why internal energy of a system is state variable but work and heat are path variables ? pls explain in details!
Answers
Internal energy does not depend on the path of the process it has a particular value at each every different states of the system or state points. It is also called as point function
take an example, take one glad of water; it has some specific internal energy within it.
Now the water poured into a pot and left to cool down in a refrigerator till it takes the form of ice.
Now ice has a different and definite internal energy.
The internal energy in this process is independent of the path followed to change the state of the system. What may be the path every substance has an define amount of internal energy at each and every state of the system and it not dependent on the path or process followed to reach that state.
A quantity of work w is measurable from physics principles.
A quantity of heat q can be measured by the first law, energy conservation.
A function of state must meet mathematical criteria, in particular the behavior with closed path integrals sums to zero. (Do not misread this, you close the loop, you do not enclose the system). In other words when you arrive back in the same state. (This extends to infinitesimal paths.)
So quite simply, q and w over such paths are not conservative. You get different results depending on the path. (Although their sum might be). You only have to show one example.