About Nepal: Science Notes For SLC Students

Virus:

Virus is a very tiny, infections and highly pathogenic organisms, which can multiply only within the living host. They don't have cellular structure. They contain hereditary material in the form of DNA or RNA enclosed in a protein coat called capsid. A mature virus is called a viron.

They exhibit properties of both living and non-living things, as Virus has no metabolic activity of its own.

They are smallest living organism with the size of about 20nm to 300 nm. Based on the host they attack, viruses are of 3 types.

· Animal Virus

· Plant Virus

· Bacteriophages

They are also classified into RNA virus and DNA virus on the basis of nucleic acid present in them.

Q. Why is a virus considered as the boarder line between living and non living things?

Virus is a living thing only inside a cell and nonliving thing outside a cell so it is considered as a chain between living and non living things.

Q. Why are viruses called obligatory parasite?

Viruses are called obligatory parasite because they are fully dependent on the other organisms.

Characteristics/ Nature of Virus

· They don't show respiration & metabolism

· They can reproduce only inside living cell.

· They don't show cellular structure.

· They are like molecules.

· They show host specificity i.e. particular virus can attack only a particular host

Transmission of Virus:

Virus transmits by means of air, water and food particles. When an infected person sneezes, virus from the water droplets enter the near by healthy person. Viruses also transmit by blood and body fluids. Some common viral diseases, organs affected medium & control measures are shown in the table below.

Name of the disease	Name of Virus	Organs effected	Medium	Control Measures
Common cold	Rhino virus	Upper respiratory tract	Saliva	Control of infection
Mumps	Parmyxo virus	Respiratory tract	Saliva	Control of infection
Measles	Parmyxo virus	Mouth, Skin, Instestine and Respiratory tract	Saliva	Control of infection
Polio	Polio virus	Pharynx, Intestine, Blood and Brain	Feaces, Saliva	Polio drops
Rabies	Rabies virus	Spinal cord, brain	Saliva	Anti rabies vaccine
AIDS	Retro virus	Skin, Blood and Brain	Unsafe sex, Blood	Avoid sex with multiple partners, Use condoms

Q. Culture of virus is impossible, why?

Culture of virus is impossible because it is very minute and it is alive inside host only.

Exercise for Practice

1. Identify the two diseases caused by virus regarding the parts affected.

i. High fever

ii. Respiratory tract becomes affected.

iii. Eyes become sensitive to sunlight

iv. Skin rashes

i. Respiratory system,

ii. fever is intermittent

iii. parotid gland

iv. Testes.

2. Write two characters of virus.

3. Why virus is called obligate parasite?

a. Why are virus known as obligatory parasites?

b. AIDS virus doesn't kill a human being but it helps other diseases. Justify this statement.

6. What is the reason for transmitting HIV virus causing cold through blood and air media respectively?

7. What causes polio?

8. Which virus causes common cold? What happens if a person suffering from AIDS catches common cold

9. Are viruses living or non-living objects? Explain.

10. Give full form of AIDS and TMV.

11. Write the full form of AIDS. State four main methods by which the virus that causes AIDS gets transmitted from one person to another

12. How does individual suffer from AIDS? Give a way of transmission of this disease. What symptoms is seen in the victim?

13. Give Characteristics of Virus

14. What is virus

15. Why do polio victims become paralyzed?

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Cell division

Cell division is a process of multiplication of cell by its division. It is of two major types. They are mitosis and meiosis.

Mitosis

Mitosis cell division is that in which one cell divides into two diploid daughter cells. It is also called educational cell division and mitotic cell division. It causes growth.

Meiosis

Meiosis cell division is that in which one cell divides into four haploid daughter cell. It is also called reductional cell division and meiotic cell division. It forms gamete.

Mitosis Cell Division

Cytokinesis

Cytokinesis is division of cytoplasm and karyokinesis is division of nucleus.

Karyokinesis

Karyokinesis of mitosis completes in the following four main phases:

Prophase
Metaphase
Anaphase
Telophase

Prophase

In prophase, nucleolus disappears, nuclear membrane is disappearing, centrosome fibres is started between the centrosomes. At the last of this phase, chromosomes make their duplicates.

Metaphase

In metaphase, nuclear membrane disappears, formation of spindle fibre is completed, chromosomes come at equator and attach and their spindle fibres by the help of their centromeres.

Anaphase

In anaphase, centromere decomposes into two parts and chromatids. Now the chromosomes move towards opposite poles due to contraction of spindle fibres towards the poles.

Telophase

Telophase, chromosomes reach at poles and form two daughter nuclei, the cell is rehydrated, finally the cell goes for cytokinesis and one cell divides into two diploid daughter cells.

Significance of mitosis

Significance of mitosis cell division are as the followings:

Helps in growth
Does asexual reproduction
placement of cell and regeneration
Mainttaindgenetic stability

Meiosis Cell Division

Karyokinesis

Karyokinesis of meiosis cell division complete in 8 phases. They are-

Meiotic first division

Prophase I
Metaphase I
Anaphase I
Telophase I

Meiotic second division

Prophase II
Metaphase II
Anaphase II
Telophase II

Prophase I

Prophase I of meiosis completes in five sub-phases. The changes that take place in the nucleus of cells in these sub-phases are mentioned below:

Leptotene [Thread stage]: Chromatin divides in particular number of chromosomes. The chromosomes are so thin that chromosomes are seen in the form of beads.

Zygotene [Paring stage] : Homologus pairs or bivalents are formed. The process of forming the pairs is called synapsis.

Pachytene [Thickening stage] : Chromosomes shorten and form tetrads. Crossing over between non-sister chromatids is started.

Diplotene [Duplication stage]: Crossing over is completed. The two homologous chromosomes tend to separate from each other but both remain attached at chiasmata.

Diakinesis [Moving apart stage] : Overlapped chromo;somes move away from each other.

Significance of meiosis is as the following:

It forms gametes for sexual reproduction.
It causes variation, which is the main unit of evolution. Variation is caused by crossing over.
It helps to make chromosomes’ number constant.

Chiasmata

Chiasmata are the points at which paired homologous chromosomes remain in contact during the prophase I of meiosis cell division.

Homologous chromosomes

Homologous chromosomes or bivalents are the pairs of identical chromosomes formed in the beginning of meiosis. One member of each pair comes from the female parent and other from the male.

Synapsis

Synapsis is the process of pairing chromosomes in the first phase of meiosis in order to make homologous pairs.

Crossing over

Crossing over is defined as an exchange of portions of chromatids between homologous chromosomes.

Chromatids

Chromatids are the thread like strands formed from a chromosome during the early stages of cell division. Each chromosome divides along its length into two chromatids, which are held together at the centromere and separate completely at a later stage.

Chromosomes

Chromosomes are the thread like structures, which are found in the nucleus of plant and animal cells (eukaryotic cells). They are made of protein and nucleic acid.

Tetrads

Tetrads are the paired chromosomes in meiosis, after each chromosome has duplicated itself, and the pair is visibly four stranded

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Atmosphere

The earth is surrounded by air. Air is also found in soil, water and porous things like sponges, bricks, chalk, etc. Air makes a very thick layer around the earth called atmosphere. It is extended from the earth’s surface to the space. Atmosphere also contains smoke, dust particles and water vapour in it. According to the height of the atmosphere and its nature, temperature and components are found different. On the basis of height, components and nature, atmosphere is divided into the following five layers.

Troposphere
Stratosphere
Mesosphere
Thermosphere
Exosphere

Troposphere

It is the lowest and the thinnest layer of the atmosphere. It is extended up to the height of 16km from the earth’s surface. The layer has the maximum pressure. It contains clouds. Changes in this layer cause the change in weather. This layer is highly influenced by our activities. Rain, hail, dew, and snow come form this layer. As the height in this layer increases, the temperature decreases. Major components of this layer are: N₂, O₂, CO₂ water-vapour, dust, etc.

Stratosphere

Stratosphere is found above the troposphere. It is extended between the height of 16 to 50km from the earth’s surface. It contains less oxygen and more ozone. The layer of ozone is called ozonosphere, which is a protective layer for us. It protects us from the harmful effects of ultraviolet rays coming from the sun. In this layer the temperature increases, with the increment of height. Jet planes fly in this layer. This layer is also influenced in some extent by our activities.

Mesosphere

Mesosphere is the third layer of the atmosphere. It is extended between 50-80km height from the earth’s surface. This layer has very less ozone in it. This layer has very strong wind in it. The wind blows eastward in winter and westward in summer. In this layer the temperature decreases when the height increases.

Thermosphere

Exosphere is the outermost layer of the atmosphere. It is extended beyond 72 km height from the earth’s surface. This layer is extended 9600km. It is the outer limit of the atmosphere. It is the uppermost layer; therefore here is so less gravity that some atoms of its components escape into the space. The major components of it are hydrogen and helium atoms.

Atmosphere Review

i. Temperature decreases as the height increases by 6.5⁰c per km.

ii. Upper most part of the troposphere is called tropopause.

iii. Troposphere is responsible to determine the weather and climate (e.g. Raining, cloud, storming, fogs etc.)

iv. Temperature increases as the height increases in stratosphere.

v. The upper layer of stratosphere is called stratopouse.

vi. The region where the ozone (O₃) lies in the stratopause is called ozone layer.

Ozone Layer

i. The ozone layer absorbs about 99% ultraviolet radiation coming from the sun.

ii. Ozonosphere is about 35 km thick.

iii. Ozone is formed the following ways by radiation

O₂ ® O+O

O₂ + O ® O₃ (Ozone)

iv. The reduction of ozone layer due to chloro fluoro carbon, methyl chloro form carbon tetra chloride and methyl bromide is called depletion of ozone layer.

Effects of depletion of ozone layer.

i. It causes skin burning, lung and breast cancer, leukemia, blindness

ii. Growth of aquatic and amphibian plants affected

iii. Loss of productivity of living beings

iv. The depletion of O₃ layer may bring about significant changes in climate and cause green house effect.

v. The depletion of ozone layers causes eye cancer in cattles.

Protection of ozone layer

i. By banning the production and use of CFC.

ii. By bringing awareness of ozone depletion in people.

iii. By recapturing CFC released from the air conditioners, refrigerators and other machineries using them.

Green House Effect

i. The house made up of green glasses which can trap the solar radiation in it is called a green house.

ii. Off season fruits and vegetables can be grown with the help of green house.

iii. Carbon dioxide, nitrous oxide, methane; CFC, water vapour protect the solar radiation to wards the earth. That means these gases help the earth to act as a green house. So these gases are called green house gases and when amount of these gases increased the temperature of the earth is also increased. This increment in temperature is called green house effect.

Effects of green house effect

i. It increases the temperature of the atmosphere

ii. On human, it helps spreading diseases like malaria and filar

iii. It helps breeding, growth and spreading of insects.

iv. Reduces crop yields by reducing the soil moisture.

v. The changes in climate pattern changes the crop pattern.

Ways of controlling green house effect

i.By increasing forestation

ii. By reducing the use of fossil fuels.

iii. By banning deforestation

iv. By banning the use of CFC

v. By using unleaded petrol in vehicles

vi. By using biogas for domastic purposes

Industrial gases

i.The gas like SO₂, CO₂, CO, NO₂ dust and smoke are produced by industries are called industrial gases.

ii. The increament in the amount of industrial gas in the atmosphere pollute the air. Which is called air pollution.

Effects of air pollution

i.Produce diseases of neck, breast and longs.

ii. Leukemia

iii. Acid rain

iv. Raise in temperature)

Exercise for Practice

Answer the following questions.

1. What is atmosphere? List the composition of different gases in the atmosphere?

2. List the layers of the atmosphere.

3. List the thickness of different layers of atmosphere.

4. What is tropopause? Write the rate of decrease of temperature of troposphere with its height.

5. Why is the troposphere denser?

6. Which layer of atmosphere is called stratosphere? List its thickness.

7. What characters does the stratosphere show?

8. Which layer of atmosphere is responsible to absorb ultra voilet rays. Name the matter that absorbes ultravoilet rays and remains in the atmosphere.

9. Why lower layer of troposphere is is hotter?

10. Write the event that happens in the troposphere.

11. Write every short note about the distribution of temperature in the stratosphere.

12. Write short note about wind below in the mesosphere.

13. Why thermosphere is called ionosphere too?

14. In which layer of atmosphere following events happen?

a. Lightning and thundering

b. Meteors produce streaks produce streaks of light.

c. Radio waves are reflected to the earth

d. Clouds are formed and sowers rain

15. Define

a. Stratopause

b. Ozonosphere

c. Mesopause

d. Mesosphere

16. Why is temperature of upper layer of stratosphere more?

17. What is ozone? Where is layer of ozone situated?

18. What is meant by ozone layer? How is it formed?

19. Which rays is absorbed by ozone layer? What are advantages to us from this act of the ozone layer?

20. Write importance of ozone layer.

21. What are measure matters which cause depletion of ozone layer?

22. How should we take care of ozone layer?

23. Ozone layer is security shield for living things in the earth . How ?

24. What is meant by ozone hole? What does it inform to living things of the earth; sign of goodness or danger to us? How?

25. What is CFC and uses of it?

26. Why is CFC banned?

27. What are the effects of depletion of ozone layer?

28. Define green house effect?

29. What is meant by natural green house and articicial green house?

30. Write the importance of natural green house?

31. Write the uses of artificial green house.

32. How is green house effect produced?

33. What are measure matters which cause increase of green house effect?

34. List the effects of increasing green house effect.

35. List human activities which increase green house effect.

36. Increasing green house effect is more dangerous to our country, Nepal, How?

37. What should be our role to decrease the green house effect?

38. Sketch a figure to show natural green house

39. Sketch articicial green house encluding its cunctioning.

40. What is industrial gas?

41. What are toxic industrial gases?

42. What is meant by industrial pollution?

43. What is meant by industrial smog?

44. What is meant by acid rain? How is acid rain formed?

45. Which industrial gases cause diseases of throat and thorax in human?

46. Which industrial gases cause increase of global temperature?

47. Which industrial gases cause acid rain?

48. What is industrial pollution?

49. List the effects of industrial pollution.

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Lens: A lens is a transparent medium bounded by two spherical surfaces

· Centre of curvature:

The centre of the curved surface of a lens is called centre of curvature. A lens has two centers of curvatures, which are denoted by c₁and c₂ as shown in figure (i).

· Principles axis:

The line passing through the two centers of curvature of a lens is known as Principle axis as shown in figure (i).

· Optical centre:

The geometrical centre of a lens is called its optical centre. A ray of light passing through it does not bend. It is denoted by ‘o’.

· Principle Focus:

When narrow parallel rays of light converge to a point or appear to come from a point after refraction. The point is known as Principle focus or focus. It is denoted by F.

· Focal length:

The distance between principle focus (F) and optical centre (O) is known as focal length. It is denoted by f. It is taken as +ve for convex lens and +ve, for concave lens.

Image formed by a convex lens:

SN	Position of		Nature & size
SN	Object	Image	Nature & size
1	At infinity	At focus F	Real, inverted and highly diminished
2	Beyond 2F	Between F & 2F	Real, inverted and diminished.
3	At 2F	At 2F	Real, inverted and of the same size.
4	Between F & 2F	Beyond 2F	Real, inverted and magnified.
5	At F	At infinity	Real, inverted and highly magnified.
6	Between F & Optical center	On the same side of lens	Virtual, erect and magnified.

· · In concave lens the image formed is virtual, erect ad diminished. When the object is at infinity the image will be formed at focus. When the object is in between optical centre and infinity, the image will be formed in between focus and optical centre on the same side of the lens as shown in the figure.

Magnification:

The process of variation of the size of image produced by a lens, of position of object is called magnification. Mathematically, it is equal to the ratio of height of the image to the height of the object. It is denoted by ‘m’

The power of lens:

The power of lens is the ability to converge or to diverge a beam of light. Mathematically, it is equal to the reciprocal of focal length (in meter) of the lens. It is denoted by P. It is unit is Dioptre (D).

Optical Instrument:

Optical instrument is that instrument in which mirrors, prisms, lens etc. are used. It is of two types.

i. Real image forming: Camera, film projector, slide projector, eye, etc.

ii. Virtual image forming: Binoculars, microscope, telescope, etc.

Camera:

It is an optical instrument used for taking photographs.

· The inner surface of the camera is made black in order to absorb maximum light which passes through the lens.

· The main parts of the human eyes are (i) cornea (ii) iris (iii) lens (iv) ciliary muscle (v) optic nerve (vi) retina, etc.

· Light coming from an object enters in the lens through the cornea. After refraction through the lens, it is focused on the retina, where inverted image of the object is formed. The op tic nerves transmit the image impulse (making it erect) to the brain.

The nearest point

The nearest point, up to which an object can be seen clearly, by an eye, is known as near point. (25cm) of that eye.

The farthest point

The farthest point, up to which objects can be seen clearly, by an eye, is known as far point. (Infinity) of that eye.

Accommodation:

The ability of the eye lens to focus objects at various positions on the retina is known as accommodation of the eye. Ciliary muscles conduct it by its contraction and relaxation.

· The size of the pupil becomes small in bright light.

· The size of the pupil becomes big in dim light.

The ordinary sun glass should not be used for a long time even in sunny days. The sunglass makes shade to the eyes due to which the size of the pupil increases eve in bright light. The ultra- violet rays passing through the glass falls on the retina. It may damage the retina.

Defect of vision:

It is a defect due to which the image of distance or nearer object is not formed at retina. Hence the object appears unclear.

Short sightedness (myopia):

The defect of vision, in which a person cannot see the distant objects, clearly, is known as short sightedness. The image of distant objects is formed in front of retina as shown in figure (i). It is caused by elongation of the eye ball or shortening of focal length of eye lens. The removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper at proper distance from his/her eyes, while reading without using lens.

Long Sightedness (hypermetropia):

The defect of vision which a person cannot see the near object clearly, is known as long sightedness. The image of near objects is formed behind the retina as shown in figure (i). It is caused by shortening of the eye ball or increasing focal length of the eye lens. The removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper a little bit away from his/ her eyes, while reading without using lens.

Differences between eye lens and camera.

Eyes	Camera
Its focal length is variable.	Its focal length is fixed for a camera.
The image is formed on the retina.	The image is formed on the film.
Retina of an eye is used again and again.	A photographic film cannot be reused.
It forms temporary image.	It forms permanent image.

Similarities between the camera and the eyes are:

i. Both of them have a convex lens and form a real and inverted image.

ii. The diaphragm of camera does the same work of pupil of eye.

iii. The shutter of camera does the same work of eyelids of eye.

Differences between a compound microscope and a telescope:

Compound Microscope	Telescope
It is used for viewing microscopic objects clearly.	It is used for viewing heavenly bodies clearly.
It is used by pathologists or scientists.	It is used by astronomers.
The focal length of objective lens is shorter than that of eye lens.	The focal length of objective lens is longer than that of eye lens.
The image formed by the objective lens lies within the focus of the eyepiece lens.	The image formed by the objective lens lies exactly at focus of the eyepiece lens.
The final image is formed beyond the objective lens.	The final image is formed at infinity.

Combination of Resistors

Series Combination	Parallel Combination
In this combination, resistors are connected from end to end	In it, resistors are connected between two common points.
In series combination, current through each resistor is the same but pd across each resistor is different.	In parallel combination, current through each resistor is different but pd across each resistor is the same.
The effective resistance ( R) of the combination is given by R = R₁+R₂+R i.e., R is more than individual resistance.	The effective resistance (R) of the combination is given by i.e., R is less than individual resistance.
Individual use of each resistor is not possible in this combination.		Individual use of each resistor is possible in this combination.
In it, brightness of the bulb changes with change in the number of bulbs.		In it, brightness of the bulb does not change with change in the number of bulbs.
Other resistors doe not work when one of them fails to work.		Other resistors do their work when any one of them fails to work.

Grouping of cells:

Series combination	Parallel Combination
Negative terminal of one cell is connected to the positive terminal of another cell and so on.	Positive terminal of the cells are connected to one point and negative terminals to another point.
Sum of potential difference across each cell is equal to the pd across the external resistance .i.e., pd across R (E) = 3Vin the given figure.	Potential difference between two common points A and B is always constant. i.e., pd across R = E in the given figure.
Value of current increases by increasing the number of cells in it because in this combination, current (I) = (internal resistance of the cell is neglected)	Value of current does not increase by increasing the number of cells in it because in this combination, (I) =E/R (internal resistance of the cell is neglected.
In this grouping the cells lasts for short time	In this grouping, the cells last for a long time.

Q] In parallel combination of electric bulbs, there is no difference in the brightness of the bulb even when their number is increased or decreased, why?

Ans] In parallel combination of electric bulbs, there is no difference in the brightness of the bulb even when their number is increased or decreased because; Pd across each bulb remains the same. Therefore for household electrical circuit, parallel circuit is used.

Electric Power:

The rate of doing work by the electrical device is known as electric power. The electric power (P) of an electrical device is equal to the product of the current (I) flowing through the device and potential difference (V) across its two ends i.e.

P = IV

Using Ohm’s laws it can be expressed as:

P =

One Kilowatt-hour is the energy supplied when one kilowatt power is used for one hour. One kilowatt hour (KWh) is also called one unit.

Energy = Power

time

1kWh = (1kW)

(1h)

=1000W

3600s

1kWh = 3.6

Total cost of electricity = total electrical energy in kWh unit cost

rate

Direct Current:

If the polarity of electrical sources does not change with time, it is called direct current (DC).A dry cell and DC generators produce this current.

Alternating Current:

If the polarity of electrical sources changes continuously with the time, it is called alternating current (AC).AC generators produces it.

Differences between AC and DC current

AC	DC
The polarity changes continuously with time in it.	The polarity does not change with time in DC, i.e., it always l flows in one direction.
It is produced generators or dynamos.	It is produced by dry cells or batteries and DC generators.
Its frequency is 50Hz (in Nepal)	It has no frequency.

Fuse:

A fuse is a safety device having short length and thin wire of low melting point used for protecting electric circuit. It works s on heating effect of current. A fuse wire is connected in series in the circuit. The maximum amount of current which can pass through a fuse without breaking the circuit called fuse rating.

Q] The fuse is connected to live wire, why?

Ans] The fuse is connected to live wire. When large current flows through the circuit, the fuse wire gets heated because of its low melting point. As a result of this the circuits is broken, and hence no current flows through the electrical device and it remains safe.

Q] A copper wire cannot be used as a fuse, why?

Ans] A copper wire cannot be used as a fuse because it has high melting point. Due to this reason it does not melt easily when current overflows in it.

The fuses normally available for household purposes are 3A, 5A, 10A, 13A, 15A, etc.

Q] What is the size of fuse for an electric device of power 1 KW on 220V?

Solution

Here, Power (P) = 1 KW= 1000W

Voltage (V) =220V

Current (I) =?

We know, P = VI

Or,

The current is 4.54A.

Therefore, the capacity of the fuse should be 5A. (Ans)

Domestic Wiring:

Electricity generated form a power station is fed into house by two insulated wires, the live wire (L) and the neutral wire (N). The live wire ‘L’ is at high voltage 220V and the neutral wire (N) is at the ground of potential OV. The wire enters the box which contains m main fuse in the live wire and meter. The two wires form the meter box enters. Main switch box is that connected to the distribution board through a main switch box. Appliances like electric bulb, fans, and three pin sockets are connected from the distribution box as shown in the figure.

The fuse in the meter box is called electricity authority’s main fuse and the fuse after the main switch box is called consumer’s fuse. The live terminal of each socket is connected to the live wire, earth wire. A switch is kept in the live wire of three pin socket.

Some important rules for household wiring are:

· The fuse and all the switches should be connected in live wire in order to break the power supply in the circuit when the fuse is broken or the switch is off.

· The fuse wire should be of proper rating and proper material.

· There should be separate fuses in each room or floor.

· There should be separate circuits. They are light circuit and power circuit. In case fuse of power circuit is blown, the light circuit still works.

· Wiring should not be done in a damp area or in the region where water leaks on the wire. It prevents leakage of electricity and the chance of electric shocks.

Hazards or dangers of electricity are:

· If a person touches a live wire, he/she gets a severe electric shock.

· Short-circuit due to damaged wiring or overloading of the circuit may cause electrical fires in a building.

· The defects in the household wiring like loose connections and defective switches, sockets and plugs can cause sparkling and leads to fires.

· The phase wire is a live wire. In the live wire the indicator of the line tester glows, while in other wires the line tester does not glow.

· The colour codes used in household wiring is given in the table:

Colour Codes

Wire	Colour codes
Phase/live	Red, Brown
Neutral	Blue, Black
Earthing	Green, Green with yellow strip

Grounding /Earthing

Grounding /Earthing is the process of connecting earth- wire to the metal body or frame of electrical appliance. It is a safety device for the consumers, which protect them from electric shocks.

Q] A person wearing shoes with rubber sole does not get electric shock even when he touches the live wire, why?

Ans] A person wearing shoes with rubber sole does not get electric shock even when he touches the live wire, because the current will not pass through the body due to insulation between the body and the ground. But if the person is on the ground (earth) or touching the neutral line, the person gets and electric shock because the earth and our body both are conductors of electricity.

Effects of Electricity

Current electricity has following effects

1. Heat effect

2. Light effect

3. Magnetic effect

4. Chemical effect

Heating Effect:

When electric current is passed through a conductor, it gets heated up. This is called heating effect of electric current.

Nichrome, a heating element, is an alloy of nickel and chromium. Its resistance is very high and it does not oxidize even at very high temperature, therefore it is used as a heating element. Its melting point is also very high.

Lighting effect:

When electric current is passed through a conductor, it becomes very hot due t which it emits light. This is called lighting effect of electric current, e.g., electric bulbs, fluorescent lamps, other lighting devices, etc.

Filament lamp:

The structure of a filament lamp is as shown in the diagram. When electric current is passed in it, due to more resistance the tungsten filament gets heated to white and emits light. Its working capacity is about 1000 hours. It converts about 10% of electrical energy into light energy. The rest changes into heat energy.

Fluorescent lamp:

The structure of fluorescent lamp is shown in the diagram. When electric current is passed in it, the choke magnifies the voltage and the starter provides the complete circuit before the mercury is ionized. After the ionization of mercury, the starter disconnects the circuit and the ionized mercury completes the circuit. It also produces ultraviolet rays. The rays make the fluorescent powder bright, which emits light. It changes more parts of electrical energy into high energy and its working capacity is about 3,000 hours only.

Q] Tungsten filament is used in an electric bulb, why?

Ans] Tungsten filament is used in an electric bulb because of its high resistance. When electric current is passed through the filament of t tungsten, it attains high temperature and emits light. Its melting point is also very high (3400^oC).

Q] Electric bulbs (or filament lamps) are filled with inert gases like nitrogen or argon, why?

Ans] Electric bulbs (or filament lamps) are filled with inert gases like nitrogen or argon in order to prevent oxidation at high temperature due to which the filament remains safe.

Q] Fluorescent lamps are filled with mercury vapour, why?

Ans] Fluorescent lamps are filled with mercury vapour in order to produce ultraviolet rays, which are emitted due to the flow of electrons through the mercury vapour. The fluorescent powder absorbs the ultraviolet rays and emits visible light.

Q] When electric current is passed in a filament lamp, the filament is heated to bright but not the other wires, why?

Ans] When electric current is passed in a filament lamp, the filament is heated to bright but not the other wires because the filament is very thin and has high resistance. Due to high resistance the filament converts electrical energy into heat and light but not the other wires.

Differences between filament lamps and fluorescent lamp

Filament lamps	Fluorescent lamps
It is an electric lamp made up of a glass bulb, inert gas, filament, etc.	It is an electric lamp made up of fluorescent powder, mercury vapour, glass tube, electrodes, etc.
It converts 10% light and 90% heat of total electrical energy.	It can convert 30% light of total electrical energy.
Its average life is shorter, i.e. about 1000 hours only.	Its average life is longer, i.e. about 3000 hours
It is less efficient and cheap	It is more efficient and expensive
Tungsten filament gets heated and emits light.	Fluorescent powder absorbs UV rays obtained from Hg and emits light.

Magnetic Effect:

When an electric current is passed through a conductor, it behaves like a magnet. This is called magnetic effect of current.

Magnetic effect of electricity was discovered by Hans Christian Oesrted in 1819 AD

Magnetic field around a straight wire is found in the form of rings. The direction of lines of force is determined by using magnetic compass or by Maxwell’s screw rule or by right hand thumb rule.

Electromagnet:

When an iron bar is wrapped with an insulated wire and the current is passed through it, the iron bar changes into a magnet. The temporary magnet so formed is called an electromagnet.

The strength of an electromagnet can be increased by

o Increasing the number of turns in the coil.

o Increasing the amount of current passing through the wire.

Decreasing the length of air gap between the poles of a ‘U’ shaped magnet.

Electromagnets are used in electric bells, electric motors, loudspeakers, the receivers of telephones etc.

Advantages of an electromagnet over a permanent magnet are

o It is temporary.

o Its strength can be altered.

o It can be modified into any shape.

Electric Bell:

When electric current is passed in an electric bell the electromagnet pulls the hammer towards the gong and the bell rings. On this condition, the circuit gets disconnected at point of contact ‘X’ and the electromagnet gets demagnetized. The soft magnetic plate touches at ‘X’ again and the circuit becomes closed. The process repeats again and again till the switch is on. It causes the bell to ring.

Chemical Effect:

When current is passed through a liquid and it under goes chemical, the effect is called the chemical effect of current.

Q] A few of sulphuric acid is added for the electrolysis of water, why?

Ans] Pure water does not conduct electricity therefore, a few of sulphuric acid is added for the electrolysis of water.

Electrolysis of water:

· O₂ gas collects at anode and H₂O gas collects at cathode.

· The volume of H₂ is twice than the volume of O₂.

· The diagram for electrolysis of water is given below:

· Water is ionized as follows:

H₂OàH⁺+OH^- (Ionization)

Reaction at anode	Reaction at cathode
OH^-becomes neutral (OH) by losing electron which later forms molecules of water and oxygen. OH^-à OH+e^- 4OHà2H₂O +O₂	H⁺ becomes neutral (H) by gaining electron which later forms molecules of hydrogen H⁺ à e^- àH 4H –> 2H₂

Q] For electrolysis of water, copper anode should not be used, why?

Ans] For electrolysis of water, copper anode should not be used because nascent oxygen formed during the process reacts with copper to form copper oxide due to which the amount of oxygen will be reduced. Therefore, copper anode should be replaced by platinum or graphite anode.

Note:

Hydrogen gas does not react with copper; therefore copper cathode may be used in electrolysis of water.

The diagram for electroplating of silver on spoon is given below:

Electroplating:

The process of coating a conducting surface with metal by passing electric current in it is called electroplating.

Some tips for electroplating

· The object to be electroplated to be electroplated should be placed at cathode.

· The electrolyte should be a water-soluble salt of the metal, which is to be polished, e.g., for copper plating copper sulphate, for silver plating silver nitrate and for gold plating AuCl₃etc.

The metal by which we have to coat is made anode.

Application of electrolysis:

· To obtain pure metals form the related ores.

· For electroplating of copper, silver, gold. etc.

· For the extraction of sodium, calcium magnesium, etc.

· In the manufacture of H₂ and O₂.

Electromagnetic Induction:

The phenomenon by which the electric current is produced in a closed coil due to relative motion between the coil and a magnet is called electromagnetic induction. The electric current so produced is called induced current.

Faraday’s Laws of Electromagnetic induction:

· Whenever the magnetic flux linked with a closed circuit changes, an induced emf is produced in the circuit.

· The induced emf lasts as long as the change in magnetic flux continues in the circuit.

· The induced emf is directly proportional to the rate of change of magnetic flux in the circuit.

· Induced emf ∞ the rate of change of magnetic flux linked with the circuit.

Magnetic flux:

Magnetic flux through a surface is the number of magnetic lines of force crossing through the surface of coil placed perpendicular to the line of forces.

Fleming’s right hand rule:

When a conductor moves across a magnetic field, it cuts the lines of force and an emf is induced current is given by Fleming’s right hand rule. The rule states that if the first three fingers of the right hand are held mutually perpendicular to each other, the fore finger, the thumb and the central finger give the direction of the magnetic field, motion of the coil and the induced current respectively.

Electrical generator or dynamo:

The electrical device to produce electric current or electricity is known as electrical generator or dynamo. It converts mechanical energy into electrical energy.

A device which generated small amount of AC is known as dynamo and the device, which generates large amount of AC, is known as generator.

Ways of increasing emf or voltage in a generator or dynamo:

The induced emf (or voltage) of generator or dynamo can be increased by:

· Increasing the number of turns in the coil.

· Increasing strength of magnetic field.

· Wounding coil on a soft iron core.

· Increasing speed of rotation of the magnet near the coil

· Decreasing the distance between the coil and the magnet.

Electric motor:

An electric motor is a device which converts electrical energy to mechanical energy. It is based on the principle of motor effect. When a current carrying conductor is kept in a magnetic field, the conductor moves.This is called motor effect.

Differences between electric motor and generator

Electric Motor	Generator
It converts electrical energy into mechanical energy.	It converts mechanical energy into electrical energy.
It is based on the principle motor effect.	It is based on the principle of electromagnetic induction
Fleming’s left hand rule gives the direction of motions of the conduction.	Fleming’s right hand rule gives the direction of induced current through the conductor.
It is used in fans, cars etc.	It is used in bicycles, power plants etc.

Fleming’s left hand rule:

The motion of an electric motor is given by Fleming’s left hand rule. The rule states that if the first three fingers are mutually perpendicular to each other, the index finger, the middle finger and the thumbs represent the direction of magnetic field, current and the motion of the conductor respectively.

Transformer:

A transformer is a device which converts low alternating voltage at high current into high alternating voltage at low current and vice versa. In practice, it is used to increase or decrease alternating emf.

The transformer which changes high voltage alternating current to low voltage alternative current is called step down transformer. They are used at power substations and in streets to lower the supplied AC voltage.

The transformer which changes low voltage alternative current to high voltage alternative current is called step up transformer. They are used at power stations.

A transformer works on the principle of mutual induction i.e., when a change in current or magnetic flux takes place in one coil, induced emf is produced in the other coil.

The coil connected to the a.c., source is called primary coil and the coil connected to the output is called secondary coil.

The current in the primary coil is called input current and the voltage across the primary coil is called the input voltage or primary voltage V_P.

The current in the secondary coil is called output current and the voltage across the secondary coil is called the output voltage or secondary voltage V_S.

The relation between primary and secondary voltage in a transformer is given by

Q] The core of a transformer is laminated, why?

Ans] The core of a transformer is laminated in order to reduce the eddy-current loss to prevent heating of transformer due to which out put is increased.

Eddy current:

Eddy current is the induced current set up in a conductor by a changing magnetic field.

Differences between step up & step-down transformer

Step up Transformer	Step down transformer
It changes low voltage ac into high voltage	It changes high voltage ac into low voltage
The number of turns in the secondary coil is more than that in the primary coil	The number of turns in the secondary coil is less than that in the primary coil
It is used between electric tower and power generating station	It is used between the high power transmission line houses

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Saturday, June 16, 2012

Science Notes For SLC Students

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