Question

For a ___________________ internal energy of a system does not remain constant.

• A. A Cyclic process
• B. An Isothermal process
• C. An Adiabatic process
• D. Any process in which the heat given out by the system is equal to the work done on the system.

Answer 1

Answer: (C)

From First Law of Thermodyamics,
$\Delta Q=\Delta U+\Delta W$ ...(1)
where $\Delta Q$ is Heat added, $\Delta U$ is change in internal energy and $\Delta W$ is work done.
For a cyclic process, $\Delta U$ is conserved and hence, constant.
For an isothermal process, $\Delta T$ is zero and from the internal energy and temperature relation we know that $\Delta U=mC\Delta T$ thus $\Delta U$ is also zero.
For Heat $=$ Work, from (1) we know that $\Delta U$ is zero hence constant.
And for an adiabatic process, $\Delta Q=0$ , thus $\Delta U=-\Delta W$ and thus change in work leads to change in internal energy. Therefore, internal energy does not remain constant.