Understanding the First Law
The First Law of Thermodynamics is a statement of energy conservation. It states that the change in internal energy of a system (ΔU) equals the net heat transfer into the system (Q) minus the net work done by the system (W):
ΔU = Q - W
ΔU
Change in Internal Energy
Q
Heat Added (+) / Removed (-)
W
Work Done by (+) / On (-)
This law implies that energy cannot be created or destroyed, only transformed from one form to another. In thermodynamic systems, energy can be transferred as heat or converted to/from work, but the total energy remains constant.
Concept Explorer
Interactive Gas Cylinder Simulation
Gas Cylinder Visualization
Adjust parameters to see energy changes
Common Thermodynamic Processes
Isochoric Process (Constant Volume)
Volume remains constant (W = 0). All heat added goes to changing internal energy:
ΔU = Q
Isobaric Process (Constant Pressure)
Pressure remains constant. Both heat and work contribute to energy change:
ΔU = Q - W
Isothermal Process (Constant Temperature)
Temperature remains constant (ΔU = 0). Heat added equals work done:
Q = W
Adiabatic Process (No Heat Exchange)
No heat exchange with surroundings (Q = 0). Work done changes internal energy:
ΔU = -W
Thermodynamic Process Diagram
Real-World Applications
Car Engines
Internal combustion engines convert chemical energy to mechanical work through thermodynamic cycles, primarily Otto and Diesel cycles, which are governed by the First Law.
Refrigerators
Refrigeration systems move heat from cold to hot spaces using work input, demonstrating the First Law through the refrigeration cycle with compressor, condenser, and evaporator.
Power Plants
Thermal power plants convert heat energy from fuel into electrical energy through steam turbines, following the Rankine cycle which applies the First Law at each stage.