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

  Edunes Online Education

Chemical Effects of Electric Current Explained
Electrolysis and its applications
Science | Class 8 | CBSE & SEBA Board

Introductions of Electrolysis

Edunes Online Education

BOARD: CBSE | CLASS: 8 | SCIENCE | Chapter 14:
📌 CHEMICAL EFFECTS OF ELECTRIC CURRENT

⚡ The Case of Distilled Water, Tap Water, Sea Water & Rainwater

💡 Core Idea: Electricity flows through water only when ions (charged particles) are present.

1️⃣ Distilled Water

Observation: Distilled water does not conduct electricity.

Think WHY:
Distilled water is pure. No dissolved salts → no ions → no charge flow.

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

By adding:

1. Salts
2. Acids
3. Bases

🧠 Memory Hook:
PURE water = NO current
ADD ions = SWITCH ON current

2️⃣ Tap Water

Tap water contains small amounts of dissolved salts, so it conducts electricity.

Think Safety:
Since tap water conducts electricity, touching electrical appliances with wet hands can cause electric shock.

🚫 Never operate heaters, irons, mixers, grinders, or switches with wet hands.

🧠 Wet hands + electricity = DANGER

3️⃣ Drinking Water

Drinking water also contains dissolved salts, hence it is a conductor of electricity.

Compare:
Distilled water (no salts) ❌
Drinking water (some salts) ✔️

4️⃣ Sea Water

Sea water contains a large amount of salts, making it a very good conductor of electricity.

Which is a better conductor: drinking water or sea water? Why?

Sea water is a better conductor because it contains more dissolved salts than drinking water.

🧠 More salts → more ions → more current

5️⃣ Rain Water

Rain water contains small amounts of acids, so it also conducts electricity.

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

No. Rain water conducts electricity, so it is unsafe for electricians to work during rain.

🧠 Rain + electricity = RISK

🧪 Chemical Effects of Electric Current

When electric current passes through a conducting liquid, it can cause chemical reactions.

When electric current is passed through acidified water using carbon electrodes:

1. Water decomposes
2. Hydrogen gas is released
3. Oxygen gas is released

🧠 Electricity doesn’t just flow — it can CHANGE substances

🔬 Electrolysis

Electrolysis: The chemical decomposition of a conducting liquid caused by the passage of electric current.

Example: Electrolysis of acidified water produces:

1. Hydrogen at the cathode
2. Oxygen at the anode

🧠 Break it to remember it:
Electro = electricity
Lysis = breaking

🧠 Activity: Demonstration of Chemical Effect of Electric Current

Describe an activity to demonstrate the chemical effect of electric current.

1. Take a beaker filled with water.
2. Add a few drops of dilute sulphuric acid.
3. Insert two carbon electrodes into the water.
4. Connect the electrodes to a battery.
5. Pass electric current through the solution.
6. Observe gas bubbles forming at both electrodes.

Think Observation → Conclusion:
Gas bubbles = chemical reaction → proves chemical effect of current.

🧠 See bubbles? ✔️ Electricity is doing chemistry!

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

🧪 Chemical Effect of Electric Current: Change in Colour

💡 Key Thinking Idea: Electric current flowing through a conducting material can trigger chemical reactions that may cause a visible colour change.

🔬 Activity: Colour Change Using a Potato

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

1. Cut a potato into two halves.
2. Take one half and insert two iron nails into it, keeping some distance between them.
3. The iron nails act as electrodes.
4. Connect the nails to a battery using wires, a switch, and a compass.
5. Close the switch to allow electric current to pass.
6. Keep the current flowing for about 30 minutes.

Observation:
The compass needle shows deflection, proving that the potato conducts electricity. After some time, a greenish-blue spot appears around the iron nail connected to the positive terminal.

Think Like a Scientist:
✔ Deflection of compass → current is flowing ✔ Colour change → chemical reaction is taking place ✔ Reaction occurs near positive electrode (anode)

🧠 Memory Hook: No reaction → no colour
Current + conductor → chemistry + colour

🧠 Conclusion from the Activity

The formation of a greenish-blue spot near the positive electrode proves that the chemical effect of electric current can cause a change in colour in a conducting medium.

⚙️ Applications of Chemical Effect of Electric Current

1. Electroplating of metals
2. Purification of metals
3. Production of metals from their ores
4. Production of chemical compounds
5. Decomposition of chemical compounds

🧠 Think Big Picture: Electricity is not only energy — it is a chemical tool.

🔩 Electroplating of Metals

Electroplating: The process of depositing a thin layer of one metal over another metal using electric current.

Commonly electroplated objects:

1. Taps
2. Utensils
3. Jewellery
4. Machine parts

Metals used include chromium, nickel, tin, copper, silver, and gold.

Why Electroplate?
✔ Prevent corrosion ✔ Improve appearance ✔ Increase durability ✔ Reduce cost (thin layer, expensive metal)

🧠 Shiny outside, strong inside — that’s electroplating!

🧲 Activity: Electroplating Copper on Iron

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

1. Take copper sulphate solution in a beaker.
2. Clean the iron object thoroughly.
3. Connect the iron object to the negative terminal of the battery.
4. Connect a copper plate to the positive terminal.
5. Dip both electrodes in the solution.
6. Allow current to pass for some time.

Observation:
A thin layer of copper gets deposited on the iron object.

Think Mechanism:
Copper ions move towards the negative electrode (iron) and get deposited as metal.

🧠 Negative attracts metal — remember this for electroplating!

⚙️ Purification of Metals by Chemical Effect of Electric Current

The chemical effect of electric current is widely used to purify impure metals. This process is called electrolytic refining.

🔩 Electrolytic Refining of Copper

1. A thick rod of impure copper is made the positive electrode (anode).
2. A thin strip of pure copper is made the negative electrode (cathode).
3. A water-soluble salt of copper (copper sulphate) is used as the electrolyte.

Experimental Setup:
• Take 250 ml distilled water in a beaker. • Dissolve two teaspoons of copper sulphate in it. • Add a few drops of dilute sulphuric acid. • Connect the impure copper rod to the positive terminal. • Connect the pure copper plate to the negative terminal. • Switch on the current and allow it to pass for about 30 minutes.

Think Process Flow:
Anode dissolves → Copper ions move → Cathode gains copper

Observation:
✔ The impure copper rod becomes thinner. ✔ The pure copper plate becomes thicker. ✔ Impurities settle at the bottom of the beaker.

🧠 Anode loses, cathode gains — remember this golden rule!

🏭 Production of Metals

The chemical effect of electric current is used to extract certain metals from their ores.

Think Why:
Some metals are too reactive to be extracted by heating — electricity does the job.

🧠 Electricity replaces fire for reactive metals.

🧫 Production of Chemical Compounds

Electrolysis is used to produce chemical compounds such as:

1. Sodium hydroxide from aqueous sodium chloride

Think Conversion:
Electricity → Chemical factory

🔬 Decomposition of Chemical Compounds

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

Example: Water → Hydrogen + Oxygen (by electrolysis)

🧠 Electricity doesn’t just move — it breaks matter.

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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.