Heat Engines - 2nd Law of Thermodynamics | Thermodynamics | (Solved examples)

The video starts by explaining why heat energy transfer occurs in a specific direction (e.g., hot tea cooling down), introducing the second law of thermodynamics. This law states that processes have a certain direction and energy possesses both quantity and quality. Entropy is mentioned as a property to identify violations of this law.

It clarifies that work can be easily converted to internal energy (heat), but converting heat back into work requires specific devices called heat engines. Heat engines receive heat from a high-temperature source, convert a portion into work, and transfer the remaining waste heat to a low-temperature sink, operating on a continuous cycle.

A steam power plant is presented as a prime example of a thermodynamic heat engine. The process involves heating water in a boiler to create steam, which drives a turbine to produce work. The steam then condenses, releasing waste heat, and the water is pumped back to the boiler to restart the cycle. The net work output is the difference between work output and work input, or heat input and heat output.

Thermal efficiency measures how well a heat engine converts heat into work. It is calculated by dividing net work output by total heat input. The video also introduces the concept of high (QH) and low (QL) temperature reservoirs for heat transfer and explains that efficiency can be expressed using these terms.

The Kelvin-Planck statement is introduced, asserting that no heat engine can ever achieve 100% thermal efficiency. This means it's impossible for a device operating in a cycle to take heat from a single reservoir and produce a net amount of work, implying that some waste heat is always produced.

The video illustrates how to calculate the rate of heat transfer to a river (low-temperature reservoir) from a steam power plant, given its net work output and thermal efficiency. It demonstrates using the efficiency equation to find heat input and then applying the net work equation to determine the waste heat rejected.

This example calculates the thermal efficiency of an automobile engine. It involves determining the rate of heat supply by using the fuel consumption rate, density, and heating value, and then applying the thermal efficiency formula with the given power output.

The final example focuses on a steam power plant, calculating the amount of coal consumed over a 24-hour period and the rate of airflow through the furnace. This involves using the plant's net power output, thermal efficiency, coal heating value, and air-to-fuel ratio.