What is the significance of the second law of thermodynamics

The ultimate attainable efficiency of any heat engine will depend on the temperatures at which heat is supplied to and removed from it. The remainder of the heat q L is exhausted to a reservoir at a lower temperature T L. In practice, T H would be the temperature of the steam in a steam engine, or the temperature of the combustion mixture in an internal combustion or turbine engine. The low temperature reservoir is ordinarily that of the local environment.

For most terrestrial heat engines, T L is just the temperature of the environment, normally around K, so the only practical way to improve the efficiency is to make T H as high as possible. This is the reason that high pressure superheated steam is favored in commercial thermal power plants.

The highest temperatures and the greatest operating efficiencies are obtained in gas turbine engines. However, as operating temperatures rise, the costs of dealing with higher steam pressures and the ability of materials such as turbine blades to withstand high temperatures become significant factors, placing an upper limit of around K on T H , thus imposing a maximum of around 50 percent efficiency on thermal power generation. For nuclear plants , in which safety considerations require lower steam pressures, the efficiency is lower.

One consequence of this is that a larger fraction of the heat is exhausted to the environment, which may result in greater harm to aquatic organisms when the cooling water is returned to a stream or estuary.

Several proposals have been made for building a heat engine that makes use of the temperature differential between the surface waters of the ocean and cooler waters that, being more dense, reside at greater depth. The specific heat capacity of water is 4. The amount of heat q H that must be extracted to cool the water by 5 K is 4. The ideal thermodynamic efficiency is given by. Comment : It may be only 1. Few toys illustrate as many principles of physical science as this popular device that has been around for many years.

At first glance it might appear to be a perpetual motion machine, but it's really just a simple heat engine. Modern "dippy birds" as they are sometimes called utilize dichloromethane as the working fluid. The liquid to which a dye is often added for dramatic effect is stored in a reservoir at the bottom of the bird. The bird's beak is covered with felt which, when momentarily dipped in water, creates a cooling effect as the water evaporates.

This causes some of the CH 2 Cl 2 vapor to condense in the head, reducing the pressure inside the device, causing more liquid to boil off and re-condense in the head. The redistribution of fluid upsets the balance, causing the bird to dip its beak back into the water. Once the head fills with liquid, it drains back into the bottom, tipping the bird upright to repeat the cycle.

We will leave it to you to relate this to the heat engine diagram above by identifying the heat source and sink, and estimate the thermodynamic efficiency of the engine. Refrigerators and air conditioners are the most commonly-encountered heat pumps. A heat pump can also be used to heat the interior of a building.

In this application, the low temperature reservoir can be a heat exchanger buried in the earth or immersed in a well.

In this application heat pumps are more efficient than furnaces or electric heating, but the capital cost is rather high.

It was the above observation by Carnot that eventually led to the formulation of the Second Law of Thermodynamics near the end of the 19th Century. One statement of this law by Kelvin and Planck is as follows:. It is impossible for a cyclic process connected to a reservoir at one temperature to produce a positive amount of work in the surroundings. To help you understand this statement and how it applies to heat engines, consider the schematic heat engine in the figure in which a working fluid combustion gases or steam expands against the restraining force of a weight that is mechanically linked to the piston.

From a thermodynamic perspective, the working fluid is the system and everything else is surroundings. Expansion of the fluid occurs when it absorbs heat from the surroundings; return of the system to its initial state requires that the surrounding do work on the system. Now re-read the above statement of the Second Law, paying special attention to the italicized phrases which are explained below:.

Note carefully that the Second Law applies only to a cyclic process — isothermal expansion of a gas against a non-zero pressure always does work on the surroundings, but an engine must repeat this process continually; to do so it must be returned to its initial state at the end of every cycle. The Second Law can also be stated in an alternative way:. It is impossible to construct a machine operating in cycles that will convert heat into work without producing any other changes.

And since heat can only flow spontaneously from a source at a higher temperature to a sink at a lower temperature, the impossibility of isothermal conversion of heat into work is implied.

Chem1 Virtual Textbook. Learning Objectives You are expected to be able to define and explain the significance of terms identified in green type. This waste heat must be discarded by transferring it to a heat sink. In the case of a car engine, this is done by exhausting the spent fuel and air mixture to the atmosphere.

Additionally, any device with movable parts produces friction that converts mechanical energy to heat that is generally unusable and must be removed from the system by transferring it to a heat sink. This is why claims for perpetual motion machines are summarily rejected by the U. Patent Office. When a hot and a cold body are brought into contact with each other, heat energy will flow from the hot body to the cold body until they reach thermal equilibrium, i.

However, the heat will never move back the other way; the difference in the temperatures of the two bodies will never spontaneously increase. Moving heat from a cold body to a hot body requires work to be done by an external energy source such as a heat pump. The Second Law indicates that thermodynamic processes, i. Perhaps one of the most consequential implications of the Second Law, according to Mitra, is that it gives us the thermodynamic arrow of time.

In theory, some interactions, such as collisions of rigid bodies or certain chemical reactions, look the same whether they are run forward or backward. In practice, however, all exchanges of energy are subject to inefficiencies, such as friction and radiative heat loss, which increase the entropy of the system being observed.

Is there any way out? Some physicists even think quantum machines might bend the rules or cause them to be cast in a new form.

That might not have much practical use on large scales, but one instance where quantum thermodynamics comes into play is at the event horizon of a black hole — so it could help solve the enduring riddle of how to unite general relativity with quantum theory.

The second law in its classical form also determines the ultimate fate of the universe. Or perhaps not. Other scenarios predict a more dramatic end. And the founder of classical statistical thermodynamics came up with a bizarre theory in Ludwig Boltzmann argued that, given enough time in a large enough universe, fluctuations might randomly create a sub-universe that looks like ours.