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21.3 Adiabatic Processes for an Ideal Gas As we noted in Section 20.6, an adiabatic process is one in which no energy is trans- ferred by heat between a system and its surroundings. For example, if a gas is com- Suppose that an ideal gas undergoes an adiabatic expansion. At any time during the process, we assume that the gas is in an equilibrium state, so that the equation of (21.18) where ( # C P /C V is assumed to be constant during the process. Thus, we see that all three variables in the ideal gas law—P, V, and T—change during an adiabatic process. Proof That PV $ % Constant for an Adiabatic Process When a gas is compressed adiabatically in a thermally insulated cylinder, no energy is int # nC V dT (Eq. 21.12). Hence, the first law of thermodynamics, " E int # Q $ W, with Q # 0 becomes Taking the total differential of the equation of state of an ideal gas, PV # nRT, we see that P dV $ V dP # nR dT Eliminating dT from these two equations, we find that P
dV $ V
dP # ! R C V P
dV dE
int # nC V
dT # !P
dV PV ( # constant SECTION 21.3 • Adiabatic Processes for an Ideal Gas 649 Relationship between P and V for an adiabatic process involv- ing an ideal gas Example 21.2 Heating a Cylinder of Helium A cylinder contains 3.00 mol of helium gas at a temperature (A) If the gas is heated at constant volume, how much energy must be transferred by heat to the gas for its temper- Solution For the constant-volume process, we have Because C V # 12.5 J/mol & K for helium and "T # 200 K, we obtain 7.50 % 10 3 J # Q 1 # (3.00 mol)(12.5 J/mol&K)(200 K) Q
1 # nC V
" T (B) How much energy must be transferred by heat to the gas at constant pressure to raise the temperature to 500 K? Solution Making use of Table 21.2, we obtain Note that this is larger than Q 1 , due to the transfer of energy out of the gas by work in the constant pressure 12.5 % 10 3 J # # (3.00 mol)(20.8 J/mol&K)(200 K) Q
2 # nC P
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