Second Law and Engines

Second Law and Engines

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The Second Law of Thermodynamics can be explained in different ways, such as the direction of a process and its irreversibility, and through entropy. Basically, it means that heat naturally flows from a hotter to a colder object, and not the other way around. Entropy in isolated systems never decreases, and this leads to a state of maximum entropy called thermodynamic equilibrium. For example, when a hot drink cools down, it gives off heat to the surroundings. The First Law of Thermodynamics says be or destroyed, only changed from one form to another. So perpetual motion is impossible. Heat engines are systems that convert thermal energy to mechanical work, like gasoline and diesel engines, jet engines, and steam turbines. They use cycles of heating and cooling to produce work. Heat sinks and heat sources are needed for thermal energy transfer, and a heat source is needed to be hotter than a heat sink to transfer energy.

Heat engine energy flow diagram
Heat engine energy flow diagram

The diagram shows that the work done by a heat engine (W) is equal to the difference between the heat transferred from the hot reservoir (QH) and the cold reservoir (QC), measured in Joules. Heat engines follow the second law of thermodynamics, which refers to the direction of heat and is expressed in terms of increasing entropy. A cyclical process can't fully convert heat into work because the system can't return to its initial state. Engine efficiency measures the amount of input energy converted into mechanical work. Maximum efficiency is when all the heat transferred from the sink is used for work, but some energy is always lost to the environment, so efficiency is always less than 100%. The Carnot engine operates on the Carnot cycle, which provides maximum efficiency. The Carnot principle states that no other engine can be more efficient than a reversible Carnot engine. Reversible engines don't lose energy under reverse operation, while irreversible engines do. The Carnot cycle is shown in a diagram where heat is transferred during isothermal paths AB and CD, and the total work done is the area inside the shape.

Carnot cycle p-v diagram
Carnot cycle p-v diagram

The Carnot cycle is a theoretical thermodynamic cycle that includes four stages: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. The Carnot efficiency or maximum efficiency is the highest possible efficiency that an ideal reversible heat engine can achieve. For engines to reach maximum efficiency, they must operate on a reversible cycle with no energy lost due to friction. The second law of thermodynamics is used in many engines, including steam engines, petrol and diesel engines, and gas turbine engines. To calculate the efficiency of a power station work must the difference in heat transfer between the source and the sink, and the fraction of the work output over the heat transfer of the source must be calculated. The output power of a heat engine is the work done per unit time, measured in Watts.

Second Law and Engines

How does the second law relate to heat engines?

A heat engine requires operation between a source and a heat sink so that heat is transferred from the source to the sink, producing work as predicted by second law.

Which engine violates the second law of thermodynamics?

A heat engine that produces higher efficiency than a Carnot engine violates the second law of thermodynamics.

What is an example of the second law?

A hot drink cooling down and transferring thermal energy to room temperature due to the lower temperature of its surroundings is an example of the second law.

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