PX154 - D2 - the first law

formula and convention

• Clausius convention:

where,
$\mathrm{\Delta }U=$ change in internal energy of the system
$Q=$ heat input into the system
$W=$ work done by the system
note negative sign
- cases:
- for $Q$
- $Q>0:$ heat input into the system
- $Q<0:$ heat output by the system
- for $W$
- $W>0:$ work done by the system
- $W<0:$ work done on the system

• international physical chemistry convention:

here, $W>0:$ work done on the system
- opposite sign convention

internal energy (U)

• $U$ may not be known precisely, but only $\mathrm{\Delta }U$ is used in the first law
• eg: for a monatomic gas: $U=K_{tr}=\frac{3}{2}nRT$
• $U$ only depends on the temperature of the gas, not its pressure or volume