Question

Internal energy is a state function but work is not a state function. Why?

Answer 1

Hint: One must be clear about the basic differences between state functions and path functions in order to understand the solution. Besides this a basic insight into theory of work and internal work will help to relate it better.

Complete answer:
State function is the thermodynamic property which describes the particular state in which the quantity is present. It is independent of the path taken or chosen by the quantity to reach that specified point. It is also known as Point Function. It only depends on the initial and final state in which the thermodynamic system is present.

Path function is the thermodynamic term used for the quantities that are path dependent i.e., these are the quantities that vary by changing the path followed to reach the specified value. It is also known as Process Function. It depends on the route followed by the particle to start from its initial state and to reach up to its final state.

Now by the differences between both functions we can easily relate internal energy and work with these functions. Internal energy or change in internal energy during a process depends upon the initial and final stage the particle is, irrespective of the path it followed to reach the stages. But work is opposite to internal energy in this part as it totally varies with the path it follows. To reach from one point to another, work done by the particle is different for the different paths it follows.

Hence, as the work changes with path and internal energy remains constant with the path followed so it is then clear that internal energy is a state function and work is a path function.

Note: Energy is the ability of the body to do any work while work is displacement in the body in the direction of applied force on the body. State functions include quantities like mass, energy, pressure, temperature, volume, altitude, entropy etc. And path functions include terms like mechanical work, heat, arc length etc.