CLASS 7TH | PHYSICS | ENERGY | REVISION NOTES

Work :

Work is said to be done only when there is displacement of the body in the direction of
force applied.
Examples : Moving Car, Squeezing tennis ball , Hitting a shuttlecock
Work Measurements :
Work is said to be completed if and only following conditions are satisfied :
(a) A force must be acting on the body
(b) The Body should move in the direction of applied force or rather it’s shape and size
should change
Consider the force (F) acting on body through a distance (S) in it’s direction , then the work
(W) is the product of force acting on it and the displacement :
W= F x s
Here the displacement is the shortest distance of a body from it’s initial position to final
position.
Work can be resulted out as Zero , if either applied force or displacement is zero.
(1) Example where displacement is zero , as result there is no work done
A boy is pushing a heavy cupboard , two boys pushing a string with same amount of
force
(2) Example where force is zero , as a result there is no work done
A child sitting and reading a book , A man is resting at sofa
Consider following events where work is said to be done or it is non-zero
• Group of girls playing badminton
• A boy is ridding bicycle
In above mentioned events work is done or completed as force on the object brings
displacement in its position.

Unit of Work :
The unit of work is product of unit of force and displacement that is Newton and Meter
respectively. Hence unit of work is newton-meter (Nm) termed as joule (j)
1 j = 1 N x 1m = 1 Nm
One joule is the amount of work done when a force of one newton moves an object by one
meter in its direction

Energy :

The capacity or ability of body to do work is called as energy stored in the body
The object which does work , losses energy , whereas the object on which work is
performed, gains the energy. Energy can not be created nor be destroyed.
Unit : same as of work : SI unit is “Joule” (j)
For heat energy is measure in “calorie” ( 1 cal = 4.184J)
Different forms of Energy:
(1) Kinetic Energy :
It is the energy possess by body or object by virtue of its motion or movement.
Mass of body is moving with speed then following kinetic equation is used
K= 1
2
mv2
One can easily conclude that from above, the heavier an object, and the faster it
moves ,the more kinetic energy do it possess.
Example: moving car , athlete is running in race
(2) Potential Energy:
The energy do possess by body by virtue of its position or configuration is termed as
its potential energy. The value of this energy is depend upon the reference point.
When it is consider in terms of height above the ground level of earth, called as
gravitational potential and given by
U=mgh
As we know , m= mass of body and g =acceleration due to gravity
Thus, mg = weight of the body and h = height above the surface

Examples of Potential Energy :
• Tones of water stored at water tank above the ground at some height
• Book kept on book-shelf
(3) Mechanical Energy :
It is the summation of kinetic energy and potential energy in an object that is
performed to do work. In simple words , it is energy because of movement or
position , or both.
Example : boy is inserting nail into wooden block by using iron hammer.
(4) Sound Energy :
Sound energy is produced when an object or body vibrates. This vibrations cause
waves of pressure that travel through a medium (air, water, wood or metal).
Sound energy is a kind of mechanical energy.
It is far less than as compared to any other form of energy .
Sound energy is measured in pascals and decibel.
The loud noise which causes paining sensation termed as threshold of pain.
(5) Heat (Thermal Energy):
Thermal energy is produced when there is increase in temperature causes particles
to move faster and collide with each other. The energy that arises from the
temperature of the hot substance is called thermal energy.
It is measured in joule (j).
(6) Temperature :
It is the quantitative expression for object how hot or cold it is. Which is expressed in
degree Celsius (oC). It is also denoted by the Fahrenheit (F).
The instrument used for measure the temperature id termed as Thermometer.
Larger the amount of water requires more heat energy to boil as compare to the
smaller amount and relatively extra time as well.

(7) Chemical Energy :
It is stored form of energy in the bonds of chemical compound. It is liberated as heat
and termed as exothermic reaction as reaction proceeds.
Examples : Biomass , batteries , natural coal and petroleum
After explosion , the chemical energy which is stored in , resulted in transformation to the
surrounding as thermal energy , sound energy and kinetic energy.
Chemical reaction causes formation of new compounds from break down of molecules and
atoms.
(8) Electrical Energy :
Electrical energy is generated when there is movement of electron from one atom to
another in presence of magnetic force.
In case of battery the electron (stored form of energy as charged particles) move
through a conducting wires.
All Energy chains start from sun:
The sun is ultimate source of energy, from where the energy cycle begins .
Example :
(a) Solar energy is get trapped into plant further which get converted to coal that we
use in power station to produce electricity and because of this our electrical
appliances such as fan , bulbs works.
(b) The water cycle is also result of solar energy.
In some cases we uses the solar energy directly, in solar cells which are part of
watches, calculators and artificial satellites. Use of solar cooker is also suited example here.

Law of conservation of Energy:
The law of conservation of energy states that energy can neither be created nor destroyed
only it will transformed from one form of energy to another form of energy. By this flow of
energy is maintained but total energy remains the same.
This indicate that in the system there is always the same amount of energy, unless it’s acted
upon it from the outside.
Examples of the conservation of energy:
• Driving car :
Consider a car fill up with gasoline and act like close system in which further events
are happening step by step. After the burning of gas the chemical enrgy stored in the
gas get transformed to heat energy , fuel makes the hot expanding of gas which
resulted in pushing up the piston in engine cylinders. This step where heat is
converted to mechanical energy. This change in piston eventually brings the motion
of car’s wheel which gives the kinetic energy.
Discovery of conservation of energy:
The experiment performed by James Prescott Joule , In his experiment . a falling weight
pulls on a rope that passes over pulley . Then the rope spin the axle which in turns a paddle
inside a sealed container of water. As the paddle rotate , the water get heat up .
In this he proved that the heat energy gained by the water was exactly same as the
potential energy lost by weight which is initially tend to fall down