# Thermal Physics, a few questions

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