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This text is meant to accompany class discussions. It is not everything there is to know about the basics of torque and the two conditions of equilibrium. It is meant as a prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)
Questions Click for the questions that go with this reading
Isobaric Process


Isobaric means there is no the change the pressure between the states. This mean P is the same for every state on process's line.

Isobaric Example
A weight is placed on top of our box with the sliding top. The weight applies a constant pressure to the system. As the weight and lid settle into a new equilibrium, the lid slides down and changes the boxes volume. The volume changes due to this constant pressure in the lid.


Isovolumetric or Isochoric Process


Isochoric means there is no the change the volume between the states. Isovolumetric means the same thing as isochoric. This means the volume is the same for every state on process's line.

Isochoric or isovolumetric process do have any work associated with them because there is no changes in volume.

Isochoric Example

The lid is locked in place. Something cold like ice comes into contact with the system. The ice draws thermal energy out of the system and the gas gets cooler. The cooler gas means the gas's molecules slow down. This means fewer collisions with the wall. Fewer wall collisions translates into a decreasing pressure while keeping the volume the same.

The system's temperature could be increased while maintaining the volume by adding fire underneath the system while locking the lid in place.


Isothermal Process


An isothermal process is where the temperature of every state along the hyperbolic curve has the same temperature.

An isothermal process follows a hyperbolic path. Isothermal processes do not change the internal energy of a gas because the temperature is unchanged. A single isothermal process is often referred to as an "isotherm."

Because the gas's temperature is the same for every state during a process, the internal energy of the gas does not change.

Isothermal Example

In this example thermal energy is drawn out of the system by having a cooler sources come in contact with the gas in the system. However, unlike the previous process, the lid is allowed to move. As the gas in the system cools, the lid changes position in order to keep the gas from changing its temperature. Because the temperature does not change, the process is called an isothermal process.

If, instead of a cool source, a hot source was placed in contact with the gas in the system, the lid would move upwards to expand the system in such a way that the temperature would be maintained.


Adiabatic Process


An adiabatic process is one where no thermal energy is exchanged between the gas and the surroundings. If like putting the process in a large Styrofoam cooler that blocks energy from entering or leaving the gas. The curve between the two states is exponential.


An adiabatic process occurs between isotherms.

An adiabatic process is one where thermal energy is not allowed into or out of the system.

Adiabatic Example
A weight applies a constant pressure to the lid. The lid slides down and compresses the gas. The gas heats up because the insulator will not let the thermal energy leave the system. The system's pressure increases and the lid is pushed up. Because the insulator prevents energy from entering the system, the gas cools during this process and the lid slides down again. Because the lid is frictionless and no thermal energy is allowed to enter or leave the system, this oscillation will go on forever. Note the addition of an insulator to the system's perimete to block the transfer of energy between the system and the surroundings.
Which process on this PV diagram is isothermal and which is adiabatic for this sealed system?

PV Diagram

Video Solution

This video can be viewed on YouTube at


Visual Summary

Number 1


Number 2


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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)