Thermal Physics, a few questions

[philip02]

First post, so hello! We've been introduced to thermal physics, with which I've had some complications:

1)An ideal gas expands isothermically, absorbing a certain amount of energy, Q, in the process. It then returns to its original volume adiabatically. During the adiabatic process, the internal energy change of the gass will be:
A: Equal to Q
B: Greater than Q

Where B is the correct answer. Could someone explain this?

Isothermic expansion means that delta Q is positive (energy is being absorbed by the system)
Adiabatic compression means that delta Q = 0.

Therefore I believe that during the adiabatic process, energy absorbed from the isothermal process is kept in the system and none is lost, therefore A, but according to the answer section this is wrong.

2) A heat engine operates between a hot reservoir (400K) and a cold reservoir (300K). IN each cycle, it would take 100J from the hot reservoir, lose 25J to the cold reservoir and do 75J of work. This heat engine would violate:

The answer: It violates the second but not the first law of thermodynamics.

I can't seem to understand this. A brief explanation would help!

Many thanks!

Kind regards,

Philip

 

[jbsteele]

The first law of thermodynamics states that energy is conserved, meaning that the total amount of energy in the system remains constant. The second law of thermodynamics states that the entropy of the universe (the total energy divided by temperature) increases over time. In this case, the heat engine is taking 100J from the hot reservoir, losing 25J to the cold reservoir and doing 75J of work, which is a net loss of 50J. This violates the second law of thermodynamics because it is decreasing the entropy of the universe even though the total energy is conserved.