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WORK AND Energy (Chapter 6)
PHYSICAL
DEFINITION OF
WORK
"Work is said to be done if a force causes a displacement
in a body in the direction of force".
OR
"The work done by a constant force is defined as the product of
the component of the force and the displacement in the direction of
displacement."
MATHEMATICAL
DEFINITION
"Work is the scalar product of force and displacement".
OR
"Work is the dot product of force and displacement".
Work is a scalar quantity.
UNIT OF WORKS
• In S.I system:
Joule (j)
• In C.G.S. system: Erg
• In F.P.S. system: ft X lb
CATEGORIES
OF WORK
(i) POSITIVE WORK:
If force and displacement are in the same direction, work will be positive or if or  <
90°
(ii) ZERO WORK:
If force and displacement are perpendicular to each other, work will be zero. i.e.
since = 90°
Work = 0
as
Work = Fd Cos
Work = Fd Cos 90°
Work = (F)(d)(0)
Work = 0
NEGATIVE WORK:
If force and displacement are in the opposite direction, work will be negative.
since = 180°
Work = - ve
as
Work = Fd Cos
Work = Fd Cos 180°
Work = (F)(d)(-1)
Work = -Fd
ENERGY
ENERGY
"The ability of a body to perform work is called Energy".
A body cannot perform work if it does not posses energy. A body cannot perform work
more than the amount of energy.
It is a scalar quantity.
UNITS OF
ENERGY
(i) Joule
(ii) Calorie
[NOTE: 1 Calorie = 4.2 joule.]
(iii) K Watt-Hour
TYPES OF
ENERGY
There are numerous types of energy such as:
Heat Energy: Heat is a form of energy given out by hot bodies. E.g. heat obtains by
burning fuel. Or sun is main source of heat energy.
Light Energy: Light is important source of energy, because plants prepare food in the
presence of light. We get light from candles, electric bulbs, and sun
Sound Energy: Is produce when body vibrates. E.g. when we knock the door a sound
is produced or vibrating string of a sitar.
Nuclear Energy: Energies released in form of nuclear radiations from the process of
fission and fusion.
Chemical Energy : The energy obtained during checmical reations. Chemical energy
obtained from food and fule.
Electrical Energy: Electricity is obtained from electrical energy. This energies is send
through wires. We get electrical energies from batteries and generators.
Mechanical energy: The energy possessed by a body due to its motion or position e.g
water running in streams, wind, moving car .
Solar Energy :
Wind Energy:
Kinetic Energy
Potential Energy
POWER
"The rate of work done of a body is called Power".
AVERAGE POWER
Average power of a body doing work is numerically equal to the total work done divided
by the time taken to perform the work.
MATHMATICALLY
Power = Work done/time
Power = Work/t
but [work = Fd]
therefore
Power = Fd/t
UNITS OF POWER
(i) watt
[1 watt = 1joule/sec ]
(ii) Kilo watt
[1Kw = 1000 watt]
(iii) Mega watt (Mw) [1Mw = 106 watt]
(iv) Horse power
[1Hp = 746w]
POTENTIAL ENERGY
INTRODUCTION
Energy stored by a body by any means is called "Potential Energy".
DEFINITION
"The energy stored by a body due to its position in gravitational field is known as
‘Gravitational Potential Energy’".
FORMULA
Consider a body of mass "m" placed at a height of "h" from the surface of earth.
Force = Weight = W
but displacement (d) = h
Work done = Fd
OR
Work done = Wh
[but W = mg]
work done = mgh
We know that the work done in lifting the body is stored in the body in the form of
Potential Energy. Thus
P.E. = mgh
Prove that K.E= ½ mv2
Let a body is moving with velocity ‘v’. An opposing force F acting through a distance S
brings it to rest.
Then Vi =V Vf =0 and a=-F/m
By third equation of motion 2aS= Vf2- Vi2
2x (-F/m) S = (0)2 - (V)2
-2(FS/m) = V2 or FS= mV2/2
Here FS = the magnitude of work = kinetic energy. Thus
K.E = ½ mV2
Inter conversion of energy: "Energy can neither be created nor it is destroyed,
however energy can be converted from one form to any other form of energy. This is
called inter conversion of energy or Law of conservation of energy.
Example-1 : Rub your hands together quickly. You will feel them warm. You have used your muscular
energy in rubbing hands as a result heat is produced. In this process of rubbing hands, mechanical
energy is converted into heat energy.
Example-2 : Some of heat energy from sun is taken up by water in the oceans. This increase the
thermal energy. Thermal energy causes water to evaporate from the surface to form water vapours.
These vapours rise up and form clouds. As the cool down they form water drops and fall down as rain.
Potential energy changes to kinetic energy as the rain falls, this rain water may reach in lakes or dams.
As rain water flow down its K.E changes into thermal energy.
Source of Energy: i. Fossil Fuels: We use fossil fuels such as coal , oil and gas to heat our houses and run
industry and transport. The carbon becomes carbon dioxide, and hydrogen becomes hydrogen oxide
called water and energy is released.
In case of coal carbon + oxygen ------ carbon dioxide + energy
In case of oil and Gas: Hydrocarbon + oxygen ______ carbon dioxide + water + energy
Nuclear Fuels: In nuclear power plants we get energy using uranium. The process taking place in
nuclear reactors is known as nuclear fission.
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 the floods by
storing water. The stored in dams is used for irrigation and also generates electrical energy without
creating much environmental problem.
Energy from the Sun: Solar energy is the energy coming from sun. The energy coming up from the sun is
used directly and indirectly. Sunlight does not pollute the environment in any way. The sunrays are the
ultimate source of life on the earth. We are dependent on the sun for all our food and fuels.
Solar house Heating: Complete solar house heating systems are successful ly 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: 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
plants, etc
Solar Cell: Solar cell can also convert solar energy into electricity. A solar cell is also called
photocell is made of silicon wafer.
Wind energy: Wind has been used as a source of energy for centuries. It has powered sailing
ships across oceans. It has been used by windmill to grind grain and pump water. Most recently
wind power is used to turn wind turbines. When many wind turbine grouped to gather on wind
farms, they generates enough power to operate a power plant to generate 1300Mwatt of
electricity.
Geothermal Energy: In some parts of the world, the earth provides us hot water from geysers
and hot spring. There is hot molten part deep in the earth called magma. Water reaching close
to magma changes into steam due to high temperature of magma. This energy is called
geothermal energy. Water is pushed down by drilling deep near hot rocks at places, where
magma is not very deep. When water reached down it changes into steam and these then can
be piped out and used to heat houses and used to generate electricity.
Bio mass Energy: Biomass is plant or animal waste can be burnt as fuel. These wastes are run
the power plants. Other forms of biomass are garbage , Farm wastes, Sugarcane and other
plants. 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.
Mass energy Equation. Einstein predicted the inter conversion of matter and energy. According
to him, a loss of mass provides a lot of energy. This happen 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 x 108ms-1) The above equation shows that tremendous amount
of energy can be obtained from small quantity of matter.
Flow Diagram of an energy converter:
In an energy convert, a part of the energy taken by the system is converted into use full work.
The remaining part of energy is dissipated as heat energy, sound energy, (noise) into
environment.
Efficiency: The ratio of the use full work done by the device or machine to total energy taken up by it is
called its efficiency.
% efficiency =( required form of output / total input energy) X 100