State the law of conservation of energy
According to law of conservation of energy:”Energy can neither be created nor is it destroyed.It can only be transformed from one form to another.A loss in one form of energy is accompanied by an equal increase in the other forms of energy.The total energy remains constant.”
According to Einstein’s mass energy relation:
Conservation of Energy formula
Total energy = kinetic energy + potential energy
law of conservation of energy equation
Consider a body of mass “m” placed at a point “p” which is at a height “h” from the ground.
P.E of the body at A =mgh
K.E of the body at point A =0
Total energy of the body at point P=K.E +P.E =0 + mgh
Total energy at P=mgh …………(1)
Just before hitting the ground the potential energy of the body will be minimum or zero while the K.E of the body will be maximum.If “v” be the velocity of the body just before hitting the ground then K.E of the body=½mv².
Total energy at Q =K.E + P.E
=mgx + mgh – mgx
Total energy at Q = mgh ————(3)
From equations (1),(2) and (3) it can be seen that the total energy of the body remains constant every where provided there is no force of friction involved during the motion of the body.
If there is some force of friction acting on the body then a friction of P.E is lost in doing work against the force of friction.Thus:
Total energy =K.E + P.E + Loss of energy or work done against force of friction.
examples of Law of conservation of energy
- When we switch on an electric bulb ,we supply electrical energy to it which is converted into heat and light energies.i.e.
Electrical energy = Heat energy + Light energy
- Fossil fuels e.g coal and petrol is stores of chemical energy.When they burn,chemical energy is converted into heat energy i.e .
Chemical energy = Heat energy + losses
- The heat energy present in the steam boiler can be used to derive a steam engine.Here heat energy is converted into kinetic (mechanical energy) ,i.e .
Heat energy =Mechanical energy + Losses
- In rubbing our hands we do mechanical work which produces heat,i.e
Mechanical energy = Heat energy + losses
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