Physics XI Notes for Physics Notes

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“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

  1. Joule
  2. Calorie [NOTE: 1 Calorie = 4.2 joule.]
  3. KWatt-Hour

Types of energy
There are numerous types of energy such as:

  • Heat Energy
  • Light Energy
  • Sound Energy
  • Nuclear Energy
  • Chemical Energy
  • Electrical Energy
  • Solar Energy
  • Wind Energy
  • Kinetic Energy
  • Potential Energy etc.

“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.
Power = Work done/time
Power = Work/t
but [work = Fd]
Power = Fd/t

Units of power

  1. Watt [1 watt = 1joule/sec ]
  2. Kilo watt [1Kw = 1000 watt]
  3. Mega watt (Mw) [1Mw = 106 watt]
  4. Horse power [1Hp = 746w]

Potential energy
Energy stored by a body by any means is called “Potential Energy”.

“The energy stored by a body due to its position in gravitational field is known as ‘Gravitational Potential Energy’”.

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
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

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