15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency

If the path BA is followed, the gas will be returned to its original state, but the environment will not, because it will have been heated in both directions.

Reversibility depends on the direction of heat transfer.
Real processes can't be reversed since dissipative mechanisms can't be completely eliminated.

There are reasons that real processes can't be reversed.

We can imagine them going in different directions.
The reverse of heat transfer occurs when it occurs from hot to cold.
It wouldn't violate the first law of thermodynamics for this to happen.
All of the processes, such as bubbles bursting, never go in reverse.

They can't go in reverse.

When we study this law, we will learn that it limits the efficiency of heat engines.
Even heat engines with the greatest theoretical efficiency can't convert all heat transfer into work because they have to use reversible processes.
The table summarizes the simpler processes.

There are different types of molecule that form a solid, liquid, or gas.

If you want to watch the phase change, add or remove heat.
You can change the temperature or volume of the container to see a pressure-temperature diagram.
Understand the interaction potential of the molecule.

During the summer, the ice falls.

The second law of thermodynamics states that it would be very unlikely for the water molecule contained in these particular Floes to reform the alligatorlike shape they formed when the picture was taken in the summer of 2009.

The direction taken by processes is dealt with in the second law of thermodynamics.

Many processes are irreversible under a set of conditions.
Complete irreversibility is a statistical statement that cannot be seen during the lifetime of the universe.
The reverse path can't be reversed if the process can only go in one direction.
The transfer of energy from higher to lower temperature is referred to as heat.

Transferring heat from a cold object to a hot one is what makes it hotter.

The reverse of mechanical energy being converted to thermal energy is impossible.
A hot stationary object never cools off.
Another example is the expansion of a puff of gas into a vacuum chamber.
The gas never regroups in the corner as it expands to fill the chamber.
The random motion of the gas molecule could take them all back to the corner, but this is never observed.

The reverse process is impossible.

The random motions of the gas molecule will never return them to the corner.

There is a law that forbids certain processes from happening.

The first law of thermodynamics would allow them to occur.
The second law of thermodynamics forbids these processes.
The second law can be stated in many ways that may seem different, but which in fact are the same.
Like all natural laws, the second law of thermodynamics gives insights into nature, and its several statements imply that it is broadly applicable, fundamentally affecting many apparently disparate processes.

There is never a heat transfer in the reverse direction.

It is impossible for a process to have a sole result of heat transfer from a cooler to a hotter object.

Let's look at a device that uses heat transfer.

Part of the heat transfer from one source to another is what heat engines do.
The work done by the engine is the difference between heat transfer from the hot object and heat transfer into the cold object.
The temperatures of the hot and cold areas are different.

There are hot and cold objects.

It is important that the work is done as efficiently as possible because of the energy intensive nature of the work.

We would like for there to be no heat transfer to the environment.
It is not possible to convert a heat transfer system to work in a way that1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556

The majority of heat engines use cyclical processes.

The second law states that heat transfer engines cannot have perfect conversion into work done.

The system is brought back to its original state at the end of each cycle.

At the beginning and end of every cycle, the system's internal energy is the same.
The first law of thermodynamics states that the net heat transfer during the cycle is the net work done by the system.

The problem is that in all processes there is some heat transfer to the environment.

The problem of getting less out than we put in is always faced in the conversion of energy to work.

conversion efficiency is the ratio of useful work output to the energy input.

All of the s are positive.

The direction of heat transfer is indicated by a sign.
It is preceded by a minus sign if it is out of the system.

A heat engine is a coal-fired power station.

It uses heat transfer from burning coal to generate electricity.
A coal power station has heat transfer from coal to the environment in a single day.
The amount of coal put into the atmosphere is implied by this.

The work output can be found if a process is used in the power station.

In this process, water is boiled under pressure to form high-temperature steam, which is used to run steam turbine-generators, and then condensed back to water to start the cycle again.

The work was found in the first part of the example.

The efficiency is given by.

We have in the process.

The amount of coal put into the atmosphere is 44 kilogrammes.

370,000 metric tons is produced every day.

The average power output is 1180 MW if all the work output is converted to electricity in a single day.

This is the size of a conventional power plant.
The efficiency is close to 42% for coal power stations.
It means that 59.2% of the energy is heat transfer to the environment, which usually results in warming lakes, rivers, or the ocean near the power station, and is implicated in a warming planet generally.
The heat transfer to the environment could be used for heating homes or industrial processes, even though the laws of thermodynamics limit the efficiency of such plants.
It is not economical to use the waste heat transfer from most heat engines because of the low cost of energy.
Coal is the least efficient fossil fuel when it comes to per unit energy output.

The four steps shown complete this heat engine's cycle, bringing the gasoline-air mixture back to its original condition.

The nearly adiabatic compression stroke of the gasoline engine is related to the adiabatic process.

Work is done on the system to increase its temperature and pressure.
Along path BC of the Otto cycle, heat transfer into the gas occurs at constant volume, causing a further increase in pressure and temperature.
The process takes place so quickly that the volume is almost constant.

Path CD in the Otto cycle is an adiabatic expansion that works on the outside world just as an internal combustion engine works on the inside.

In the Otto cycle, heat transfer from the gas at constant volume reduces its temperature and pressure, returning it to its original state.

In an internal combustion engine, this process corresponds to the exhaust of hot gases and the intake of air-gasoline mixture at a considerably lower temperature.

Along this final path, heat transfer occurs.

The area inside the closed path on the diagram is the net work done by the process.

It is absolutely necessary for heat transfer from the system to occur in order to get a net work output.
In the Otto cycle, heat transfer occurs.
The return path is the same if there is no heat transfer.
The less work needs to be done to compress the gas the lower the temperature.
The engine does more work and is more efficient because the area inside the closed path is greater.
The more work output there is, the higher the temperature along path CD is.
The temperatures of the hot and cold reservoirs are related to efficiency.
In the next section, we will look at the absolute limit to the efficiency of a heat engine and how it relates to temperature.

In the four-stroke internal combustion gasoline engine, heat transfer into work takes place.

The work on the gas in the cylinder is done by the rotating crankshaft, which is connected to the piston.

The work on the gas is done.
The air-fuel mixture is ignites, converting chemical potential energy into thermal energy, which leads to a great increase in pressure.
The gas works when it exerts a force through a distance in a nearly adiabatic process.

The compression stroke of an internal combustion engine starts at point A. PathsAB and CD correspond to the compression and power strokes of an internal combustion engine.

The internal combustion engine's cycle can be accomplished by Paths BC and DA.
There is a net work output because more work is done by the gas along path CD.