Short Answer
The First Law of Thermodynamics applies to a closed system by stating that the total energy remains constant, and any heat added to the system is either stored as internal energy or used to perform work. A closed system allows energy transfer in the form of heat and work but does not allow mass transfer.
For a closed system, the First Law is expressed as:
Q=ΔU+WQ = \Delta U + WQ=ΔU+W
where Q is the heat supplied, ΔU is the change in internal energy, and W is the work done. This principle is used in piston-cylinder devices, boilers, and pressure vessels to analyze heat and energy interactions.
Detailed Explanation
First Law of Thermodynamics in a Closed System
A closed system is a system that allows energy transfer but does not permit the transfer of mass across its boundary. The First Law of Thermodynamics governs how energy behaves in such a system.
Energy Balance in a Closed System
The First Law equation for a closed system is:
Q=ΔU+WQ = \Delta U + WQ=ΔU+W
Where:
- Q = Heat energy supplied to the system (Joules)
- ΔU = Change in internal energy of the system (Joules)
- W = Work done by or on the system (Joules)
This equation means:
- If heat is added to the system, it increases internal energy or is used to perform work.
- If work is done on the system, it increases the internal energy.
- If the system does work on the surroundings, internal energy decreases.
Different Processes in a Closed System
- Constant Volume Process (Isochoric Process)
- W = 0, so the equation simplifies to Q = ΔU.
- All heat added increases internal energy since no work is done.
- Example: Gas in a rigid container being heated.
- Constant Pressure Process (Isobaric Process)
- Some heat is used to increase internal energy, and some is used to do work.
- Example: Steam formation in a boiler.
- Adiabatic Process (No Heat Transfer)
- Q = 0, so the equation becomes ΔU = -W.
- Work is done at the cost of internal energy.
- Example: Compression or expansion of gas in an insulated cylinder.
- Cyclic Process (Energy Balance Over a Cycle)
- Since ΔU = 0, the equation simplifies to Q = W.
- Heat input is completely converted into work output.
- Example: Carnot cycle, Rankine cycle.
Applications of the First Law in a Closed System
- Internal Combustion Engines: Fuel burns inside a closed cylinder, releasing heat and doing work.
- Piston-Cylinder Devices: Work is done as gas expands or compresses inside the cylinder.
- Refrigeration and Air Conditioning: The refrigerant undergoes heat transfer within a closed loop.
Conclusion
The First Law of Thermodynamics applies to a closed system by ensuring that the heat added is either converted into work or stored as internal energy. In a closed system, energy transfer occurs without mass flow, making this principle essential in thermodynamic analysis of engines, compressors, and boilers.