First law of thermodynamics
First law of thermodynamics is similar to the principle of conservation of energy which states "Energy can neither be created nor be destroyed but can be transformed from one form to another". First law of thermodynamics can also be stated as, "heat applied to a thermodynamics system is used to do work whereas some are used in increasing the internal energy of a gas". In simple words, when some quantity of heat is applied to a system some part of it is used in work done in expansion and rest is used in increasing the internal energy of the gas. IF the quantity of supplied heat be dQ, then, from first law of thermodynamics,
dQ = dU + dW ….. (i)
Where,
- dQ is the heat supplied to the system by the surroundings
- dU is the change in internal energy of the system
- dW is the work done by the system by the surroundings
Since dW = P dV
dQ = dU + P dV ….. (ii)
Thus, the above equations are the mathematical representations of the first law of thermodynamics.
For a cyclic system the change in the internal energy is zero because the system is brought back into the initial position. So,
dQ = dU + dW
dQ = dW
Limitations of First Law of Thermodynamics
- First Law of thermodynamics does not indicate the direction of heat transfer. Heat always flows from the body at a higher temperature to the body at a lower temperature. But first law doesn’t indicate why heat doesn’t flow from a colder body to a hotter body.
- When a bullet strikes a target, the kinetic energy of the bullet is converted into heat energy. But the first law does not indicate to why heat energy developed in the target cannot be converted back into mechanical energy of the bullet enabling it to fly.
- The first law of thermodynamics does not give to what extent mechanical energy is obtained from heat energy.
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