LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 10 : REACHING THE AGE OF ADOLESCENCE

DIFFERENT STAGES OF LIFE AND ROLE OF HORMONES:

The human beings can reproduce only after a certain age because their reproductive systems start working only after a certain age. 

It is the hormones which bring about changes that make a child grow into an adult. 

1. A young human being below the age of full physical development is called a child.

2. A mature human being who is fully grown and developed is called an adult.

3. The transitional period of physical and mental development which occurs between childhood and adulthood is called adolescence.

4. The period of life of a person when the body undergoes a lot of changes leading to reproductive maturity is called adolescence. 

5. A person who is in the process of growing from a child to an adult is called an adolescent. Adolescence is usually begins around the age of 10 or 11 and lasts up to 18 or 19 years. They are also known as teenagers. 

6. Adolescence is the time between childhood and adulthood. 

7. Puberty is the time when adolescent become sexually mature. The period during which adolescent boys and girls reach sexual maturity and become capable of reproduction is called puberty. 

8. Puberty tends to start earlier in girls than in boys. Girls attained puberty at a lower age of 10 to 13 years, while boys reach puberty at a comparatively higher age of 12 to 14 years. 

9. During the puberty development of secondary sexual characteristics occur in both boys and girls. 

10. All the changes which occur in boys and girls at puberty are brought about by various hormones. 

11. The most conspicuous change during puberty is the sudden increase in the height of of boys and girls. 

12. The height of a person depends on the genes inherited from the parents. 

13. When puberty sets in, a time of rapid changes in body shape and appearance starts in boys and girls which make the boys and girls look different from one another. 

14. Some of the changes in body shape and appearance in boys and girls brought about by the onset of puberty are as follows:

a. Boys develop broader shoulders and wider chests than girls.

b. Girls develop broader hips than boys. Due to this, the region below the waist becomes wider in girls. 

c. Boys develop more muscle than girls. So, the body of boys looks more muscular than that of girls. 

d. Girls develop breasts. This also changes the body shape of grown up girls and makes them look different from boys. 

e. Boys develop Adam's Apple which makes them look different from girls. 

f. Boys develop facial hair like moustache and beard, girls do not have facial hair. Growth of facial hair makes boys look different from girls. 

15. Changes at puberty:

a. Increasing height: 

When puberty sets in there is a rapid increasing height of boys and girls. Initially girls grow faster than boys. The rate of growth in height varies in different persons. 

Full height of a person = (present height of a person/percentage of full height at present age) x 100

Problem1: A 10 year old boy 125 cm tall. Is the present height of the boy is 75% of his full height, calculate the full height which the boy will eventually reach at the end of growth period. 

Solution: present height of the boy = 125 cm

                Percentage of full height = 75%

So, full height of the boy = (125/75) x 100 cm

                                           = 166.67 cm

b. Change in body shape and appearance

a. Boys develop broader shoulders and wider chests than girls.

b. Girls develop broader hips than boys. Due to this, the region below the waist becomes wider in girls. 

c. Boys develop more muscle than girls. So, the body of boys looks more muscular than that of girls. 

d. Girls develop breasts. This also changes the body shape of grown up girls and makes them look different from boys. 

e. Boys develop Adam's Apple which makes them look different from girls. 

f. Boys develop facial hair like moustache and beard, girls do not have facial hair. Growth of facial hair makes boys look different from girls. 

c. Change in voice

The growth of voice box in boys is much more than the growth of voice box in girls. The bigger boys box in boys gives deeper voice to the boys. The bulge or projection the front of throat or neck in grown up boys is called Adam's Apple. The smaller voice box in girls gives shrill voice or high pitched voice to the girls.

d. Development of pimples and acne on the face.

Pimples are small red spots on the face of a person. Acne is a skin condition marked by the eruption of numerous red pimples on the face. The pimples and acne are formed due to the increased activity of of sebaceous glands and sweat glands present in the skin. 

e. Development of sex organs

The testes begin to produce male sex cells called sperms at puberty. The ovaries start releasing mature eggs or ova at puberty. 

f. Reaching mental, intellectual, and emotional maturity. 

Adolescence is a period which brings maturity is a person's way of thinking. 

The changes which take place in body and mind during adolescence are a natural part of the growing up process. Every human being has to pass through this stage. 

Adolescence is a period which also brings intellectual maturity. Adolescence is the time in one's life when the brain has the greatest capacity for learning.

16. HORMONES:

Hormones are the chemical substances whitch co-ordinate the activities of living organisms and also their growth. 

The hormones involved in the development and control of the reproductive organs and secondary sexual characteristics are called sex hormones. 

The male sex hormone is known as testosterone and female sex hormone is known as oestrogen. 

17. Exocrine glands and endocrine glands

A gland is a structure which secretes a specific substance or substances in the body. A gland which secretes is product into a duct or tube is called and exocrine gland. For example, salivary glands secrets is product into a duct called salivary duct. Sweat glands and sebaceous glands are also exocrine glands. 

A gland which does not have a duct and secrets is product directly into the bloodstream is called an endocrine gland. An endocrine gland makes a chemical substance called hormone. Pituitary gland, thyroid gland, adrenal glands are example of endocrine gland. 

17: SEX HORMONES:

The onset of puberty is controlled by hormones. The testes make male sex hormone called testosterone and ovaries make female sex hormone called oestrogen. The production of sex hormone is controlled by pituitary gland. 

18. FUNCTIONS OF OF TESTOSTERONE AND OESTROGEN:

Testosterone: Testosterone hormone performs the following functions:

a. Testosterone hormone produces male secondary sexual characteristics in boys at puberty.

b. Testosterone hormone causes the growth and development of male sex organs at puberty.

c. Testosterone hormone causes growth spurt in boys at puberty.

Oestrogen: Oestrogen hormone perform the following functions. 

a. Oestrogen hormone produces female secondary sexual characteristics in girls at puberty.

b. Oestrogen hormone causes the growth and development of female sex organs at puberty. 

c. Oestrogen hormone brings about the monthly preparation of uterus for pregnancy.

19. Menstrual cycle:

In females the reproductive phase of life begins at puberty at the age of 10 to 12 years and generally lasts till the age of approximately 45 to 50 years. 

PREGNANCY:

With the onset of puberty, the eggs begin to mature in the ovaries of a woman. One mature egg is released by one of the over is of the women once in about 28 to 30 days. During this period, the inner lining of uterus grows and becomes thick and spongy, and prepares itself to receive the fertilized egg. So, in case the fertilization of egg cell occurs by a sperm, the fertilized Excel begins to divide to form an embryo. The embryo then gets embedded in the thick uterus lining. This results in pregnancy. 

MENSTRUATION:

If fertilization does not occur, then the egg released by the ovary dies within a few days and the thick lining breaks down. Since the thick uterus lining contains a lot of blood vessels therefore the breaking down of uterus lining produces blood along with other tissues. This blood and the other tissues come out of vagina of women in the form of a bleeding.

The bleeding from the uterus which occurs in a woman every month is called menstrual flow or menstruation. Menstruation occurs once in in about 28 to 30 days.

The first occurrence of menstruation activity is called Menarche. 

The permanent stoppage of menstruation in a woman is called menopause.

The process of ovulation and menstruation in women is called menstrual cycle and it is controlled by hormones. 

20. The two chromosomes that determine the sex of an offspring are called sex chromosomes. The two sex chromosomes are named X chromosome and Y chromosome. The baby develops from XX combination of sex chromosomes in zygote is called a girl. The baby develops from XY combination of sex chromosome in zygote is a boy. It is the sperm of man which determines the sex of the child. The egg cells of woman cannot decide the sex of the child because all the egg cells contain the same sex chromosome, X chromosome. 

Lecture-1 : CLASS-X: SCIENCE : Chapter: COLOURFUL WORLD

CLASS X  |    SCIENCE    |   Colourful World

      notes prepared by subhankar Karmakar

                                                                         

GLASS PRISM:

A prism is a wedge shaped portion of a transparent refracting medium bounded by two plane faces inclined to each other at a certain angle.

The two plane faces (ABED and ACFD) inclined to each other are called refracting faces of the prism.

The line (AD) along which the two refracting faces meet is called the refracting edge of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

The angle A included between the two refracting faces is called angle of the prism.

Any section of the prism cut by a plane perpendicular to the refracting edge is called principal section of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

ANGLE OF DEVIATION FOR A PRISM:

The angle of deviation D is the angle between the incident ray and the emergent ray. 

Difference between when light travels through a glass slab and light travels through a prism:

 

In refraction through a glass slab , the emergent ray is parallel to the incident ray, but in refraction throigh a glass prism, the emergent ray is not parallel to the incident ray.

DISPERSION OF LIGHT:

The phenomenon of splitting of white light into its component colours on passing through a refracting medium is called "dispersion of light." The pattern of the coloured bands obtained on the screen is called "spectrum."

CAUSES OF DISPERSION:

The dispersion of white light occurs because different colours of white light travel at different speeds through the glass prism. 

"More is the speed, less is the deviation"

(i) Red colour travels with maximum speed in glass prism, hence, red colour deviates least or minimum.

(ii) Violet colour travels with minimum speed in glass prism, therefore, violet colour deviates maximum.

RECOMBINATION OF LIGHT:

Opposite of dispersion is called recombination of light. 

Where as splitting of white light into seven colours is called dispersion, but when seven colours combined together to give white light, it is called "recombination" of light.

RAINBOW FORMATION:

The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun's light by raindrops in the atmosphere. Therefore, we can say, a rainbow is produced by the dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum of seven colours.

(i) Why planets don't twinkle

In comparison to the stars planets  are very near to us. Therefore size of the planets seem to bigger to us. The continuously changing atmosphere is unable to cause variations in the light coming from a big sized planet because of which the planet does not twinkle at all.

(ii) Atmospheric Refraction:

The refraction of light caused by the earth's atmosphere give two different layers of  varrying optical densities is called atmospheric refraction

The different phenomena occur due to optical refraction of light

1) twinkling of stars

2) the stars seem higher then they actually are

3) advance sunrise and delayed sunset

(iii) Why does twinkling of a star  occur?

Twinkling of a star occurs due to atmospheric refraction of star's light. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. Therefore the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and hence the star appears to twinkle at night.

In the fig the actual position of a star is given, but due to atmospheric refraction the light from the star gradual curved as shown in the figure. When it ultimately reach our eyes, we see the stars at a height which is more than its actual height. The light bends due to the difference of optical densities of different layers of atmosphere. Hence ,"The Stars Seem Higher Than They Actually Are."

(iv) Advance Sunrise and Delayed Sunset :

We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.

So we observe an advance sunrise and a delayed sunset.

When the sun is below the horizon actually the rays of sunlight get bend due to atmospheric refraction and reaches our eyes and we observe the sun rise although the sun is still below the horizon. 

Similarly when the Sun already goes below the horizon after actual sunset, but still rays from the sun get bend due to atmospheric refraction and we observe that sunset still not happen. Therefore, we see a delayed sunset.

SCATTERING OF LIGHT

Q1. What is scattering of light? 

Ans. Scattering of light means to through light in various random directions. When light rays fall on various types of suspended particles in its path, it is scattered in random directions. Depending on the size of particles, different colour of light are scattered. The most prominent examples of scattering of light are (i) blue colour of the sky, (ii) red colour of the sun at sunrise as well as at sunset. 

Q2. What is Tyndall effect?

Ans. The scattering of light by particles in its path is called tyndall effect. When a beam of sunlight enters a dusty room through his window the tiny dust particles present in the air of room scatter the beam of light all around the room, and thus its path becomes visible to us. 

Q3) On what factor the colour of scattered light depends upon. Explain.

Ans. The colour of scattered light depends upon the size of the scattering particles in the atmosphere. Like, the larger particles of dust and water droplets present in the atmosphere scatter the light of all the colours due to which the scattered light appears to be white. As larger particles scatter all the colours present in the white light. But air molecules in the upper atmosphere where the amount of dust particles are low, they are very small compared to air particles and hence they scatter only light with low wavelength like blue. Due to this reason, the atmosphere looks blue to us.

Q4. Why the colour of the sky is blue?

The scattering of blue component of the white sunlight by air molecules of small size are responsible for blue colour of the sky. As we know scattering of light depends upon the size of the scattering particles. Where as dust particles are larger and scattered all the colours of visible light but air molecules like nitrogen and oxygen are much smaller in size hence they only scatter colours with low wavelengths like blue. It is the real reason behind the fact that our sky looks blue in colour.

Q5. Why the sun appears red at sunrise and sunset.

Ans. The sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.

Q6. Perform an experiment to study the scattering of light.

Ans.

 To show the scattering of light we can perform an experiment. For this we shall take a source of light S placed at the focus of a convex lens L1 so as to produce a parallel beam of light rays. A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc d having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to focus the light rays to form an image on the screen R. If we dissolve about 200 grams of sodium thiosulphate in water of the glass tank. Then add 1 or 2 ml of concentrated sulphuric acid to the water, we will see the fine microscopic particles of sulphur begin to form in water and colloidal solution is obtained. As the sulphur particles begin to form in water we will see the blue light coming from the sides of the glass tank. This is due to the scattering of short wavelength blue light by the minute colloidal sulphur particles. This is how the sky looks blue.

©Subhankar Karmakar

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 9 : REPRODUCTION IN ANIMALS

CLASS VIII   |    SCIENCE    |    CHAPTER 9

      notes prepared by subhankar Karmakar

                                                                         

Reproduction in Animals:

REPRODUCTION: 

The production of new organisms from the existing organisms of the same species is known as reproduction. It is essential for the survival of species on this earth. The process of reproduction ensures continuity of life on earth. 

METHODS OF REPRODUCTION: 

There are two main methods of reproduction in living organisms.

a. Asexual reproduction

b. Sexual reproduction

Special reproductive cells are called sex cells or gametes.  

ASEXUAL REPRODUCTION: 

The production of a new organism from a single parent without the involvement of sex cells or gametes is called asexual reproduction. 

In asexual reproduction, a part of the parent organism separates of and grows into a new organism. Thus, in asexual reproduction, only one parent is needed to produce a new organism. 

Some of the examples of asexual reproduction are binary fission in amoeba and budding in hydra.

SEXUAL REPRODUCTION:

The production of a new organism from two parents by making use of their sex cells or gemetes is called sexual reproduction. 

In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called zygote.

This zygote then grows and develops to form a new organism. 

Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female. 

Male and female parents have special organs in which produce male sex cells and female sex cells respectively.

The humans, fish, frog, cats and dogs all produced by the method of sexual reproduction.

In asexual reproduction, the young one produced is an exact copy of the parent whereas in sexual reproduction, the young one produced is not an exact copy of the parents.

MALE AND FEMALE:

An animal having male sex cells called sperms in its body is called male and having female sex cells called eggs or ova in its body is called female. 

GAMETES:

The cells involved in sexual reproduction are called gametes. There are two types of gametes, (a) male gametes and (b) female gametes. The male gametes in animals is called sperm and female gamete in animals is called egg or ovum.

SPERMS: 

The sperms are extremely small cells and can be seen only through microscope. It is is about 0.05 millimetre long. It has a head, middle piece and a tail. Sperm is a single cell with all the  cell components like nucleus, cytoplasm and cell membrane. 

The purpose of tail of sperm is to move it by waving its tail from side to side. 

EGGS or OVA:

The eggs or ova are also very small and we need a microscope to see the eggs are ova. The egg or ovum is also a single cell having a nucleus, cytoplasm and a cell membrane. Outside the cell membrane, an egg or ovum has a thin layer of jelly called jelly coat which allows only one sperm to enter into it during fertilisation. 

The nuclei of sperm and egg contain chromosomes which carry genes and transmit the characteristics of parents to the offspring. The fusion of a male gamete called sperm with the female gamete called egg gives rise to a new cell called zygote. Thus, the new cell which is formed by the fusion of a male gamete and female gamete is called zygote.

Zygote is a single cell which contains one nucleus. The nucleus of zygote is formed by the combination of nuclei of sperm and egg.

The process of fusion of gametes is called fertilisation. 

FERTILISATION:

Sexual reproduction will occur when a male gamete combines with a female gamete. the fusion of male gamete called sperm with female gamete called ovum to form a zygote  during sexual reproduction is called fertilisation. Therefore we can say the zygote is actually a fertilised egg or ovum. 

All the multicellular animals start their life from a single cell called zygote through sexual reproduction.

There are two types of fertilisation process known as internal fertilisation and external fertilisation.

INTERNAL FERTILISATION:

The fertilisation takes place inside the female body is called internal fertilisation. The process of fertilization in humans the fusion of male gamete of sperm and female gamete or a occurs inside the body of a female animal is called internal fertilisation. 

EXTERNAL FERTILISATION:

The fertilization which takes place outside the female body is called external fertilisation. In external fertilisation, the male and female animals released their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilised them outside the body of female frog or fish. 

External fertilisation is very common in aquatic animals such as frog fish and starfish etc. 

Although the animals like frog and fish which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs or fishes. 

This is because of the following reasons.

(i) frog of fish lay eggs or release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall and hence all the eggs do not get fertilised. 

(ii) many of the eggs of frogs are eaten by other animals which live in water. 

(iii) many of the larvae of frog and fish which are hatched from the fertilized eggs are eaten up by other animals in water and fail to develop into adult frogs or fishes.

SEXUAL REPRODUCTION IN ANIMALS

The whole process of sexual reproduction in animals involves the formation of sperms and eggs, joining together of sperm and egg to form a fertilized egg called zygote and growth and development of zygote to form a baby animal. 

The sexual reproduction in animals takes place in the following steps:

(a) the male parent produces male gametes called sperms.

(b) the female parent produces female gametes called eggs or ova.

(c) The sperm enters into egg. The nucleus of sperm fuses with the nucleus of egg cell to form a new cell called zygote. 

(d) The zygote divides repeatedly to form a hollow ball of hundreds of cells which is called embryo. 

(e) Embryo grows and becomes a foetus (in which all the main body features of the baby animal have formed).

(f) Foetus grows and develop to form a new baby animal. 

SEXUAL REPRODUCTION IN HUMANS

In human beings, there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation and make a zygote; and for the growth and development of a zygote into a baby. 

THE MALE REPRODUCTIVE SYSTEM

The human male reproductive system consists of the following organs: Testes, Scrotal sacs, Epididymis, Sperm ducts, Seminal vesicles and Penis. 

Testes are the real reproductive organs in men. Testes make male gametes called sperms. Millions of sperms are produced in the testes. The testes are enclosed in two small bags of skin called scrotal sacs. 

The sperms formed in testes come out and go into a coiled tube called Epididymis. From epididymis, the sperms are carried by a long tube called sperm duct into organs called seminal vesicles. The seminal vesicles join to another tube called urethra coming from the bladder. Urethra carries the sperms to an organ called penis which opens outside the body. These sperms are carried in a liquid called semen. When the penis introduces sperms into the woman, one of the sperms be able to fuse with an egg cell and bring about fertilisation. 

THE FEMALE REPRODUCTIVE SYSTEM

The human female reproductive system consists of the following organs: Ovaries, Oviducts, Uterus and Vagina. 

Ovaries are the real reproductive organs in a woman. Ovaries make the female gametes called eggs or ova. Just above the ovaries, there are tubes called oviducts or fallopian tubes. In human beings, one mature egg is released into oviduct every month by one of the ovaries. The fertilisation of egg by a sperm takes place in the oviduct. 

The process of reproduction in humans takes place in two steps:

1. Fertilisation, and

2. Development of embryo.

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      Notes prepared by Subhankar Karmakar

---

• FACTORS AFFECTING FRICTION:

It has been found that the friction between two surfaces depends on two factors:

(i) the nature of the two surfaces (smoothness or roughness of the two surfaces)

(ii) the force with which two surfaces are pressed together.

• 1. Dependence of friction on the nature of two surfaces:

Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other. When the two surfaces in contact are smooth, then the friction between them will be small. As the degree of roughness of the two surfaces in contact increases, the friction also increases. 

• 2. Dependence of friction on the forces with which two surfaces are placed together.

The friction between two surfaces depends on the force with which the two surfaces are pressed together. Greater the weight of an object which moves over another surface, more is the force with which the two surfaces are pressed together and greater will be the friction between them. 

• Types of friction:

There are three types of friction. 

• 1. Static friction
• 2. Sliding friction and
• 3. Rolling friction

• 1. Static friction: 

The maximum frictional force present between two any two objects when one object just tends to move or slip over the surface of the other object, is called static friction. In the case of static friction, the object is actually not moving or sliding over the other object, it only takes to move or slide. It is the maximum frictional force. 

• 2. Sliding friction:

the frictional force present when one object moves slowly or slides over the surface of another object is known as sliding friction. Is the sliding friction is smaller than the static friction, it is easier to keep an object moving which is already in motion than to move the same object from rest or stationary position. Sliding friction is smaller than the static friction. 

• Why does sliding friction is smaller than the static friction?

When an object has already started moving or sliding the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely. Since the interlocking of the two surfaces is less when an object has already started moving, therefore, the sliding friction is smaller than the static friction. 

• 3. Rolling friction:

when an object like a wheel rolls over the surface of another object the resistance to its motion is called rolling friction. Therefore it is always easier to roll than to slide an object over another object. Show rolling friction is much less than the sliding friction. Rolling reduces friction.

heavy machines can be easily move from one place to another by placing round logs of wood under them and then pushing with the force of hands. 

• Maximum Friction : Static Friction 
• Minimum Friction : Rolling Friction

Static friction > sliding friction > rolling friction.

Lecture-4 : CLASS-X: SCIENCE : Chapter: Electricity

CLASS X  |    SCIENCE    |    ELECTRICITY

      notes prepared by subhankar Karmakar

                                                                         

Ohm's Law: 

According to Ohm's Law, at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends. 

If Ｉis the current flowing through a conductor and V is the potential difference (voltage) across its ends, then according to Ohm's Law:

Ｉ ∝ V  (at constant temperature)

This can also be written as:  V ∝ Ｉ

or V = RＩ

where R is a constant called "resistance" of the conductor. The value of this constant depends on the nature, length, area of cross section and temperature of the conductor. 

V/Ｉ = R

The ratio of potential difference applied between the ends of a conductor and the current flowing through it is a constant quantity called resistance.

Therefore, 

(i) The current is directly proportional to potential difference, and

(ii) The current is inversely proportional to resistance. 

a. "Current is proportional to the potential difference" - it means if the potential difference across the ends of o conductor is doubled, the current flowing through it also gets doubled, and if the the potential difference is halved the current also gets halved.

b. "The current is inversely proportional to resistance" - it means if the resistance is doubled, the current gets halved, and if the resistance is halved the current gets doubled.

Resistance of a conductor:

The property of a conductor due to which it opposes the flow of current through it is called resistance.

The resistance of a conductor depends on length, thickness, nature of material and temperature of the conductor. 

SI unit of resistance:

The SI unit of resistance is ohm. It is generated by the symbol Omega, Ω. The unit of resistance can be defined as following. "1 ohm is the resistance of a conductor such that when a potential difference of 1 volt is applied to its ends a current of 1 ampere flows through the conductor". 

Numericals on Ohm's law:

1. Potential difference between two points of a wire carrying a 0.4 ampere current is 0.6 volt. Calculate the resistance between these points.

Soln: From Ohm's law we know that resistance of a conductor is equal to the ratio of the potential difference applied across the conductor and the current flowing through it. Therefore, R = V/Ｉ

or, resistance, R = 0.6/0.4 = 3/2 = 1.5 Ω

2. A simple electric circuit has a a 12 volt battery and a resistor of 60 ohms. What will be the current in the circuit? The resistance of the connecting wires is negligible. 

Soln. From Ohm's law we know that current flowing through the circuit, Ｉ is equal to the ratio of the potential difference across the ends of the conductor V and the resistance R attached to it. 

Therefore, Ｉ = V/R = 12/60 = 0.2 A (ampere).

3. An electric appliance draws a current of 2.2 ampere from a 220 volt supply line. What current will this electric iron draw when connected to 110 volt supply line?

Soln. From Ohm's law we know that resistance of a conductor is equal to the ratio of the potential difference applied across the conductor and the current flowing through it. As the resistance of the appliance is same.

R = V₁/Ｉ₁ = V₂/Ｉ₂

V₁ = 220 V, Ｉ₁ = 2.2 A,  V₂= 110 V, Ｉ₂ = ?

Ｉ₂ =  V₂Ｉ₁/V₁ = 110x2.2 /220 = 1.1 A

4. If 20 C of charge pass a point in a circuit in two second, what is the current flowing?

Soln. Charge passing through a conductor per unit time is called current. Therefore, current, Ｉ = q/t, q = charges passing through the conductor, t = time. 

Ｉ = q/t = 20 /2 = 10 A

5. If a potential difference of 20 volt causes a current of 1 ampere to flow for 2 minute how much energy is transferred?

Soln. Q = Heat generated, Ｉ= Current = 1 A;

          R = Resistance = V/Ｉ , t = time = 120 s

Q = Ｉ²Rt = ＩVt = 1 x 20 x 120 = 2400 Joules.

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      Notes prepared by Subhankar Karmakar

---

• FRICTION : 

The force which always opposes the motion of one object over another object in contact with it, is called friction. 

• friction occurs between the two surfaces which are in contact with each other. 

• friction is a force which occurs when the two objects tend to slide over each other and even when there are actually sliding over each other. 

• Direction of force of friction:

The force of friction always opposes the motion of one object over another object. The force of friction acts in a direction opposite to the direction in which an object moves. Therefore, the force of friction acts in a direction opposite to the direction of motion of an object.

• Cause of friction:

Friction is caused by the interlocking of irregularities in the surfaces of the two objects which are in contact with each other. 

When we try to move an object over the other, we have to apply a force to overcome interlocking of the irregularities in the their surfaces. More the roughness of a surface, larger is the number of irregularities on its surface and hence greater will be the friction. Thus, the force of friction is greater if very rough surfaces are involved. Friction will be less if the surface is smooth and polished. 

• Spring balance:

The spring balance is a device which is used for measuring force acting on an object. With the help of a spring balance we can measure the frictional force. 

The spring balance contains a coiled spring which gets stretched when a force is applied to its free end having a hook. The extent by which the spring gets structured is a measure of the force applied. Larger the stretching of spring, greater will be the magnitude of the force applied. the stretching of spring or magnitude of force is indicated by a pointer attached to the spring which moves on a graduated scale. The reading on the scale of spring balance give us the magnitude of force. When the spring balance is held vertically, it is used to measure the weight of an object hung from its hook. and when spring balance is held horizontally it can be used to measure the force being applied to pull the object on a horizontal surface.

• Factors affecting friction:

It has been found by experiment that friction between two surfaces depends on two factors.

• The nature of the two surfaces (smoothness or roughness of the two surfaces).
• The force with which two surfaces are pressed together.

The force of friction does not depend on the amount of surface area of the two objects which is in contact with each other. 

Dependence of friction on the nature of two surfaces:

• Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other. 
• If the two contact surfaces are smooth, then the frictional force is less where as when the two contact surfaces are rough, frictional forces are more. 

LECTURE 3 : CLASS XII: PHYSICS : ELECTRIC FIELD

CLASS XII   |    PHYSICS    |    CHAPTER 1

      notes prepared by subhankar Karmakar

(the physical quantity written in bold letters are vectors )

ELECTRIC FIELD: 

The electric field or electric intensity or the electric field strength E at a point is defined as the force experienced by a unit positive test charge placed at that point, without disturbing the position of source charge. 

• Electric field E is a vector quantity. 

• SI unit of electric field is N/C. 

• Dimension of electric field is

 [E] = force/charge = [MLT⁻²]/[AT] = [MLT⁻³A⁻¹]

Electric field is an example of vector field:

As the value of electric field E is different at different point, so we can say each point having a position vector r , therefore, the vector E is a function of position vector of a point. Hence, we can say electric field is an example of vector field. 

Principle of superposition of electric fields:

It says the electric field at any point due to a group of charges is equal to the the vector sum of the electric field produced by the each charge individually at that point, when all other charges are assume to be absent.