In an adiabatic expansion of a gas,the initial and final temperatures are ${T_1}$ and ${T_2}$ respectively. What is the change in internal energy of the gas?

• A
  $\frac{R}{{\gamma - 1}}({T_2} - {T_1})$
• B
  $\frac{R}{{\gamma - 1}}({T_1} - {T_2})$
• C
  $R({T_1} - {T_2})$
• D
  Zero