Welcome to our exploration of the first law of thermodynamics. This fundamental principle states that energy cannot be created or destroyed, only transferred or transformed from one form to another. In our diagram, we see a system that can exchange heat and work with its surroundings, while its internal energy changes accordingly.
The first law of thermodynamics can be expressed mathematically as delta U equals Q minus W. Here, delta U represents the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system. When heat flows into the system, Q is positive. When the system does work on its surroundings, W is positive, which decreases the internal energy.
Let's apply the first law to a practical example. Consider a gas in a cylinder that receives 500 joules of heat from its surroundings. As the gas heats up, it expands and pushes the piston upward, doing 300 joules of work on the surroundings. Using the first law equation, the change in internal energy equals 500 joules minus 300 joules, which gives us 200 joules. This positive value means the internal energy of the gas has increased.
The first law applies to all thermodynamic processes, but some special cases simplify the equation. In an isothermal process, temperature remains constant, so the change in internal energy is zero. In an adiabatic process, no heat is exchanged with the surroundings, so Q equals zero. In an isochoric process, volume remains constant, so no work is done and W equals zero. Each case demonstrates how energy conservation still holds under different constraints.
The first law of thermodynamics has countless applications in engineering and science. Heat engines convert thermal energy to mechanical work, following the principle that energy input equals work output plus waste heat. Refrigerators use work to move heat from cold to hot reservoirs. Chemical reactions, biological processes, and power generation all obey this fundamental law. In summary, the first law states that energy cannot be created or destroyed, only transformed, expressed mathematically as delta U equals Q minus W.