Class 10 Science Chapter 16 Sustainable Management of Natural Resources

Natural resources 

 Natural resources are the resources available in a nature like air, water, sunlight, soil, minerals, forests, wild life etc.Natural resources are of two main types. They are renewable and non-renewable natural resources.

 i) Renewable natural resources 

 Those resources which can be replenished in a short period of time like air, water, sunlight, forests etc.

ii) Non-renewable natural resources :-

 Those resources which cannot be replenished in a short period of time like minerals (coal, petroleum, natural gas, metals etc.) because they take millions of years to be formed. Human activities produce a lot of waste materials which are thrown away into the environment. These wastes cause pollution of natural resources like air, water and soil

The Three R’s to save the environment :-

 The three R’s to save the environment are Reduce, Recycle and Reuse.

 i) Reduce :- means using less of natural resources and avoiding wastage of natural resources.

ii) Recycle :- means the materials like paper, plastic, glass, metalse tc used for making things can again be used for making new thingsi nstead of synthesising or extracting new paper, plastic, glass orm etals.

iii) Reuse :- means using things again and again like the plastic bottles in which we buy jams, pickles etc can be again used for storing things in the kitchen.

Need for management of natural resources :-

 All the things we use and consume are obtained fromn atural resources. Due to increase in population, industrialisation and urbanisation the demand for natural resources is increasing and their availability is limited . So there is a need for proper management of natural resources.

 The proper management of natural resources consists of :-

 i) Judicious use of natural resources and avoiding wastage of natural resources.

 ii) Long term planning for the use of natural resources so that it last not only for the present but also for future generations.

iii) The exploitation of natural resources should not be for the benefit of a few people but should be distributed equally for all.

iv) While extracting and using natural resources we should also plan for the safe disposal of wastes so that no damage is caused to the environment.

Forests :-

Importance of forests :- 

 i) Forests help to preserve biodiversity.

 ii) Forests are natural habitats of plants and animals. 

 iii) Forests provide timber, wood, fuel, medicines, fodder, etc.

 iv) Forests help to maintain ecological balance.

 v) Forests help to control climate and rainfall.

 vi) Forests help to prevent soil erosion and controls floods.

vii) Forests help to maintain the oxygen–carbon dioxide balance in nature.

Stake holders of forests :-

 People who are associated with forests directly or indirectly are :-

 i) People living in and around forests depend on forests for their livelihood.

 ii) Industrialists who use the raw materials from forests for manufacturing 

 paper, medicines, furniture etc. 

iii) Forest Department of the Government who owns the forests and controls the 

 resources from the forests.

iv) Nature and wild life organisations who want to conserve and preserve 

 forests.

Conservation of forests :-

 Forests can be conserved by :-

 i) Afforestation – planting of more trees.

 ii) Preventing or reducing deforestation.

iii) Preventing over grazing by cattle.

iv) By setting up wildlife sanctuaries, national parks, biosphere reserves etc.

 v) Undertaking social forestry programs like Van Mahotsav, Chipko

 movement for planting and protecting trees on a large scale.

Wildlife :-

a) Importance of wildlife :-

 i) Wildlife helps to preserve biodiversity.

 ii) Wildlife helps to maintain food chains and food web.

iii) We get useful products from wildlife like food, medicines, leather, bones,                honey, lac etc.

b) Conservation of wildlife :-

 i) Preserving the natural habitats of animals.

 ii) Banning poaching of animals.

iii) Protecting endangered species of animals.

iv) Setting up of wildlife sanctuaries, national parks, biosphere reserves etc.

Water :-

a) Uses of water :-

 Water is a basic necessity for all living things. We use water for our daily needs, for agriculture, transportation, construction of buildings, roads, dams etc. Water is a natural habitat for aquatic organisms.

Human activities are affecting the availability of water and causing pollution of water bodies.

b) Dams :-

 Advantages of dams :-

 i) Irrigation of crops.

 ii) Producing electricity.

iii) Supplying water to towns and cities.

iv) To control floods.

 Disadvantages of dams :-

 i) Social problems :- It displaces a large number of people who have to be rehabilitated.

ii) Economic problems :- It is expensive and uses a huge amount of public money.

iii) Environmental problems :- It causes deforestation and loss of biodiversity.

Water harvesting (Rain water harvesting) :-

 Water harvesting is collecting and storing rain water for future use.

 The common methods of water harvesting are :-

 i) Digging pits, ponds, lakes etc.

 ii) Building small earthen dams or concrete check dams.

iii) Construction of dykes.

iv) Construction of reservoirs.

 v) Construction of rooftop collecting units in houses.

 Advantages of underground water :-

 i) It does not evaporate easily.

 ii) It spreads out and recharges wells.

iii) It provides moisture for irrigation of crops.

iv) It does not get polluted easily.

v) It does not provide breeding ground for mosquitoes and houseflies.

Coal and petroleum :-

 Coal and petroleum are fossil fuels formed by the decomposition of dead plants and animals inside the earth after several millions of years. They are non-renewable sources of energy. Petroleum reserves may last for about 40 years and coal reserves may last for about 200 years. Coal and petroleum contain carbon, hydrogen, nitrogen and sulphur. When they are burnt, they release carbon dioxide and oxides of nitrogen and sulphur. Carbon dioxide is a green house gas which causes global warming. Oxides of nitrogen and sulphur combines with moisture in the air and produces acid rain

Class 10 Science Chapter 15 Our Environment

Class 10 Science

Chapter 15 Our Environment 

Introduction

→ Everything that surrounds us is environment. It includes both living (biotic) and non-living (abiotic) components.

→ Interaction between these biotic and abiotic components form an ecosystem.

→ In an ecosystem living components depend on each other for their food which give rise to food chains and food webs in nature.

→ Human activities lead to environmental problems such as depletion of ozone layer and production of huge amount of garbage.

Ecosystem

→ All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem. E.g., forest, pond etc.

Types of ecosystem

It is of two types

(i) Natural ecosystem: The ecosystem which exist in nature on its own.

Example: forest, lake, ocean.

(ii) Artifical ecosystem: Man-made ecosystems are called artificial ecosystem.

Example: crop field, aquarium, garden.

(i) Abiotic Components: All the non-living components such as air, water, land, light, temperature etc. form the abiotic components.

(ii) Biotic Components: All the living components such as plants, animals, bacteria, fungi etc. form the biotic components.

• On the basis of nutrition biotic components are further divided into:

→ Producers: All green plants and blue-green algae can produce their own food using abiotic components (photosynthesis), hence called producers.

→ Consumers: Include all animals which depend on producers directly or indirectly for their food.

Division of Consumers

(i) Herbivores: Plant eaters. Example:  goat, deer.

(ii) Carnivores: Flash eaters. Example: tiger, crocodile.

(iii) Omnivores: Eats both plants and animals. Example: human.

(iv) Parasites: Live on the body of host and take food from it. Example: lice, cascuta.

Decomposers: Include organisms which decompose the dead plants and animals. Example: bacteria, fungi. These help in the replenishment of natural resources.

Food Chain

→ Food chain is a series of organisms in which one organism eats another organism as food. For example:

• Grass → Deer → Lion

→ In a food chain various steps where transfer of energy takes place is called a trophic level.

Flow of energy between trophic levels

→ Flow of energy in a food chain is unidirectional.

→ Green plants capture 1% of sunlight and convert it into food energy.

→ 10 percent law : Only 10% of energy is transferred to the next trophic level. The remaining 90% energy is used in life processes (digestion, growth, reproduction etc.) by present trophic level.

→ Due to this gradual decrease in energy, food chains contain 3-4 trophic levels.

Trophic levels

trophic level, step in a nutritive series, or food chain, of an ecosystem. 

• Decrease in energy1 kJ ↣ 10 kJ ↣ 100 kJ ↣ 1000 kJ

Biological magnification: The concentration of harmful chemicals increases with every next trophic level in a food chain. This is called biological magnification.

→ Maximum concentration of such chemicals get accumulated in human bodies as human occupy the top level in any food chain.

Food web

→ In nature large numbers of food chains are interconnected forming a food web.

• Environmental problems:

Changes in the environment affect us and our activities change the environment around us. Human activities leads to pollution, deforestation etc.

Ozone layer

→ Ozone layer is a protective blanket around the earth which absorbs most of the harmful UV (ultraviolet) radiations of the sunlight, thus protecting living beings from many health hazards such as skin cancer, cataract, destruction of plants etc.

→ Ozone (O3) layer is present at higher levels of atmosphere (i.e. stratosphere). It is a deadly poison at ground level.

Formation of ozone molecule

(i) The high energy UV radiations break down the O2 molecules into free oxygen (O) atoms.

O →(UV)-->  O + O (atoms)

(ii) These oxygen atoms then combine with oxygen (O2) molecule to form the ozone molecule.

O2 + O → O3 (ozone)

Depletion of ozone layer

→ The decrease in the thickness of ozone layer over Antarctica was first observed in 1985 and was termed as ozone hole.

→ This decrease was linked to excessive use of synthetic chemicals like chlorofluorocarbons (CFCs) which are used in refrigerators, ACs, fire-extinguishers, aerosols sprays etc.

→ United Nations Environment Programme (UNEP) succeeded in forging an agreement to stop CFC production at 1986 levels (KYOTO PROTOCOL) by all countries.

Garbage disposal

→ Improvements in lifestyle have resulted in accumulation of large amounts of waste materials.

Types of materials in Garbage

(i) Biodegradable: Substances which can be decomposed by the action of micro-organisms are called biodegradable wastes.

Example:  fruit and vegetable peels, cotton, jute, dung, paper, etc.

(ii) Non-biodegradable wastes:

Substances which cannot be decomposed by the action of micro-organisms are called non-biodegradable wastes.

Example: plastic, polythenes, metals, synthetic fibres, radioactive wastes, pesticides etc.

→ Micro-organisms release enzymes which decompose the materials but these enzymes are specific in their action that’s why enzymes cannot decompose all the materials.

Methods of waste disposal

(i) Biogas plant: Biodegradable waste can be used in biogas plant to produce biogas and manure.

(ii) Sewage treatment plant: The drain water can be cleaned in sewage treatment plant before adding it to rivers.

(iii) Land fillings: The wastes are buried in low lying areas and are compacted by rolling with bulldozers.

(iv) Composting: Organic wastes are filled in a compost pit and covered with a layer of soil, after about three months garbage changes to manure.

(v) Recycling: Non-biodegradable wastes are recycled to make new items.

(vi) Reuse: It is a conventional technique to use an item again. Example: newspaper for making envelops.

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Class 10 Science Chapter 14 Sources of Energy

Class 10 Science

Chapter 14 Sources of Energy 

Introduction

→ Energy comes in different forms and one form can be converted into another.

→ A source of energy is one which provide adequate amount of energy in a convenient form over a long period of time.

Need of energy

→ For making food

→ For lightning

→ For transport

→ For running machines

→ For industrial activities and agricultural work

Qualities of a Good Source of Energy

(i) Which would do a large amount of work per unit mass.

(ii) Cheap and easily available.

(iii) Easy to store and transport.

(iv) Safe to handle and use.

(v) Does not cause environmental pollution.

• Fuels: The materials which are burnt to produce heat energy are known as fuels.

Example: wood, coal, LPG, kerosene.

Characteristics of a Good Fuel

→ High calorific value (give more heat per unit mass).

→ Burn without giving out any smoke or harmful gases.

→ Proper ignition temperature.

→ Cheap and easily available.

→ Easy to handle, safe to transport

.→ Convenient to store.

→ Burn smoothly.

Sources of Energy

Conventional Sources of Energy

→ Sources of energy which are known to most of the people.Example:

fossil fuels, bio massFuels

Fossil Fuels

→ Fuels developed from the fossils e.g., coal, petroleum.

→ Take millions of years to form.

→ Available in very limited amount.

→ These are non-renewable sources of energy.

• India has about 6% share in the world reserved coal, that may last 250 years more at the present rate of consumption.

Pollution Caused by Fossil Fuels

→ Released oxides of carbon, nitrogen and sulphur (acidic in nature) which causes acid rain that damages trees, plants, reduces fertility of soil.

→ Produces large amount of CO2 in the atmosphere which causes green house effect leading to excessive heating of the earth.

Controlling Pollution Caused by Fossil Fuels

→ Increasing the efficiency of the combustion process.

→ Using various techniques to reduce the escape of harmful gases and ashes into the surroundings.

thermal power plant

→ A power plant which uses heat energy to generate electricity.

→ Burning of fossil fuels produces steam to run turbines.

→ Set up (power plants) near the coal and oil fields to minimize the cost of transportation and production.

→ Transmission of electricity is more efficient.

Hydro Power Plants

→ Convert the potential energy of falling water into electricity.

→ Hydro power plants are associated with Dams.

→ Around 25% of our country’s energy requirement is met by Hydro Power plants.

Advantages of Hydro power plants

→ No environmental pollution.

→ Flowing water is a renewable source of electric energy.

→ Construction of dams prevents flooding of rivers, provide water for irrigation.

Disadvantages of Hydro Power Plants

→ Large areas of agricultural land, a vast variety of flora and fauna, human settlements get submerged in the water of reservoir formed by the dam.

→ Large ecosystems are destroyed.

→ Vegetation that submerged under water rots under anaerobic conditions and produces large amount of methane which is a green house gas.

→ Creates the problems of satisfactory rehabilitation of displaced people.

Biomass

→ The dead parts of plants and trees and the waste materials of animals and man are called Biomass.

(i) Wood: It is a biomass and used as a fuel for a long time.

 Disadvantages of using wood as fuel

→ Produces a lot of smoke on burning.

→ Do not produce much heat.

→ Thus by improvement in technology we can improve the efficiency of traditional sources of energy.

For example: wood can be converted into much better fuel called charcoal.

(ii) Charcoal: When wood is burnt in limited supply of air, then water and other volatile materials gets removed and charcoal is formed.

→ Wood on limited supply of Oxygen gas get converted into Charcoal.

Charcoal is better fuel than wood because

→ It has higher calorific value than wood.

→ Does not produce smoke while burning.

→ It is a compact fuel, easy to handle and convenient to use.

(iii) Cowdung: It is biomass but it is not good to burn cow dung directly as fuel because :

→ produces lot of smoke.

→ cowdung does not burn completely, produces lot of ash as residue.

→ low calorific value.

→ by making bio gas (or gobar gas) from cow dung, we get a smokeless fuel.

(iv) Bio gas: It is produced in a biogas plant.

→ Anaerobic micro organisms decomposes the complex compound of the cow dung+water slurry.

→ It takes few day for the decomposition process and generate gases like methane, CO2, hydrogen and hydrogen sulphide.

→ Bio gas is stored in the gas tank above the digester from which they are drawn through pipes for use.

Advantages of Bio gas

→ It is an excellent fuel as it contains upto 75% methane (CH4 ).

→ It burns without smoke.

→ Leaves no residue like ash in wood & coal burning.

→ Heating capacity is high.

→ It is also used for lighting.

→ Slurry left behind is used as excellent manure rich in nitrogen and phosphorus.

→ Safe and efficient method of waste disposal.

(v) Wind energy

→ Unequal heating of the landmass and water bodies by solar radiations generate air movement and causes wind to blow.

• Uses of kinetic energy generated by wind energy

→ to generate electricity by turning the rotor of the turbine.

→ to lift water from the well.

→ to run the flour mills.

→ The output of a single wind mill is quite small so a number of windmills are erected over a large area called wind energy farm.

→ The minimum wind speed for wind mill to serve as a source of energy is 15-20 KmPH.

• Advantages of wind energy

→ Eco-friendly.

→ Efficient source of renewable energy.

→ No recurring expenses for production of electricity.

• Disadvantages of wind energy

→ Wind energy farms need large area of land.

→ Difficulty in getting regular wind speed of 15-20 KmPH.

→ Initial cost of establishing wind energy farm is very high.

→ High level of maintenance of blades of wind mill.

• Denmark is called the ‘Country of Winds’.

• India is ranked 5 th in harnessing wind energy for the production of electricity.

• In India largest wind energy farm has been established near Kanyakumari in Tamil Nadu and it generates 380 MW of electricity.

Alternate or Non-conventional Sources of Energy

→ Day by day, our demand for energy increases, so there is a need for another source of eenergy

Reasons for alternate sources of energy

→ The fossil fuel reserves in the earth  limited which may get exhausted soon if we use them at the current rate.

→ Reduce the pressure on fossil fuels making them last for a much longer time.

→ To reduce the pollution level and to save the environment.

(i) Solar Energy

→ Sun is the ultimate source of energy.

→ Energy obtained from the sun is called solar energy.Solar constant = 1.4 KJ/s/m2

→ Outer edge of the earth receives solar energy equal to 1.4 KJ/s/m2 or 1.4 KW/m2[... 1 KJ/s = 1 KW]

Solar energy devices:

Solar cooker

Solar cookers and water heaters use solar energy to operate.

Black surfaces absorb more energy than other surfaces and solar cookers use this property by coating their insides black.

They use reflecting surfaces like mirrors to focus the sun’s rays.

The device is covered with a glass plate thereby establishing the greenhouse effect by trapping heat inside the cooker.

Solar cell

A device that converts solar energy into electricity is known as a solar cell.

A typical solar cell produces a voltage of 0.5 1 V and 0.7 W of electrical power. A large number of such cells can combine to form a solar panel which can generate power large enough for practical uses.

Advantages:

(i) no moving parts

(ii) require little maintenance

(iii) can be set up in remote areas without the hassle and expenses of transmission lines.

Disadvantages:

(i) Requires a special grade silicon which is not easily available

(ii) Usage of silver for interconnections makes it expensive.

Uses: traffic signals, calculators, artificial satellites and space probes

Geothermal Energy

→ ‘Geo’ means ‘earth’ and ‘thermal’ means ‘heat’.

→ Geothermal energy is the heat energy from hot rocks present inside the earth.

→ When underground water comes in contact with ‘hot spot’, steam is generated. Steam trapped in rocks is routed through pipes to a turbine and used to generate electricity.

Advantages of Geothermal energy

→ Economical to use geothermal energy.

→ Does not cause any pollution.

Limitations of Geothermal energy

→ Geothermal energy is not available everywhere.

→ Deep drilling in the earth to obtain geothermal energy is very difficult and expensive.

→ In New Zealand and USA, there are no. of power plants based on geothermal energy are operational.

Nuclear Energy

→ The energy released during a nuclear reaction is called nuclear energy.

→ It can be obtained by two types of nuclear reactions :

(i) Nuclear fission

(ii) Nuclear fusion

(i) Nuclear Fission

• ‘Fission’ means split up.

→ The process in which the heavy nucleus of a radioactive atom (such as uranium, plutonium or thorium) split up into smaller nuclei when bombarded with low energy neutrons, is called nuclear fission.

→ A tremendous amount of energy is produced.

→ U-235 is used as a fuel in nuclear reactor in form of uranium rods.

Working of Nuclear Fission

→ In a nuclear reactor self sustaining chain reaction releases energy at a controlled rate, which is used to produce steam and further generate electricity.Major Nuclear Power Plants

(i) Tarapur (Maharashtra)

(ii) Rana Pratap Sagar (Rajasthan)

(iii) Kalpakkam (Tamil Nadu)

(iv) Narora (U. P.)

(v) Kakrapar (Gujrat)

(vi) Kaiga (Karnataka)

(ii) Nuclear Fusion

→ When two nuclei of light elements (like hydrogen) combine to form a heavy nucleus (like helium) and tremendous amount of energy is released is called nuclear fusion.

→ Very-very high temperature and pressure is needed for fusion.

→ Hydrogen bomb is based on this phenomenon.

→ Nuclear fusion is the source of energy in the sun and other stars.

Advantage of Nuclear Fusion

→ Production of large amount of useful energy from a very small amount of nuclear fuel.

→ Does not produce green house gases like CO2.Limitations of Nuclear Fusion

→ Environmental contamination due to improper nuclear waste storage and its disposal.

→ Risk of accidental leakage of harmful radiations.

→ High cost of installation.

→ Limited availability of nuclear fuel.

Environmental Consequences

→ Exploiting any source of energy disturbs the environment in some way or the other.

→ The source we would choose depends upon following the factors:

(i) Ease of extracting energy from the source.

(ii) Cost of extracting energy from the source.

(iii) Efficiency of technology available to extract energy.

(iv) The environmental damage caused by using that source.

→ In other words, no source of energy is said to be pollution free. Some source are cleaner than the other.

→ For example, solar cells may be pollution free but the assembly of the device would havecause some environmental damage.

Non-renewable Sources of Energy:

Sources that will get depleted some day.

For example : Fossil fuel

Renewable Sources of Energy:

Energy sources that can be regenerated and that will last for ever.

For example : Wind energy, water energy

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Class 10 Science Chapter 13 Magnetic Effects of Electric Current

Class 10 Science

Chapter 13 Magnetic Effects of Electric Current 

Introduction

→ Magnet is any substance that attracts iron or iron-like substances.

→ An electric current-carrying wire behaves like a magnet.

→ Electromagnets and electric motors involve the magnetic effect of electric current, and electric generators involve the electric effect of moving magnets.

→ Compass needle get deflected on passing an electric current through a metallic conductor.

Properties of Magnet

(i) Every magnet has two poles i.e. North and South.

(ii) Like poles repel each other.

(iii) Unlike poles attract each other.

(iv) A freely suspended bar magnet aligns itself in nearly north-south direction, with its north pole towards north direction.

Characteristics of Field Lines

→ Field lines arise from North pole and end into South pole of the magnet.

→ Field lines are closed curves.

→ Field lines are closer in stronger magnetic field.

→ Field lines never intersect each other as for two lines to intersect, there must be two north directions at a point, which is not possible.

→ Direction of field lines inside a magnet is from South to North.

→ The relative strength of magnetic field is shown by degree of closeness of fieldlines.

Magnetic Field of a Bar Magnet

→ H. C. Oersted was the first person to state that electric current has magnetic field.

Right Hand Thumb Rule

→ Imagine you are holding a current carrying straight conductor in your right hand such that the thumb is pointing towards the direction of current.

→ Then the fingers wrapped around the conductor give the direction of magnetic field.

Magnetic Field due to Current through a Straight Conductor

→ It can be represented by concentric circles at every point on conductor.

→ Direction can be given by right hand thumb rule or compass.

→ Circles are closer near the conductor.

→ Magnetic field ∝ Strength of current.

→ Magnetic field ∝ 1/Distance from conductor

Magnetic Field due to Current through a Circular Loop

→ It can be represented by concentric circle at every point.

→ Circles become larger and larger as we move away.

→ Every point on wire carrying current would give rise to magnetic field appearing as straight line at centre of the loop.

→ The direction of magnetic field inside the loop is same.

Factors affecting magnetic field of a circular current carrying conductor

→ Magnetic field ∝ Current passing through the conductor

→ Magnetic ∝ 1/Distance from conductor

→ Magnetic field ∝ No. of turns in the coil

→ Magnetic field is additive in nature i.e., magnetic field of one loop adds up to magnetic field of another loop. This is because the current in each circular turn has some direction.

Solenoid

→ A coil of many circular turns of insulated copper wire wrapped closely in a cylindrical form.

→ Magnetic field of a solenoid is similar to that of a bar magnet.

→ Magnetic field is uniform inside the solenoid and represented by parallel field lines.

Direction of magnetic field

(i) Outside the solenoid: North to South

(ii) Inside the solenoid: South to North

→ Solenoid can be used to magnetise a magnetic material like soft iron.

Electromagnet

→ It is a temporary magnet, so, can be easily demagnetised.

→ Strength can be varied.

→ Polarity can be reversed.

→ Generally strong magnet.

Permanent Magnet

→ Cannot be easily demagnetised.

→ Strength is fixed.

→ Polarity cannot be reversed.

→ Generally weak magnet.

Force on a Current carrying Conductor in a Magnetic Field

Andre Marie Ampere suggested that the magnet also exerts an equal and opposite force on a current carrying conductor.

→ The displacement in the conductor is the maximum when the direction of current is at right angle to the direction of magnetic field.

→ Direction of force is reversed on reversing the direction of current.

Fleming’s Left Hand Rule

→ Stretch the thumb, fore finger and middle finger of your left hand such that they are mutually perpendicular.

→ If fore finger points in the direction of magnetic field, middle finger in the direction of current then thumb will point in the direction of motion or force.

→ Heart and brain in the human body have significant magnetic field.

• MRI (Magnetic Resonance Imaging): Image of internal organs of body can be obtained using magnetic field of the organ.

• Galvanometer: Instrument that can detect the presence of current in a circuit. It also detects the direction of current.

Electric motor

→ An electric motor is a rotating device that converts electrical energy to mechanical energy.

→ An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field.

→ The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle.

→ The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively.

→ Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.

→ The force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards.

→ Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise.

→ At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA.

→ The split ring acts as a commutator which reverse the direction of current and also reverses the direction of force acting on the two arms AB and CD.

→ Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down.

→ Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

• Commutator: A device that reverses the direction of flow of current through a circuit is called a commutator.

• Armature: The soft iron core, on which the coil is wound including the coils is called armature. It enhances the power of the motor.Commercial use of motors

(i) an electromagnet in place of permanent magnet

(ii) large number of turns of the conducting wire in the current-carrying coil

(iii) a soft iron core on which the coil is wound.

Electro Magnetic Induction

→ When a conductor is placed in a changing magnetic field, some current is induced in it.

→ Such current is called induced current and the phenomenon is called electromagnetic induction.

(i) Magnet moved into the coil: Momentary deflection in G indicating presence of current.

(ii) Magnet kept stationary inside the coil: No deflection.

(iii) Magnet is withdrawn: Momentary deflection in G but in opposite direction of first case.

(i) Switched on: Momentary deflection in G.

(ii) Steady current: No deflection.

(iii) Switched off: Momentary deflection in G but in opposite direction of the first case.

Fleming’s Right Hand Rule

→ Hold the thumb, the fore finger and the middle finger of right hand at right angles to each other.

→ If the fore finger is in the direction of magnetic field and the thumb points in the direction of motion of conductor, then the direction of induced current is indicated by middle finger.

• Working principle of electric generator.

• Used to find direction of induced current.

Electric Generator

→ An electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

→ An electric generator consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.

→ The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated.

→ The inner side of these rings are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.

→ The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.

→ Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

→ When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet.

→ After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA.

→ The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes.

• To get a direct current (DC), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down.

The direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

Alternate Current (A. C.)

→ The current which reverses its direction periodically.

→ In India, A. C. reverses its direction in every 1/100 second.

Time period = 1/100 + 1/100 = 1/50 s

Frequency = 1/time period = 1/50 = 50 Hz

Advantage of A.C.

→ A. C. can be transmitted over long distance without much loss of energy.

Disadvantage of A.C.

→ A. C. cannot be stored.

Direct Current (D. C.)

→ The current which does not reverse its direction.

→ D. C. can be stored.

→ Loss of energy during transmission over long distance is high.

→ Sources of D. C.: Cell, Battery, Storage cells.

Domestic Electric Circuits

→ There are three kinds of wires used:

(i) Live wire (positive) with red insulation cover.

(ii) Neutral wire (negative) with black insulation cover.(iii)Earth wire with green insulation cover.

→ The potential difference between live and neutral wire in India is 220 V.

→ Pole ⇒ Main supply ⇒ Fuse ⇒ Electricity meter ⇒ Distribution box ⇒ To separate circuits

Earth Wire: Protects us from electric shock in case of leakage of current especially in metallic body appliances. It provides a low resistance path for current in case of leakage of current.

• Short Circuit: When live wire comes in direct contact with neutral wire accidentally. The resistance of circuit becomes low which can result in overloading.

• Overloading: When current drawn is more than current carrying capacity of a conductor, it results in overloading.

Causes of overloading

(i) Accidental hike in voltage supply.

(ii) Use of more than one appliance in a single socket.

Safety devices

(i) Electric fuse

(ii) Earth wire

(iii) MCB (Miniature Circuit Breaker)