Work,Energy
and Power
Work
• In physics, work is the product of force and displacement.
• Work done on an object is defined as the product of the
magnitude of the force acting on the body and the
displacement in the direction of the force. W = F.s
• If a force acting on a body causes no displacement, the
work done is 0. For example, pushing a wall.
Practice Question:
In case of negative work, what is the angle between the force and displacement?
(a) 0 degree
(b) 45 degree
(c) 90 degree
(d) 180 degree
Calculate Work done
What is Energy?
• Energy is defined as the ability to do work. Its unit is the same as that of work.
• SI unit of energy or work = Joule (Nm)
• Energy has different forms: Light, heat, chemical, electrical or mechanical.
Type of Energy
• Kinetic Energy
• Potential Energy
Kinetic Energy: The energy exists due to the motion of an object is known as Kinetic
Energy. For example, a moving van, flowing water, etc.
K.E.=1/2×m×v2
Where,
K.E. Kinetic Energy
m Mass of the object
v The velocity of the object
Practice Question:
If the velocity of a body is doubled its kinetic energy
(a) Gets doubled
(b) Becomes half
(c) Does not change
(d) Becomes 4 times
Potential Energy: This is the energy stored in an object due to its position and height. It is
measured by the amount of work done. For example, a book on a table, water stored in a
lake, etc.
P.E.=m×g×h
Where,
P.E. Potential Energy
m Mass of the object
g Acceleration due to gravity
h Height
Law of Conservation of Energy
• Law of conservation of energy states that energy can neither be created nor destroyed, but can
be transferred from one form to another. The total energy before and after the transformation
remains constant.
• Total energy = KE + PE
The Law of Energy Conversion
Using Microphone Sound Energy to Electric Energy
Photosynthesis in Plants Solar Energy to Chemical Energy
In Electric lamp Electric Energy to Heat and Light Energy
Burning of wood Chemical energy to Heat and Light Energy
In Fuel cells Chemical Energy to Electric Energy
In steam engine The heat energy to Mechanical Energy
In Electric heater Electric Energy to Heat
In Geothermal Power Plant Heat Energy to Electrical Energy
In Thermocouple Heat Energy to Electrical Energy
In Hydroelectric Dams Gravitational potential energy to Electric
Energy
In Electric Generator Kinetic energy / Mechanical work to Electric
Energy
In Windmills Wind Energy to Mechanical Energy or Electric
Energy
Power
• The rate of doing work or the rate of transfer of energy is called power. It is denoted by P
• Average power = Total energy consumed/Total time taken
• The commercial unit of power is kWh i.e. energy used in 1 hour at 1000 Joules/second.
• 1kWh=3.6×10^6 J
• The bigger unit of power is Kilowatt and represented by kW.
• 1kW = 1000W
• Some another units to measure power are:
• 1 Megawatt = 106 watt
• 1 horse power = 746 watt
Practice Question:
A 60 watt radio set lasts for 50 hours. What is the electrical energy consumed by him in
"kilowatt-hour"?
A. 2 KWH
B. 3 KWH
C. 4 KWH
D. 4KWH
Question Set
• One horse power is equal to
(a)790 watts
(b)700 watts
(c)720 watts
(d)746 watts
• When a body falls freely under gravity, then the work done by the gravity is
(a) Positive
(b) Negative
(c) Zero
(d) Infinity
• Energy possessed by a body due to its stationary is called its
(a) heat energy
(b) kinetic energy
(c) potential energy
(d) chemical energy
• During inter conversion of energy from one form to other forms, total energy at any
time remains
(a) constant
(b) zero
(c) increasing
(d) decreasing
• A moving car possesses
(a)sound energy
(b)mechanical energy
(c)heat energy
(d)chemical energy
Question Answer
1 d
2 a
3 c
4 a
5 b
