LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

Lightning:

Lightning is an electric Spark which is caused by the accumulation of electric charges in the clouds. 

Electric charges:

Electric charges are the property of matter. When Amber is rubbed with silk cloth, both the Amber and the silk cloth become charged. Similarly when a rubber balloon is rubbed with a woolen cloth The rubber balloon gets electric charges due to friction. 

Uncharged and charged objects:

An object having no electric charge on it is called an uncharged object. An object having electric charge on it is called a charged object. 

If we take a glass rod and bring it near some tiny pieces of paper it will not have any effect on them, but if the glass rod is first rubbed with a piece of silk cloth and then brought near the tiny pieces of paper then the glass rod attracts the tiny pieces of paper towards itself. It means that when the glass rod is rubbed with silk cloth then it gets electric charge. 

The process of giving electric charge to an object is called charging the object. 

Charging an object by rubbing:

The charging of an object by rubbing it with another object is called charging by friction. The electric charges generated by rubbing are static electric charges. These electric charges remain bound on the surface of the charged object. 

Friction charges both the objects which are rubbed together. 

When two objects are rubbed together then both the objects get charged by friction. 

When a glass rod is rubbed with silk cloth, glass rod becomes positively charged and the silk cloth becomes negatively charged. 

Two types of electric charges and their interaction:

There are two types of electric charges positive charges and negative charges. 

A positive charge repels another positive charge but attracts a negative charge. Similarly a negative charge repels another negative charge, but attracts a positive charge. 

Q. When a charged glass rod rubbed with silk cloth is brought near a charged plastic straw rubbed with polythene there is attraction between the two. What is the nature of charge on the plastic straw?

Ans: when a glass rod is rubbed with silk cloth, glass rod becomes positively charged, if it attracts a plastic straw, then plastic straw must be negatively charged. 

Electroscope:

The electric charge on an object can be detected by using an instrument called electroscope. 

The electroscope is a device for detecting electric charge on an object. 

Working of an electroscope:

Charge a glass rod by rubbing its own and with a piece of silk cloth. Touch the charged glass rod with the top and of metal clip. We will see that the two aluminium leaves move away from each other. We say that the aluminium leaves diverge or open up. 

When we touch the top end of metal clip with the positively charged glass rod, then some of its positive charge is transferred to the top and of metal clip. Since the metal clip is a good conductor of electricity, it conducts the positive electric charge to the two aluminium leaves held on its other end. In this way the two aluminium leaves get charged with the same kind of electric charge. And therefore they repel each other. 

Electric discharge: production of sparks

The passage of electric current in air due to movement of electric charges is called electric discharge. During electric discharge the positive and negative electric charges cancel out each other and and electric Spark and the crackling sound are produced. In nature, electric discharge within a cloud during thunderstorm produces use electric sparks known as lightning along with a loud sound called thunder. And electric discharge between a thunder cloud and the earth also produces lightning followed by thunder. 

LIGHTNING:

The bright flash of light which we see in the clouds is called lightning. 

A cloud is a visible mass of condensed water vapour floating in the atmosphere high above the ground. A heavy dark rain cloud is also called storm cloud. When a storm cloud develops in the sky strong winds move upwards through the cloud and make the water drops present in the cloud to rub against one another. This rubbing together of water drop produces extremely large electric charges in the cloud due to friction. The small water drops acquire a positive charge and being lighter move to the upper part of the cloud with rising wind. On the other hand the larger water drops acquire a negative charge and being here come down in the lower part of the cloud. In this way the top of the cloud becomes positively charged where is the bottom of the cloud becomes negatively charged. 

When the amount of opposite electric charges on the top and bottom of a storm cloud becomes extremely large then electric charges start flowing with high speed through the air between them. When the positive and negative charges of a cloud meet they produce and intense Spark of electricity between the cloud in the sky. This electric Spark as a flash of lightning in the sky. The electric sparks of lightning heat the nearby air in the sky to very high temperatures due to this heat the air in the sky expands repeatedly and produces a loud sound which we call thunder. 

Lightning is usually occurs within a cloud in the sky. It is called sheet lightning. Lightning also occurs between a cloud and the earth or tall all objects of the earth. It is then called fork lightning.

Storm clouds carry electric charges. Now if a storm cloud having negative charges at its bottom passes over a tall building it induces positive charges on the roof of the building. When the electric charges on the bottom of the cloud become extremely large, then this tremendous electric charges present on the bottom of the charged cloud suddenly starts to flow to the roof of the building and we see a flash of lightning coming towards the building.

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

When we take off woolen or synthetic clothes like polyester or nylon class sometimes we hear a crackling sound and if it is a dark or as during night we can even see tiny sparks. 

Dangers of lightning:

When lightning strikes the earth it can cause a lot of destruction by damaging property building, trees and killing people. The damage caused to buildings and other structures by lightning can be prevented by installing lightning conductors on them. 

Lightning conductor:

Lightning conductor is a device used to protect a building from the effects of lightning. 

The tall buildings and other tall structures are protected from lightning strikes by using a device calling lightning conductor. A lightning conductor is made of a thick strip of metal made of copper. The top end of lightning conductor is pointed like a sharp spike and it is fixed above the highest point of the building.

 From the top of a building the thick metal strip runs along the outer wall of the building to the ground. The lower end of metal strip is joined to a metal plate and buried deep in the ground near the base of the building. 

If lightning strikes it will hit the top of the lightning conductor rather than the building. The electric energy of lightning passes through the metal strip and gets discharged safely into the ground through the buried metal plate. Since no electric energy produced by lightning passes through the building no damage is caused to it.

Measures to protect ourselves from lightning:

We can take the following measures to protect ourselves from lightning strikes during a thunderstorm. 

1. No open space is safe during lightning and thunderstorms. A house is a safe place during a lightning. 

2. Open vehicles like motorbikes, scooters, tractors and construction machinery are not safe during lightning and thunderstorm. 

3. When in open space, a person should never stand under a tree to take shelter during a thunderstorm because there is danger of lightning striking the tree and burning it up. 

4. When in open space, a person should not lie on the ground during thunderstorm and lightning. A person should squat low on the ground during lightning. 

5. We should avoid raising an umbrella over our head during lightning. 

6. The TV antennas and dish antennas fixed on the tall buildings are especially prone to lightning strikes. We should, therefore, switch off our TV sets during frequent lightning otherwise TV sets may get burnt. 

7. Lightning can strike metal pipes fixed in buildings. So, during a thunderstorm when lightning is taking place, we should avoid touching the metal pipes fixed in a house or building.

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 14 : CHEMICAL EFFECTS OF ELECTRIC CURRENT

The case of distilled water, tap water, sea water and rainwater

Distilled water, tap water, sea water and rain water

1. Distilled water is non conductors of electricity

2. When some salts are dissolved in distilled water, it becomes a good conductor of electricity

Q. How can we make distilled water a conductor of electricity?

Ans. We can make distilled water a conductor by adding

(i) salts (ii) acids (iii) bases

Tap water:

As tap water contains small amount of various salts dissolved in it, it is a conductor of electricity. 

We should never operate an electric appliances like heater, iron, mixer, grinder etc with wet hands nor we should touch electric switches. 

In case of a fire, before fireman use big water hoses to throw water on a burning houses, the electric supply to that home is cut off first. 

Drinking water:

Drinking water contains some amount of salts in it hence it is also a conductor of electricity.

Sea Water: Sea water is salty, it means a large amount of salts are dissolved in water, so they are very good conductor of electricity. 

Rain water:

Due to the presence of small amount of acids in rain water, it is also a conductor of electricity. 

Which one is better conductor drinking water or sea water and why?

Sea water contains more salts than that of drinking water, so sea water is a better conductor. 

Q. Is it safe to carry out electric repairs during a rainy day?

Ans: As rain water is a good conductors of electricity, it is not safe for an electrician to carry out electric repair during a rainy day 

Chemical effects of electric current

When electric current is passed through acidified water by using carbon electrodes, then there is a chemical reaction takes place and Hydrogen and Oxygen will be released as water will be decomposed into hydrogen and oxygen. 

ELECTROLYSIS

The chemical decomposition produced by passing electric current through a conducting liquid is called electrolysis. For example, When electric current is passed through acidified water by using carbon electrodes, then there is a chemical reaction takes place and Hydrogen and Oxygen will be released as water will be decomposed into hydrogen and oxygen. 

Describe an activity to demonstrate the chemical effect of current. 

Take two carbon rods and cleaning their metal caps they are fitted with wires and connected to a battery with a switch on the positive side. The carbon rods will act as electrodes, one is cathode (negative electrode) and anode (positive electrode). Take 250 ml of acidified water in the beaker and emerge the carbon rods into them. When we close the switch, electric current will pass through the acidified water and bubbles of gases will be produced at the two electrodes. The formation of gas bubbles at the two electrodes shows that a chemical change or chemical reaction takes place. This is a description of an activity which shows the chemical effects of electric current. 

"If electric current is passed through acidified water, then bubbles of oxygen gas and hydrogen gas are produced at the two electrodes."

1. Oxygen will be produced at positive electrode (anode)

2. Hydrogen will be produced at negative electrode (cathode)

The fresh fruits and vegetables conduct electricity to some extent due to the presence of various salt solution in them. 

Describe an activity to demonstrate the change in colour caused by the chemical effect of electric current.

We cut a potato into two halves. Taking one piece of cut potato and inserting to iron nails into it a little distance apart from one another. The iron nails are the two electrodes in this case. We connect the two terminals of a battery to the two iron nails by including a compass and a switch in the circuit as shown in the figure. When we pass the electric current through cut potato piece by closing the switch, we observe a deflection in the compass needle which indicates that potato is conducting electricity to some extent. Latest continue to pass electric current through water two piece for about half an hour, we shall notice a greenish blue spot on the cut surface of potato around the iron nail which is connected to the positive terminal of the battery. There is however no coloured spot around the other name which is connected to the negative terminal of the battery. The formation of a greenish blue spot around the positive electrode inserted in the surface of a cut potato shows that the chemical effect of current can bring about change in the colour of a conducting solution.

Applications of the chemical effect of electric current

1. Electroplating metals

2. Purification of Metals

3. Production of certain metals from their ores

4. Production of chemical compounds

5. Decomposing chemical compounds

Electroplating of metals

The process of depositing a thin a layer of a desired metal over a metal object with the help of electric current is called Electroplating.

The metal objects like taps, utencils, jewelry and other objects are electroplated with chromium, tin, nickel, silver, gold or copper. 

Activity for electroplating:

How does copper can be electroplated on the surface of an object made of iron?

We shall take a beaker and pour copper sulphate solution in it. Now we take a copper plate and connected to the positive terminal of the battery with a switch. Again we take iron object like an iron key and connect it to the negative terminal of the battery. Therefore, copper plate and iron key will act as electrodes. Now when we close the switch , electric current will flow through the copper solution and copper will be diposited on the surface of the iron key. Thus the iron key will be electroplated with copper. During the copper plating of an iron key, coppee metal transferred from copper plate to the iron key through the copper sulphate solution. This process is also known as copper plating.

The following points should be remembered while electroplating:

1) The metal object on which electroplating is to be done is made the negative electrode or cathode. Therefore it must be connected to the negative terminal of the battery.

2) The metal whose layer is to be deposited is made the positive electrode or anode. It is connected to the positive terminal of the battery. 

3) A water soluble salt of the metal to be deposited is taken as the electrolyte the electrolyte contains the metal to be deposited in the form of a soluble salt. For example, copper plating, we need copper sulphate solution. 

Uses of electroplating:

1. Electroplating is used to cover iron and steel objects with a thin layer of chromium metal. This chromium layer gives an attractive shiny surface and also protects iron and steel objects from rusting. For example, bicycle handlebars, bicycle bells wheel rims, bathroom fittings LPG stoves motorcycle parts are often electroplated with chromium. 

2. The less reactive and shiny metals like chromium, tin and nickel are electroplated on more reactive and dull looking metals like iron and Steel to protect them from corrosion and give them an attractive finish. 

3. Electroplating is used to give objects made of metal a coating of more expensive metal to make them look more attractive. For example, less expensive metals are electroplated with more expensive metals like silver and gold to make jewellery.

Purification of metals:

The chemical effect of electric current is also used in the purification of impure metals.

Impure metals are collected from their naturally occurring compounds called ores. In the purification of an impure metal by using the chemical effect of electric current or electrolysis, 

a. Thick rod of a metal is made positive electrode or anode and it is connected to the positive terminal of the battery.

b. A thin strip of pure metal is made negative electrode or cathode and it is connected to the negative terminal of the battery.

c. A water soluble salt of the metal to be purified is taken as electrolyte.

Take 250 ml of distilled water in a clean beaker. Dissolve two teaspoonfuls of copper sulphate in it. Add a few drops of dilute sulphuric acid to copper sulphate solution. A thick rod of a copper metal is made positive electrode by connecting it to the positive terminal of the battery. 18 plate of pure copper metal is made negative electrode or cathode by connecting it to the negative terminal of the battery. Switch on the electric current by closing the switch. Allow the current to pass for about half an hour. It will be observed that the A copper rod goes on becoming thinner and thinner where is the pure copper plate goes on becoming thicker and thicker. This is because the A copper metal of Android goes on dissolving in copper sulphate solution where is the pure metal from copper sulphate solution goes on depositing on copper plate cathode. Impurities present in a rod of copper fall to the bottom of the beaker. 

Production of metals:

The chemical effect of electric current is used in the production or extraction of certain metals from their naturally occurring compounds called ores. 

Production of compounds:

The chemical effect of electric current or electrolysis is used in the production of various chemical compounds like sodium hydroxide is produced by the electrolysis of an aqua solution of sodium chloride. 

Decomposition of compounds:

The chemical effect of electric current or electrolysis is used to decompose various chemical compounds into their elements.

LECTURE - 1 : CLASS IX : SCIENCE : CHAPTER 3 : GRAVITATION

CLASS IX   |    SCIENCE    |    CHAPTER 3

      notes prepared by subhankar Karmakar

                                                                         

• GRAVITATION

We know that a force is needed to produced motion in a body.

A stone dropped from a height falls towards the earth because the earth exerts a force of attraction cal gravity on the stone and pulls it down.

The earth attracts all pools all the objects towards its centre the force with which the earth pulls the objects towards it is called the gravitational force of earth or gravity.

The gravitational force of earth or gravity of earth is responsible for holding the atmosphere above the Earth for the rain falling to the earth and for the flow of water in the rivers it is also the gravitational force of earth or gravity of earth which keeps us firmly on the ground.

According to Newton every object in this universe attracts every other object with a certain force the force with which two objects at each other is called gravitational force or gravity. 

If the masses of the objects are small then the gravitational force between them is very small and which cannot be detected easily.

If one of the objects is very big having a very large mass then the gravitational force becomes and its effect can be seen easily. 

• UNIVERSAL LAW OF GRAVITATION

Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. 

According to the universal law of gravitation,

1. The force between two bodies is directly proportional to the product of their masses. 

              F ∝ m_₁ m_₂

2. The force between two bodies is inversely proportional to the square of the distance between them. 

             F ∝ 1/r²

Therefore, F ∝ m_₁ m_₂/r²

and we can write F = Gm_₁ m_₂/r²

Where G is a constant known as universal gravitational constant. 

"The value of gravitational constant G does not depend on the medium between the two bodies. It also does not depend on the masses of the bodies are the distance between them."

If the distance between two bodies the gravitational force becomes on fourth and if we half the distance between two bodies then the gravitational force becomes 4 times. 

Newton's law of gravitation is called universal law of gravitation because it is applicable to all the bodies having mass whether the bodies are big or small head of the body is a terrestrial or celestial. 

The gravitational constant G is numerically equal to the force of gravitation which exists between two bodies of unit masses kept at a distance from each other. The value of universal gravitational constant G has been found to be 6.67 x 10⁻¹¹ Nm²/kg².

All the various objects on this earth attract one another constantly they do not cause any motion because the gravitational force of attraction between them is very small, but when both the objects or bodies are very big, having very large masses, then the gravitational force of attraction between them becomes extremely large. 

For example, two 1 kg masses, separated by 1 metre experiences a force of 6.67 x 10⁻¹¹ N, which is extremely small, but the gravitational attraction between earth and moon is equal to 2.01 x 10²⁰ N, which is very very large. 

It is gravitational force between the sun and the earth which keeps the earth in uniform circular motion around the sun. 

The tides in the sea formed by the rising and falling of water level in the sea, are due to the gravitational force of attraction which the sun and the moon exert on the water surface in the sea. 

QUESTIONS:

1. State the universal law of gravitation. Name the scientist who gave this law.

2. Define gravitational constant. What are the units of gravitational constant?

3. What is the value of gravitational constant G

(i) on the earth, (ii) on the moon?

4. Which force is responsible for the moon revolving around the earth?

5. The force between two masses of equal magnitude "m" kept at a distance "d" is F, if the masses are doubled, and the distance between them is halved, what will be the new force?

6. The mass of the earth is 6 x 10²⁴ Kg and that of the moon is 7.4 x 10²² kg. If the distance between the earth and the moon be 3.84 x 10⁵ km, calculate the force exerted by the earth on the moon. G = 6.67 x 10⁻¹¹ Nm²/kg²

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 14 : CHEMICAL EFFECTS OF ELECTRIC CURRENT

CLASS VIII   |    SCIENCE    |    CHAPTER 14

      notes prepared by subhankar Karmakar

                                                                         

Chemical effects of electric current

The materials which allow electric current to pass through them easily are called good conductors of electricity. 

The materials which do not allow electric current to pass through them easily are called poor conductors of electricity or non conductors of electricity.

The liquids that conduct electricity solutions of acids bases and salts in water. There are some important differences in the conduction of electricity by solids such as metals and liquids such as solutions of acids bases and salts. 

1. In solids like metals, electricity is carried by electrons but in liquids electricity is carried by ions positively charged ions and negatively charged ions. 

For example, in a solid like copper metal, electricity is carried by electrons but in a liquid like copper sulphate solution, electricity is carried by copper ions and sulphate ions. 

2. When electricity is passed through a solid then no chemical change takes place but when electricity or electric current is passed through a liquid then a chemical change takes place. 

For example, when electricity is passed through a copper wire, no chemical change takes place in it, but when electricity is passed through acidified water, then a chemical change takes place in which water is decomposed into hydrogen and oxygen gases. 

The liquids which conduct electricity are called conducting liquids. 

The chemical changes which takes place in conducting liquids on passing electric current through them are called chemical effects of electric current. 

ELECTROLYTES:

A liquid or solution of substance which can conduct electricity is called an electrolyte. The solutions of acids, bases and salts in water are called electrolytes. 

Electrolytes are of two types: strong electrolytes and weak electrolytes. 

Strong electrolyte is a liquid or solution which conducts electricity very well. Strong electrolyte is a very good conductor of electricity because it contains a lot of ions in it. Some examples of strong electrolytes are sulphuric acid solution, hydrochloric acid solution, nitric acid solution, sodium hydroxide solution, potassium hydroxide solution, common salt solution etc. 

Weak electrolyte is a liquid or solution which conducts electricity to a lesser extent. A weak electrolyte is a weak conductor of electricity because it contains less number of ions. Examples of weak electrolytes are: vinegar, lemon juice, carbonic acid solution, ordinary water and rainwater. 

ELECTRODES:

"A solid electrical conductor through which an electric current enters or leaves something like to a dry cell or an electrolytic cell is called an electrode."

Electrodes are of two types: Anode and Cathode. 

The positively charged electrode is called anode and the negatively charged electrode is called cathode. 

Metal rods and carbon rods can be used as electrodes. 

ELECTROLYTIC CELL:

"An arrangement having two electrodes kept in a conducting liquid or electrolyte in a vessel is called an electrolytic cell."

For example, if we keep two carbon electrodes in a beaker containing acidified water it will be an electrolytic cell. 

An experiment to test whether a liquid conducts electricity or not:

We take a small beaker and fixed two iron nails on a rubber cork about 1 cm apart and place this cork in the beaker. We connect the two nails to the two terminals of a battery by including a torch bulb and a switch in the circuit. Now we shall consider three cases. 

(1) Good conductors of electricity:

Now we pour a solution of dilute hydrochloric acid in the beaker carefully. Now we pass electric current through the hydrochloric acid solution by closing the switch. As soon as we switch on the current, the bulb starts glowing brightly. The glowing bulb in this case tells us that hydrochloric acid solution conduct electricity and it is a good conductor of electricity. 

(2) Non conductors of electricity:

If we take sugar solution instead of hydrochloric acid solution, the bulb does not glow. It says that, sugar solution does not conduct electricity. It is not an electrolyte. 

(3) Weak conductors of electricity: 

if we take lemon juice or vinegar instead of hydrochloric acid solution, the bulb glows dimly. It indicates that vinegar and lemon juice conduct electricity but they are weak conductors of electricity. 

Detection of weak current flowing through a liquid:

The weak electric current flowing through liquids can be detected in two ways. 

1. By using a LED in the circuit.

LED is a semiconductor device which glows even when a very weak current passes through it. There are two wires or leads attached to an LED. The longer lead should be connected to the positive side of the battery. 

We take a small beaker and fixed two iron nails on a rubber cork about 1 cm apart and place this cork in the beaker. We connect the two nails to the two terminals of a battery by including a LED and a switch in the circuit. 

If we pour vinegar or lemon juice in the beaker and closes the switch, LED will glow. It proves that the vinegar or lemon juice solution can conduct weak electricity through it.

2. By using a compass surrounded by turns of circuit wire. 

We take out the cardboard tray from the inside of the discarded Matchbox. Place small compass inside this cardboard tray. Wrap and electric wire a few times around the cardboard tray so as to make a type of coil of wire around the compass. The Matchbox tray containing the compass inside it and having wound up around it is connected in place of torch bulb in the circuit of the liquid to be tested for conductivity. Even if a weak electric current flows through the liquid in the circuit the magnetic needle of compass will show deflection. If a compass surrounded by wound up electric wire of a circuit including a liquid in it shows deflection, it will mean that the liquid conducts electricity. 

LECTURE - 3 : CLASS VIII : SCIENCE : CHAPTER 13 : SOUND (SOUND WAVE)

CLASS VIII   |    SCIENCE    |    CHAPTER 13

      notes prepared by subhankar Karmakar

                                                                         

Sound Wave

Introduction to waves:

The sound is produced by vibrating objects.

They travel from one place to another in the form of waves. Hence, the name sound waves.

A repeated back and forth motion is called vibrations or oscillations.

Every vibration or oscillation has three characteristics, amplitude (A), time period (T) and frequency (f). 

Amplitude (A): The maximum displacement of a vibrating object from its central position is called the amplitude of vibrations or oscillations. 

Time Period (T): The time taken by A vibrating object to complete one vibration or oscillation is called its time period. Time period is measured in second.

Time period (T) = Total time (t) / total no. of vibrations (N)

T = t/N ------ (i)

Frequency (f): The number of vibrations made in one second is called the frequency of vibration. The frequency is measured in Hertz (Hz). 

Frequency (f) = Total no. of vibrations (N) /total time (t)

f = N/t ------ (ii)

Time period = 1 / frequency

Frequency = 1/Time period

T = 1/f ------ (iii)

f = 1/T ------ (iv)

1. Suppose an object makes 200 vibration in 5 second, find time period and frequency. 

Soln: Total number of vibration, N = 200

          Total time, t = 5 s

Frequency, f = N/t = (200/5) Hz = 40 Hz

Time period, T = t/N = (5/200) = 0.025 s

2. The frequency of a vibration is 50 Hz. Calculate the number of oscillations in 5 second. 

Soln. f = N/t 

          N = ft = 50 x 5 = 250 vibrations

3. The frequency of a vibration is 1 Kilo Hertz (KHz), calculate is time period. 

Soln. f = 1 KHz = 1000 Hz

Time period, T = 1/f = 1/1000 = 0.001 s

Wave and particle motion of waves: 

Mechanical waves are waves that travel through a material medium.

It is of two types: 

Depending on the direction of motion of the particle of the medium and the wave propagation, waves are classified into two categories. 

1. Transverse wave

2. Longitudinal wave

Transverse waves: 

Particle motion is to perpendicular the direction of wave motion.

This type of wave is a mechanical wave called a transverse wave. e.g., Light, or even  Mexican wave in a stadium.

Longitudinal waves: 

When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.

It is also a mechanical wave.

Example: sound wave is longitudinal wave.

Three characteristics of sound:

There are are three characteristics of a sound. They are 1. Loudness, 2. Pitch, 3. Quality.

Loudness: 

Sounds are produced by a vibrating objects. If more energy is supplied to an object by plucking it or hitting it more strongly then the object will vibrate with a greater amplitude and produce a louder sound. 

The Loudness of sound depends on the amplitude of vibrations of the vibrating object. Greater the amplitude of vibration, louder the sound will be. 

The loudness of sound is directly proportional to the square of amplitude of vibrations.

The loudness of sound is expressed in the unit called decibel (dB). 

Sound above 80 dB, is considered harmful.

Pitch: Pitch is that characteristic of sound by which we can distinguish between different sounds of the same loudness. Pitch is also called shrillness. A man's voice is flat, having a low pitch whereas a woman's voice is shrillness, having a high pitch. 

Pitch of a sound depends on the frequency of vibration. It is directly proportional to the frequency of the vibrations. If the frequency of vibration is low, the sound produced has a low pitch, where as if the frequency of vibration is high the sound produced has a high pitch. 

Sound having high frequency or high peach is said to be shrill. The voice of a woman is shriller than that of a man. The voice of small baby e is even more shrill than that of a woman. 

Quality of a sound: 

Quality is that characteristic of sound which enables us to distinguish between the sounds produced by different sound producing objects even if they are of same loudness and pitch. Quality of sound is also known as Timbre.

The quality of sound produced by different musical instruments or different singers is different because they produce sound waves of different shapes. 

Audible and inaudible sounds

All the vibrating bodies or objects do not produce audible sound. An object must vibrate at the rate of at least 20 times per second to be able to produce audible sound. 

Audible range = 20Hz to 20kHz = 20000 Hz known as the Sonic range.

The sound having very low frequencies which cannot be heard by human ear are chord infrasonic sounds.

Below 20 Hz (inaudible) → infrasonic range.

The sounds having to high frequencies which cannot be heard by human ear are called ultrasonic sound.

Above 20 kHz (inaudible) → Ultrasonic range.

Animals and ultrasonic sounds:

The human beings can neither produce ultrasonic sound nor can they hear ultrasonic sound. But many animals can produce ultrasonic sound for different purpose. 

Bats produce ultrasonic sounds during screaming, also they can hear ultrasonic sounds. Bats use ultrasonic sounds to locate something during night. 

Dogs, monkeys, deer and leopards can also hear ultrasonic sounds. 

Uses of ultrasonic sound:

(i) Ultrasound is used as a diagnostic tool in medical science to investigate inside of the human body.

(ii) Ultrasound is used to study the growth of foetus (developing baby) inside the mother's womb. 

(iii) Ultrasound is used in the treatment of muscular pain and a disease called arthritis (which is inflammation of joints).

(iv) Ultrasound is used to measure the depth of sea (or ocean). It is also used to locate under-water objects like shipwrecks, submarines and shoals of fish, etc.

The ultrasound equipment works at sound frequencies higher than 20000 Hz. 

NOISE AND MUSIC

"The unpleasant sounds around us are called noise."

Noise is produced by the irregular vibrations of the sound producing source. Some examples of noise are as follows. Running of mixture and grinder in the kitchen produces noise. Blowing of horns of the motor vehicles causes noise. Bursting of crackers produces noise. Shattering of glass produces noise. 

"The sounds which are pleasant to hear are called musical sounds or music."

NOISE POLLUTION:

"The presence of loud, unwanted and disturbing sounds in our environment is called noise pollution." Some of the major sources of noise pollution are as follows.

1. The motor vehicles running on the road produces noise pollution by blowing horns and sounds of their engines. 

2. The bursting of crackers on various social and religious occasions produces noise pollution.

3. The various machines in factories make loud sounds and cause noise pollution.

4. The takeoff landing and flying of aeroplanes produces noise pollution.

5. The playing of loudspeakers and bands at marriages and other social functions causes of noise pollution. 

6. The construction of buildings produces a lot of noise pollution in the surroundings. 

HARMS OF NOISE POLLUTION

Excessive loud noise is harmful to us. The various harms of noise pollution are as follows:

1. Loud noise can cause great harm to our ears. Loud noise can even damage the ear permanently and cause deafness.

2. Loud noise can cause a person to lose concentration in his work or studies.

3. Loud noise can cause an ailment called hyper-tension. 

4. Noise can cause irritation and headache.

5. Loud noise during night time disturbs our sleep. 

MEASURES TO CONTROL NOISE POLLUTION:

We can control the noise pollution to some extent by taking the following measures.

1. We should not play radio, stereo systems and television too loudly.

2. The horns of motor vehicles should not be blown unnecessarily. 

3. The bursting of crackers should be avoided.

4. The noise making factories and airports should be shifted away from the residential areas of the city. 

5. Loudspeaker should be played at low volume during marriages and other social functions. 

6. Trees should be planted along the roads and around buildings to reduce the noise pollution from the roads and other activities from the reaching the residents of the area. 

HEARING IMPAIRMENT:

The partial hearing loss of a human being is called hearing impairment. A person having partial hearing loss can hear sounds properly by using hearing aid. Hearing aid is a small sound amplifying device worn on the ear by a partially deaf person.

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 13 : SOUND

CLASS VIII   |    SCIENCE    |    CHAPTER 13

      notes prepared by subhankar Karmakar

                                                                         

SOUND 

Sound is a form of energy. Sound is that form of energy which makes us hear. A vibrating object always produces sound. Each sound is special to the object which produces it. 

In some cases, the vibrations of a sound producing object are quite large which we can see with our eyes. But in other cases, the vibrations of the sound producing object are so small that we cannot see them easily, we have to feel the vibrations of such an object by touching it gently with the fingers of our hand. 

Like, sound is produced by a vibrating bicycle Bell. Sound is produced when a stretched rubber band vibrates. Sound is produced when a sitar string vibrates. Sound is produced when our vocal cords vibrate. Sound is produced when the air column enclosed in a tube vibrates. Sound is produced when the membrane of a tabla vibrates. Sound is produced when the stretched membrane of a drum vibrates. Sound of a radio or television or speakers is produced by the vibrations of the the cone of the speakers. 

Sound can be produced by the following methods:

1. By vibrating strings (in guitar, sitar etc)

2. By vibrating air columns (in flutes)

3. By vibrating membranes (in tabla, drums)

4. By vibrating plates (in bicycle bells)

Propagation of sound:

When an object vibrates then the air around it also starts vibrating in exactly the same way and carries sound to our ears through the vibrations of its molecules. 

Sound produced by humans:

The human beings produce sound by using the voice box which is called larynx. It is also known as the voice box. The human voice box or larynx contains two ligaments known as vocal cords. The vocal cords act like a kind of strings. Sound is produced by the vibrations of vocal cords. 

The lungs pass a current of air between the two vocal cords. This air makes the vocal cords vibrate and the vibrating vocal cords produce sound. 

When the muscles attached to the vocal cords contract and stretch, the vocal cords become tight and thin, and a sound of high frequency is produced. When the muscles relax, the vocal cords become loose and thick and a sound of low frequency is produced. The vocal cords of a man are about 20 mm long. The vocal cords of a omen are about 5 mm shorter than man. Due to the shorter vocal cords, the frequency or pitch of a woman's voice is higher than that of a man. Small children have very short vocal cords due to which the frequency of each of their voice is also very high. 

SOUND NEEDS A MEDIUM FOR PROPAGATION:

Sound needs a solid, liquid or gas for transmission. Therefore, we can say, sound needs a material medium like solid liquid or gas to travel and be heard. Sound cannot travel through vacuum or empty space, as there is no molecules which can vibrate and carry sound waves in empty space. 

Sound cannot be heard on the surface of moon because there is no Air on the moon to carry the sound waves. The astronauts who land on moon talk to each other through wireless sets using radio waves. 

SPEED OF SOUND

We know sound can travel through solids, liquids and gases. The speed of sound is different in different materials. Sound travels slowest in gases, faster in liquids and fastest in solids. Sound travels at a speed of 340 m/s, in water at 1500 m/s and in Iron 5000 m/s. 

The speed of sound in air is 340 m/s whereas light travels at 300000 m/s. It is due to the very high speed of light that we see the flash of lightning first and it is due to comparative low speed of sound that the thunder is heard a little later. 

STRUCTURE OF HUMAN EAR

The ears are the sense organs which help us in hearing sound. 

• Pinna: The funnel shaped outer part of the ear is called pinna. Sound waves enter our ear through pinna. 

• Ear canal: Pinna is attached to about 2 to 3 centimetre long passage called ear canal.

• Ear drum: Ear canal is attached with a thin elastic and circular membrane called ear drum. 

• Three tiny ear bones: There are three small and delicate bones called Hammer, anvil and stirrup in the middle part of the ear which are linked to one another. The three tiny bones in the middle ear act as a system of levers and amplify sound vibrations coming from the ear drum before passing them on to the inner part called cochlea. 

The free end of stirrup touches the membrane over the oval window. 

Cochlea: The inner part of ear has a coiled tube called "cochlea". One end of cochlea is connected to middle part of ear through the elastic membrane over the oval window. Cochlea is filled with a liquid. The liquid present in cochlea contains nerve cells which are sensitive to sound. The other end of cochlea is connected to auditory nerve which goes into the brain. 

WORKING OF EAR:

The sound waves are collected by the pinna. Then it passes through ear canal and fall on ear drum. It makes ear drum vibrating back and forth, which makes hammer to vibrate. Vibrations then amplified and passes through anvil and stirrup. This amplified vibrations passes through oval window to cochlea. This makes liquids in cochlea to vibrate and generates electrical impulses in the nerve cells. These electrical impulses are carried by auditory nerve to brain and the brain interprets these impulses as sound and thus we get the sensation of hearing. 

LECTURE - 3 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      notes prepared by subhankar Karmakar

                                                                         

FRICTION: A NECESSARY EVIL

Frictional force is very important in our life. Sometimes friction is useful and we want to keep it and sometimes it is harmful and we want to reduce it. 

ADVANTAGES OF FRICTION:

1. Friction enables us to walk without slipping

We are able to walk on ground because friction between the sole of our shoes and ground prevents us from slipping over the ground. Walking on slippery ground is difficult because the frictional force on slippery ground is much less which may not be sufficient to prevent us from slipping.

When we accidentally step on a banana peel thrown on the road we easily slip and fall down.

2. Friction enables a car to move on road without skidding.

The friction between tyres of a car and the road enables a car to move forward on road without skidding. If there were no friction between car tyres and road, then the wheels of car would spin at the same place but the car would not move forward at all the car would stay where it was.

3. Friction enables us to apply brakes and slow down or stop a moving car.

The brakes of a car work by friction the friction between break pad and ud rim prevents the wheel from moving ahead. If there were no friction then once a vehicle started moving it would never stop. 

4. Friction enables us how to write and draw on paper.

We are able to write and make drawings on paper because there is friction between the tip of pencil or pen and paper. 

We cannot write with a pencil on a glass sheet because the glass surface is very smooth due to which the friction between the tip of pencil and glass surface is much less. This friction is not sufficient to rub off black graphite particles from the tip of pencil.

5. Friction enables us to pick up and hold things in our hands.

We can hold a glass tumbler in our hands because of friction between the glass tumbler and our hands. If the outer surface of a glass tumbler is oily or greasy then it becomes difficult to hold it because a film of oil on the outer surface reduces friction.

6. Nails can be fixed in a wall or wood due to friction.

When we hammer a nail into the wall, it is the friction between the surface of nail and wall which holds the nail tightly in the wall. 

(i) nails and screws are also held in wood by friction.

(ii) friction enables knots to be tied in strings.

(iii) Friction enables a person to climb a tree or pole.

(iv) Friction enables a ladder to be leaned against a wall. 

(v) Friction helps in the construction of buildings.

(vi) Friction enables the belts to drive machines in factories. 

7. Friction enables us to light a matchstick.

When we rub a matchstick against the rough side of a Matchbox than friction between the head of matchstick and rough side of matchbox produces heat. This heat burns the chemicals present on the head of matchstick due to which the matchstick lights up. The burning of matchstick would not be possible without friction. 

8. Friction enables us to cut wood with a saw.

We are able to cut wood because there is friction between the the saw blade and log of wood. 

DISADVANTAGES OF FRICTION:

1. Friction wears away the soles of our shoes.

When the soles of our shoes rub against the rough surface of road, then tiny pieces of the soles keep on breaking off slowly due to which the soles of our shoes wear out gradually. 

2. The tyres of vehicles wear out gradually due to friction.

The rubbing of tyres with road keeps on breaking tiny pieces of rubber from the tyre's surface gradually. Ultimately, all the treads present on the surface of a tyre are worn out and the tyre becomes baldy. Such baldy tyres must be replaced by new tyres. 

3. Friction wears out the rubbing machine parts.

Due to friction, the rubbing parts of a machine like ball bearings of bicycles wear out gradually.

4. Friction wear out the brake pads of vehicles gradually.

When the brakes of a vehicle are applied a lot of friction is produced between the brake pads and moving part of the wheel. This friction wears out the brake pads gradually.

5. Friction wears out steps of staircases in buildings and foot over bridges.

When a lot of people use the staircase in a building everyday, the friction between soles of their shoes and the stone steps wear away hard stone steps very very slowly.

6. Friction produces heat which may damage machines.

When the moving parts of of a machine rub together, a lot of heat is produced due to friction between them. This heat may damage the machine gradually. 

7. Friction reduces the efficiency of machines.

Due to friction a lot of energy is wasted as heat. This wastage of energy reduces the efficiency of a machine.

8. Friction slows down motion. 

Friction reduces the motion of moving parts of a machine. Infact, all the moving things are slowed down by friction.

METHODS OF INCREASING FRICTION:

1. Grooves are made in the soles of shoes to increase friction and prevent slipping.

2. Treads are made in the tyres of vehicles to increase friction and prevent skidding of vehicles on wet roads.

3. Spikes are provided in the shoes of players and athletes to increase friction and prevent slipping.

4. Gymnasts apply some coarse substance on their hands to increase friction for better grip.

5. Machine belts are made of special materials to increase friction and drive machine wheels properly. 

METHODS OF REDUCING FRICTION:

1. Friction can be reduced by making the surfaces smooth by polishing.

2. Friction can be reduced by applying lubricants to the rubbing surfaces.

The substances which reduces friction are called lubricants. When a few drops of oil are poured on the Hinges of a door the friction is reduced and the door moves smoothly. Friction can be reduced but friction can never be entirely eliminated.

3. Friction can be reduced by using wheels to move objects.

Friction can be reduced by attaching wheels or rollers to a heavy suitcase or any other heavy object which is to be moved. 

4. Friction can be reduced by using ball bearings between the moving parts of machines.

Ball bearing is a device which consists of a ring of small metal balls. Ball bearings are designed to make the moving parts of a machine to roll over each other rather than slide. Ball bearing makes the wheel roll smoothly over the axle. In most of the machines, friction is reduced by using ball bearings. 

FLUID FRICTION: FRICTION EXERTED BY LIQUIDS AND GASES:

Liquids and gases are called fluids. Water and air are the most common fluids. There is friction whenever an object moves through a fluid. It is called fluid friction. 

Air exerts frictional force on cars, buses, aeroplanes, rockets and birds etc. moving through it. Water exerts frictional force on objects like boats, speed boats, ships, submarines and fish etc. which move through it. 

The frictional force exerted by a fluid is called drag or drag force. 

The magnitude of frictional force or drag exerted by a fluid on an object moving through it depends on four factors. 

1. Speed of the object

2. Shape of the object

3. Size of the object

4. Nature of the fluid.

• Higher the speed of an object moving through a fluid, greater will be the frictional force or drag.

• The objects having streamlined shapes face much less frictional force or drag when moving through a fluid then the objects which do not have Streamline shapes.

• Larger the size of an object moving through a fluid greater will be the frictional force or drag acting on it. 

• Higher the viscosity or thickness of fluid greater will be the frictional force or drug acting on an object moving through it.

Streamline Shape: 

A streamlined body is a shape that lowers the friction drag between a fluid, like air and water, and an object moving through that fluid. It offers a minimum resistance to air and water by its particular type of body shape .

The aeroplanes and ships have streamlined shaped bodies to reduce drag. The streamlined shaped body experiences minimum resistance when travelling through water or air. It helps to reduce friction. Fishes, birds and rockets also have streamline shapes. 

DISADVANTAGES OF FLUID FRICTION

The main disadvantages of fluid friction are as follows. 

1. Fluid friction reduces the speed of objects moving through the fluids. 

2. When objects move through fluids they lost some of their energy in overcoming the fluid friction. This decreases their efficiency.

METHOD OF REDUCING FLUID FRICTION

The fluid friction can be reduced or minimised by giving special shape called streamlined shape to the objects which move through the fluids. 

Cars are built with streamlined body shape to reduce air resistance caused by air. 

An aeroplane has a streamlined shape to reduce air friction that it encounters when trying at high speed through the sky.