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Notes

Q1.     Define work?

Ans:    Work:

Work is done when a force acting on a body displaces it in the direction of a force. Work is a scalar quantity. It depends on the force acting on a body, displacement of the body and the angle between them.

Work done = Force x displacement

Or                                                                 W = F S                              (i)

W = Fx x S

W = (F cos q) S

W = FS cos q                                            (ii)

Unit of work:

SI unit of work is joule (J). it is defined as

The amount of work is one joule when a force of one newton displaces a body through one metre in the direction of force.

Thus                              1 J = 1 N x 1 m

Q2. Derive the relation for work done by a force inclined with the displacement?

Ans:    Let the force F is making an angle q with the surface on the body is moved.

Resolving F into its perpendicular components Fx and Fy as;

F   =    F cos q

F   =    F sin q

In case when force and displacement are not parallel then only the x-component Fx parallel to the surface causes the body to move on the surface and not the y – component Fy .

Hence                                                       W   =   Fx x S

W   =    (F cos q) S

W     =    FS cos q

1. The rope is parallel to the road

Solution:        Force = 100 N

Distance = S = 10 m

Work = W = ?

W = F x S

W = 100 x 10 = 1000 J

1. The rope is making an angle of 300 with the road.

Solution:            Force = 100 N

Distance = S = 10 m

q    =   300

Work = W = ?

W     =    FS cos q

W = 100 x 10 cos(300)

W = 1000 x (0.866) = 866 J

Q3.      Define energy, give two types of mechanical energy.

Ans:    Energy:

A body possesses energy if it is capable to do work.

Types of mechanical energy:

Mechanical energy possessed by a body is of two types: kinetic energy and potential energy.

Q4.      Why do we need energy?

Ans:    See Q # 6.4 from Exercise

OR

Prove that K.E. =   m v 2.

Ans:    See Q # 6.6 from Exercise.

Q6.      Define potential energy and derive its relation?

Ans:     See Q # 6.7 from Exercise.

Q7.      List the different forms of energy with examples?

Ans:     Forms of Energy:

Energy exists in various forms. Some of the main forms of energy are given:

1. Mechanical energy
2. Heat energy
3. Electrical energy
4. Sound energy
5. Light energy
6. Chemical energy
7. Nuclear energy

Q8.     Describe mechanical energy with examples?

Ans:    Mechanical energy:

The energy possessed by a body both due to its motion or position is called mechanical energy.

Examples:

Water running down a stream, wind, a moving car, a lifted hammer, a stretched bow, a catapult or a compressed spring etc. possesses mechanical energy.

Q9.     Describe heat energy with examples?

Ans:    Heat energy:

Heat is a form of energy given out by hot bodies. A large amount of heat is obtained by burning fuel. Heat is also produced when motion is opposed by frictional forces. The foods we take provide us with heat energy.

Examples:

The Sun is the main source of heat energy.

Q10.   Describe electrical energy with examples?

Electrical energy:

Electricity is one of the widely used forms of energy. Electrical energy can be supplied easily to any desired place through wires.

Examples:

We get electrical energy from batteries and electric generators. These electric generators are run by hydro power, thermal or nuclear power.

Q11.     Describe sound energy with examples?

Ans:      When you knock at the door, you produce sound. A sound is a form of energy.

Examples

It is produced when a body vibrates; such as vibrating diaphragm of a drum, vibrating strings of a sitar and vibrating air column of wind instruments such as flute pipe etc.

Q12.      Describe light energy with examples?

Ans:       Light energy:

Light is an important form of energy. Name some sources of light that you come across.

Plants produce food in the presence of light. We also need light to see things.

Examples:

We get light from candles, electric bulbs, fluorescent tubes and also by burning fuel. However, most of the light comes from the Sun.

Q13.      Describe chemical energy with examples?

Ans:       Chemical energy:

Chemical energy is present in food, fuels and other substances. We get other forms of energy from these substances during chemical reactions.

The burning of wood, coal or natural gas in air is a chemical reaction which releases energy as heat and light. Electric energy is obtained from electric cells and batteries as a result of a chemical reaction between various substances present in them. Animals get heat and muscular energy from the food they eat.

Q14.      Describe nuclear energy with examples?

Ans:       Nuclear energy:

Nuclear energy is the energy released in the form of nuclear radiations in addition to heat and light during nuclear reactions such as fission and fusion reactions. The heat energy released in nuclear reactors is converted into electrical energy.

Examples:

The energy coming from the Sun for the last billions of years is the result of nuclear reactions taking place on the Sun.

DO YOU KNOW

Our body gets energy stored in the food we take to perform various activities.

DO YOU KNOW

A nuclear power plant uses the energy released in a nuclear reactor such as fission to generate electric power.

Q15.     How is energy converted from one form to another? Explain.

Ans:      See Q # 6.10 from Exercise.

DO YOU KNOW?

Pole vaulter:

A pole vaulter uses a flexible vaulting pole made of special material. It is capable to store all the vaulter’s kinetic energy while bending in the form of potential energy. The vaulter runs as fast as possible to gain speed. The kinetic energy gained by the pole vaulter due to speed helps him/her to rise as the vaulter straightens. Thus, he attains height as the pole returns the potential energy stored by the vaulter in the pole.

Q16.     List major sources of energy?

Ans:      Major sources of energy:

1. Fossil fuels
2. Nuclear fuels
3. Energy from biomass
4. Energy from water
5. Energy from the sun
6. Solar house heating
7. Solar cells
8. Wind energy
9. Geothermal energy

Q17.      Differentiate energy resources as renewable and non-renewable resources of energy with examples of each?

Ans:       Renewable resources of energy:

Renewable sources of energy are those which can be reused. They do not get extinguished. They are environmentally friendly; they do not cause pollution.

Examples:

Solar energy, wind energy and tidal energy.

Non- renewable resources of energy:

Non-renewable sources of energy are those which cannot be reused. They get extinguished. They cause pollution and are environmentally harmful.

Q18.       List non-renewable sources of energy?

Ans:        i.       Fossil fuels                      ii.             Nuclear fuels

OR

Q20        Why fossil fuels are called non-renewable form of energy?

Ans         See Q # 6.8 from Exercise

OR

List the environmental issues associated with fossil fuels:

Ans:        Environmental issues associated with fossil fuels:

Fossil fuels release harmful waste products. These wastes include carbon mono-oxide and other harmful gases, which pollute the environment. This causes serious health problems such as headache, tension, nausea, allergic reactions, irritation of eyes, nose and throat. Long exposure of these harmful gases may cause asthma, lungs cancer, heart diseases and even damage to brain, nerves and other organs of our body.

OR

List the environmental issues associated with nuclear fuels?

Ans:       Nuclear fuels:

In nuclear power plants, we get energy as a result of the fission reaction. During fission reaction, heavy atoms, such as Uranium atoms, split up into smaller parts releasing a large amount of energy.

Nuclear power plants give out a lot of nuclear radiations and a vast amount of heat. A part of this heat is used to run power plants while a lot of heat goes waste into the environment.

Q23.      List non-renewable sources of energy?

Ans:       i.      Energy from water

1. Energy from the sun

iii.     Solar house heating

1. Solar cells
2. Wind energy
3. Geothermal energy

vii.     Energy from biomass

Q24.      How energy is obtained from water?

Ans:       Energy from water:

Energy from water power is very cheap. Dams are being constructed at suitable locations in different parts of the world. Dams serve many purposes. They help to control floods by storing water. The water stored in dams is used for irrigation and also to generate electrical energy without creating many environmental problems.

Q25.      How energy is obtained from the sun?

Ans:       Energy from the sun:

Solar energy is the energy coming from the Sun and is used directly and indirectly. Sunlight does not pollute the environment in any way. The sunrays are the ultimate source of life on Earth. We are dependent on the Sun for all our food and fuels. If we find a suitable method to use a fraction of the solar energy reaching the Earth, then it would be enough to fulfil our energy requirement.

Q26.       Explain the functioning of the solar house heating system?

Ans:        Solar house heating:

Complete solar house heating systems are successfully used in areas with a minimum amount of sunshine in winter. A heating system consists of:

1. A collector
2. A storage device
3. A distribution system

Working of solar heating system:

A Solar a solar collector made of glass panels over blank metal plates. The plates absorb the Sun’s energy which heats a liquid flowing in the pipes at the back of the collector. The hot water can be used for cooking, washing and heating the buildings.

Uses of solar energy:

Solar energy is used in solar cookers, solar distillation plants, solar power plant, etc.

Q27.     Describe the process of electricity generated by solar cells?

Ans:      Solar cells:

Solar energy can also be converted directly into electricity by solar cells. A solar cell also called photo cell is made from a silicon wafer.

When sunlight falls on a solar cell, it converts the light directly into electrical energy. Solar cells are used in calculators, watches and toys. Large numbers of solar cells are wired together to form solar panels.

Solar panels can provide power to telephone booths, light houses and scientific research centres. Solar panels are also used to power satellites.

Q28.      Describe the process of electricity generated by wind energy?

Ans:       Wind energy:

The wind has been used as a source of energy for centuries. It has powered sailing ships across the oceans. It has been used by windmills to grind grain and pump water.

More recently, wind power is used to turn wind turbines. When many wind machines are grouped on wind farms, they can generate enough power to operate a power plant.

In the United States, some wind farms generate more than 1300 MW of electricity a day.

Q29.      Describe the process of electricity generated by geothermal energy?

Ans:       Geothermal energy:

In some parts of the world, the Earth provides us with hot water from geysers and hot springs. There is a hot molten part, deep in the Earth called magma. Water reaching close to the magma changes to steam due to the high temperature of magma. This energy is called geothermal energy.

Geothermal well can be built by drilling deep near hot rocks at places, where magma is not very deep. Water is then pushed down into the well. The rocks quickly heat the water and change it into steam. It expands and moves up to the surface. The steam can be piped directly into houses and offices for heating purposes or it can be used to generate electricity.

Q30.     Describe the process of electricity generated by biomass?

Ans:      Energy from biomass:

Biomass is plant or animal wastes that can be burnt as fuel. Other forms of biomass are garbage, farm wastes, sugarcane and other plants. These wastes are used to run power plants. Many industries that use forest products get half of their electricity by burning bark and other wood wastes. Biomass can serve as another energy source, but problems are there in its use.

When animal dung, dead plants and dead animals decompose, they give off a mixture of methane and carbon dioxide. Electricity can be generated by burning methane.

Q31.     State mass-energy equation E = mc2.

Ans:      Mass-energy equation:

Einstein predicted the interconversion of matter and energy. According to him, a loss in the mass of a body provides a lot of energy. This happens in nuclear reactions. The relation between mass m and energy E is given by Einstein’s mass-energy equation.

E = mc2

Here c is the speed of light (3×108 ms – 1 ). The above equation shows that a tremendous amount of energy can be obtained from a small quantity of matter.

Q32.      Explain by drawing flow diagram electricity from fossil fuels.

Ans:      Electricity from fossil fuels:

Most of the electricity is obtained using fossil fuels such as oil, gas and coal. Fossil fuels are burnt in thermal power stations to produce electricity. Various energy conversion processes involved in producing electricity from coal are described in a block diagram.

Q33.      List the environmental issues associated with energy. Highlight the solution to these issues?

Ans:       Energy and environments:

Environmental problems such as pollution that consist of noise, air pollution and water pollution may arise by using different sources of energy such as fossil fuels and nuclear energy.

Pollution:

Pollution is the change in the quality of the environment that can be harmful and unpleasant for living things.

Thermal Pollution:

A temperature rises in the environment that disturbs life is called thermal pollution. Thermal pollution upsets the balance of life and endangers the survival of many species.

Air pollutants:

Air pollutants are unwanted and harmful. Natural processes such as volcanic eruptions, forest fires and dust storms add pollutant to the air. These pollutants, rarely build up to harmful levels. On the other hand, the burning of fuel and solid wastes in homes, automobiles and factories releases a harmful amount of air pollutants.

The pollution produced by power plants and fission plants:

All power plants produce waste heat, but fission plants produce the most. The heat released into a lake, a river or an ocean upsets the balance of life in them. Unlike other power plants, nuclear power plants do not produce carbon dioxide. But they do produce dangerous radioactive wastes.

Controlling pollution issues:

In many countries’ governments have passed laws to control air pollution. Some of these laws limit the amount of pollution that, power plants, factories and automobiles are allowed to give off. To meet these conditions for automobiles, new cars have catalytic converters. These devices convert some polluting gases. The use of lead-free petrol has greatly reduced the amount of lead in the air. Engineers are working to improve new kinds of car engines that use electricity or energy sources other than diesel and petrol.

Many individual communities have laws which protect their areas from pollution. Individuals can help to control air pollution simply by reducing the use of cars and other machines that burn fuel. Sharing rides and using public transportation are ways to reduce the number of automobiles in use.

Q34.      Explain by drawing energy flow diagrams through steady-state system such as filament lamp a power station, a vehicle travelling at a constant speed on a level road?

Ans:       Flow diagram of an energy converter:

In an energy converter, a part of the energy taken (used up) by the system is converted into useful work. Remaining part of the energy is dissipated as heat energy, sound energy (noise) into the environment.

Energy flow diagrams given below show the energy taken up by an energy converter to transform it into other forms of energy.

OR

How can you find the efficiency of a system?

Ans:       Efficiency:

The ratio of the useful work done by a device or machine to the total energy taken up by it is called its efficiency.

OR

Efficiency of a system is the ratio of the required form of energy obtained from a system as output to the total energy given to it as input.

Efficiency     =   required form of output / total input energy                     (i)

% Efficiency    = required form of output / total input energy * 100                    (ii)

Ideal system/Ideal machine:

An ideal system is that which gives an output equal to the total energy used by it. In other words, its efficiency is 100 %.

People have tried to design a working system that would be 100 % efficient. But practically such a system does not exist.

 Efficiencies of some typical devices/machines Energy Input Device or machines Useful Work done % Efficiency 100 J Electric lamp 5 J 5 % 100 J Patrol Engine 25 J 25 % 100 J Electric Motor 80 J 80 % 100 J Electric Fan 55 J 55 % 100 J Solar Cell 3 J 3 %

Q36.     What is meant by the term power? Define watt.

Ans:      See Q # 6.15 from Exercise.

SUMMARY

1. Work: Work is said to be done when a force acting on a body moves it in the direction of the force.
• Work = FS
• SI unit of work is joule (J).
1. Energy: When we say that a body has energy, we mean that it can do work. SI unit of energy is also joule, the same as work.
2. Forms of Energy: Energy exists in various forms such as mechanical energy, heat energy, light energy, sound energy, electrical energy, chemical energy and nuclear energy etc. Energy from one form can be transformed into another.
3. Kinetic energy: The energy possessed by a body due to its motion is called kinetic energy.
4. Potential energy: The energy possessed by a body due to its position is called potential energy.
5. Law of conservation of energy: Energy cannot be created nor destroyed, but it can be converted from one form to another.
6. Processes in nature are the result of energy changes. The heat from the Sun causes the water of oceans to evaporate to form clouds. As they cool down, they fall down as rain.
7. Einstein Equation: Einstein predicted the interconversion of matter and energy by the equation E = mc2.
8. Nonrenewable resources: Fossil fuels are known as nonrenewable resources because it took millions of years for them to attain the present form.
9. Sunlight and water power are the renewable resources of energy. They will not run out like coal, oil and gas.
10. Environmental problems such as polluting emission consisting of noise, air pollution and water pollution may arise by using different sources of energy such as fossil fuels, nuclear energy.
11. Efficiency: The ratio of the useful work done by a device or machine to the total energy taken up by it is called its efficiency.
12. Power: Power is defined as the rate of doing work.
13. Watt: The power of a body is one watt which is doing work at the rate of one joule per second.