First Law - Closed Systems: Screencasts

Introduces the first law for a closed system and considers cases of constant pressure and constant volume.

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Applies the first law to a closed system for an adiabatic reversible process for an ideal gas.

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Important Equations:

Energy Balance (ignoring kinetic and potential energy):

$\Delta U = Q + W_{EC} + W_S$

where $$Q$$ is the heat added to the system
$$W_{EC}$$ is the expansion/compression work (moving a piston in a cylinder)
$$W_S$$ is shaft work
For most processes of importance in chemical engineering, kinetic and potential energies can be ignored.

Adiabatic/reversible compression/expansion for an ideal gas:

$\frac{T_2}{T_1} = \left( \frac{P_2}{P_1}\right) ^\frac{R}{C_P}$

where $$T_1$$ and $$T_2$$ are the absolute temperatures of the gas at the initial and final states, respectively
$$P_1$$ and $$P_2$$ are the initial and final pressures, respectively
$$R$$ is the ideal gas constant
$$C_P$$ is the heat capacity at constant pressure

Heat added to the system:  $$Q > 0$$
Heat removed from the system:  $$Q < 0$$
Adiabatic system:  $$Q = 0$$
Work added to the system:  $$W > 0$$
Work removed from the system:  $$W < 0$$