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.