and eyes), dark urine, extreme fatigue, nausea, vomiting and abdominal pain.

10. Influenza/Flu

Influenza is a viral infection that affects mainly the nose, throat, bronchi and occasionally, lungs. Infection usually lasts for about a week and is characterized by sudden onset of high fever, aching muscles, headache and severe malaise, non-productive cough, sore throat and rhinitis.

The virus is transmitted easily from person to person via droplets and small particles produced when infected people cough or sneeze.

Influenza tends to spread rapidly in seasonal epidemics. Most infected people recover within one to two weeks without requiring medical treatment.

However, in the very young, the elderly and those with other serious medical conditions, the infection can lead to severe complications of the underlying condition, pneumonia and death.

11. AIDS

The Human Immunodeficiency Virus (HIV) is a retrovirus that infects cells of the immune system, destroying or impairing their function.

As the infection progresses, the immune system becomes weaker, and the person becomes more susceptible to infections. The most advanced stage of HIV infection is Acquired Immuno Deficiency 9/ndrome (AIDS).

It can take 10-15 years for an HIV-infected person to develop AIDS; antiretroviral drugs can slow down the process even further.
HIV is transmitted through unprotected sexual relations, transfusion of contaminated blood, sharing of contaminated needles and between a mother and her infant during pregnancy, childbirth and breastfeeding.

12. COVID-2019

Coronaviruses are a large family of viruses that cause illnesses ranging from the common cold to Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS- CoV).

A novel coronavirus (nCoV) is a new strain that has not been previously identified in humans. Coronaviruses are zoonotic. SARS-CoV was transmitted from civet cats to humans and MERS- CoV from dromedary camels to humans with infected body fluids.

Common modes of transmission for these viruses include receipt of contaminated blood or blood products, invasive medical procedures using contaminated equipment and hepatitis B transmission from mother to baby at birth, from family member to child, and also by sexual contact.

Acute infection may occur with limited or no Common signs of infections including respiratory symptoms, fever, cough, shortness of breath and breathing difficulties.

In severe cases, the infection can cause pneumonia, severe acute respiratory syndromes, kidney failure and death. Standard recommendations to prevent infection spread include regular hand washing, covering mouth and nose when coughing and sneezing, avoiding close contact with people or social gatherings etc.

Epidemic due to a non-infectious disease

Epidemics of non-infectious disease are often caused by exposure to industrial products, intermediates or byproducts, either in the workplace or as a result of the contamination of a wider environment.

The Global Status Report on Non-communicable Diseases 2010 is the first report on the worldwide epidemic of cardiovascular diseases, cancer, diabetes and chronic respiratory diseases, along with their risk factors and determinants.

Non-communicable diseases killed tens of millions of people in 2008, and a large proportion of these deaths occurred before the age of 60, so during the most productive period of life. The magnitude of these diseases continues to rise, especially in low- and middle-income countries.



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