A simple thermodynamic question

[arkofnoah]

Homework Statement

When a sealed Thermos bottle of hot coffee is shaken, what changes, if any, takes place in the temperature of the coffee and its internal energy? Justify your answer.

Homework Equations

Internal energy = random kinetic energy + random potential energy

The Attempt at a Solution

The answer given says that because work is done on the system there is an increase in internal energy. But isn't the work done used to increase the mechanical energy of the system, not the internal energy. There should be no increase in the temperature, shouldn't it?

 

[Red_CCF]

Homework Statement

When a sealed Thermos bottle of hot coffee is shaken, what changes, if any, takes place in the temperature of the coffee and its internal energy? Justify your answer.

Homework Equations

Internal energy = random kinetic energy + random potential energy

The Attempt at a Solution

The answer given says that because work is done on the system there is an increase in internal energy. But isn't the work done used to increase the mechanical energy of the system, not the internal energy. There should be no increase in the temperature, shouldn't it?

Logically I would think that by shaking the bottle the particles kinetic energy increases (they move faster due to the shaking) thus increasing temperature?

 

[arkofnoah]

but if i remember correctly the internal energy of a system excludes the energies due to the state of the body as a whole such as the (translational) kinetic energy due to the motion of the body and the (gravitational) potential energy due to the height of the body. Internal energy includes only 'random' energy, doesn't it?

 

[Mapes]

But isn't the work done used to increase the mechanical energy of the system, not the internal energy. There should be no increase in the temperature, shouldn't it?

After the thermos and the coffee inside have stopped moving, the mechanical energy is the same as what it was originally. Friction has converted the additional mechanical energy into thermal energy (internal energy).