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CBSE - IX
Energy
ENERGY
The energy of an object is its ability to do work. Energy is the cause and work is its effect. Therefore both work and energy have the same units, which is joule (J) in the SI system and erg in the CGS system. Energy is also a scalar quantity. Energy exists in many forms. To name some are mechanical energy which is either in the form of potential energy or kinetic energy or a combination of the both, electrical energy, light energy, thermal energy etc. Potential energy is the energy possessed by a body by virtue of its position or state. It is further classified into gravitational potential energy (GPE) and elastic potential energy (EPE). GPE is by virtue of height of a body from a reference level, given by the expression 'mgh' (m being mass of the body, g is acceleration due to gravity and h the height of the body from the reference level) whereas EPE of a body is by virtue of its stretched state. Kinetic energy (KE)
is the energy possessed by a body by virtue of its motion and is given by, 
. The law of conservation of energy, which is the fundamental law, says that energy can neither be created nor destroyed, the sum total energy existing in all forms in the universe remains constant. Energy can only be transformed from one form to another.
Electrical energy commercially is measured in the units of kilowatt hour (kW h).
Power is defined as the rate of doing work. Power is measured in watt which is equal to joule per second. Power can also be measured as the product of force and velocity of an object. Energy can be expressed in terms of product of power and time.
1 kW h = 3.6 x 106 J
