Difference Between Adiabatic, Isothermal and Isobaric

What is Adiabatic?

An adiabatic system or process is one in which there is no net change in heat energy. Adiabatic processes are related to the First Law of Thermodynamics. This law states that when heat energy is placed into a system, it will either change the internal energy of the system or it will do work. This is related to the law of the conservation of energy which states that matter and energy cannot be created or destroyed. In the context of thermodynamics, heat energy in a system must do something. It will either change the internal energy of the system, do work, or some combination of both. It cannot just disappear.

 In an adiabatic system, pressure, volume, and temperature will change in such a way that the heat energy remains constant. Adiabatic processes are most clearly seen in gases. Adiabatic heating in a gas will cause the temperature to increase as pressure on the gas increases. If the pressure on the gas decreases, this will cause the temperature to drop, resulting in adiabatic cooling. With adiabatic heating, gas will be compressed and thus work will be done on the gas by the environment. If adiabatic cooling occurs, this will result in the gas expanding and the gas will do work on the environment. 

An example where adiabatic processes are important is in the context of a piston, such as a piston in a diesel engine. As pressure from the piston increases, the gas will contract. With decompression, the gas will expand again, moving the piston. This is controlled by adiabatic processes.

Adiabatic processes are important in meteorology. If a parcel of air rises, the pressure on the parcel of air will decrease and this will cause the air temperature to decrease due to adiabatic cooling. On the other hand, if an airmass is pushed against the ground, it will cause the pressure on the airmass to increase, warming up the airmass. Because air pressure decreases with altitude, the temperature will decrease with height in the atmosphere. The rate at which the temperature decreases with increasing altitude is known as the adiabatic lapse rate.

What is Isothermal?

An isothermal process is one where the temperature remains constant even if the pressure and volume change. In thermodynamics, pressure, temperature, and volume are related by Boyle’s gas law. If one is held constant, the others will change in proportion to each other. If the temperature of a gas is held constant, the pressure and volume of the gas will be inversely proportional. 

An example of an isothermal process is a change of phase. When a substance, such as water, reaches its melting point or boiling point, the pressure and temperature will remain constant as the phase, volume, and heat energy change.

Isothermal processes form the basis of heat engines which are used in electrical power plants, cars, airplanes, rockets, and other machines that are important for modern civilization. Isothermal processes are also important in biology, geology, space science, planetary sciences, and many other fields.

What is Isobaric?

In an isobaric process, the pressure in a system remains constant. Under isobaric conditions, volume and temperature are directly related. If temperature increases, so must the volume. This can be illustrated by placing a balloon in a freezer. The pressure both inside the balloon and outside will remain constant, but the balloon will begin to shrink in volume as it cools down. 

Another example is a weighted piston that is moved by heated gas in a cylinder. As the gas is heated, the gas temperature rises, and the gas expands, pushing on the piston. If the piston were fixed and could not move, the pressure in the gas would rise instead of the gas expanding and the system would not be isobaric.

Isobaric processes are important in the construction of heat engines since certain heat engines rely on isobaric processes to convert heat energy into mechanical energy. 

Similarities between adiabatic vs. isothermal vs. isobaric

Adiabatic, isothermal, and isobaric processes are all related to pressure, temperature, and volume. They are also all most well illustrated with gases. All three types of processes are also most relevant in planetary atmospheres.

Differences between adiabatic vs. isothermal vs. isobaric

Although these processes have similarities, they also have important differences. These include the following.

• The temperature of a gas will decrease as the gas expands in an adiabatic system, whereas the temperature will remain constant as the gas expands in an isothermal system and increase as the gas expands in an isobaric system.
• In an adiabatic or isothermal system, the volume of a gas is inversely proportional to temperature, whereas it is directly proportional to temperature in an isobaric system.
• The pressure of a gas is inversely proportional to volume in an isothermal system, whereas it does not change in an isobaric system, and the pressure on a gas is inversely proportional to volume in an adiabatic system.
• Heat does not change in an adiabatic system, whereas it does change in an isothermal or isobaric system.

Adiabatic vs. isothermal vs. isobaric

Summary

In an adiabatic system, there is no net change in heat. When a gas expands, the temperature will drop, leading to adiabatic cooling. If a gas is compressed, the temperature will increase, leading to adiabatic heating. Adiabatic processes are important in atmospheric science. In an isothermal process, temperature is constant, and pressure and volume are inversely related to each other. An example of an isothermal process is a change of phase. During the change of phase, the temperature of a substance will not change even though its heat and volume change. In an isobaric system, the pressure remains constant and volume will increase or decrease with temperature. If a volume of gas is placed in a freezer, for example, the volume of gas will decrease in size since the pressure is constant while its temperature is dropping.

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