16.3 The First Law of Thermodynamics

The First Law of Thermodynamics

The First Law of Thermodynamics

Increase in internal energy = work done on the system + energy transferred to the system by heating

The First Law of Thermodynamics

$\Delta U=q+W$ Where:

• $\Delta U$ = Increase in internal energy of the system ($\text{J}$)
• $q$ = Thermal energy transferred (heating) to the system ($\text{J}$)
• $W$ = Work done on the system ($\text{J}$)

Increase in Internal Energy

• Internal energy increases
• Thermal energy is supplied to the system: $+q$
• Work is done on the system: $+W$
• Since work is done on the system, gas is compressed therefore $-\Delta V$

Decrease in Internal Energy

• Internal energy decreases.
• Thermal energy is removed from the system: $-q$
• Work is done by the system: $-W$
• Since work is done by the system, gas expands therefore: $+\Delta V$

Warning

The First Law of Thermodynamics is defined in terms of increase in internal energy and work done on system.