Heat and Work – Isobaric, Isochoric, Isothermal, and Adiabatic Processes | MCAT Physics Prep

Need help preparing for the Physics section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know Heat and Work – the Isobaric, Isochoric, Isothermal, and Adiabatic Processes. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!

The thermodynamic laws refer to things called systems and surroundings, but what exactly are those? A thermodynamic system is tricky to define, because it is essentially whatever we define it to be for a given problem. We could say our system of interest is an organism, a container of gas, or even a car. By definition, anything that is not the system is called the surroundings. The conservation of energy theorem suggests that energy is neither created or destroyed. Therefore, if we measure the energy of a system to increase or decrease, energy must have been gained from or lost to the surroundings, respectively.

The transfer of energy between a system and its surroundings can be described by the transfer of heat and work. An equation describing this is , where ∆E represents the gain or loss of energy in a system. Temperature is proportional to the energy of molecules in a system, so temperature can be measured to determine whether a system gains or loses energy. Q stands for heat and W stands for work. Heat can flow into a system, which we define as positive Q, or out of a system, which is negative Q. Work doesn’t flow like heat. Instead, a change in the work of a system involves a change in the volume of the system.

If the volume of a system is increasing, the molecules in the system are pushing against the walls of their container to expand the system. Therefore, the molecules are giving off energy, and the system has an overall decrease in energy. If the volume decreases, something external to the system is pushing against the system to compress it. Therefore, by the surroundings acting to compress the system, the system experiences an overall increase in energy. If the system is decreasing in volume and gaining energy, we can say the system experienced positive work (+W).

Pressure Volume Diagrams

When heat and work are changing in a system, the pressure and volume of the system are going to be directly affected. We can represent these differences using a pressure volume diagram, of which there are four different types, depending on the type of process that has occurred: isobaric, isochoric, isothermal, or adiabatic. A pressure volume diagram has volume on its x-axis and pressure on its y-axis.

If we want to determine the work done on a system, we need to calculate the area under the curve on a PV diagram. Work is equal to the force applied to an object over a distance, and force is equal to the pressure applied over an area. By combining the ideas in these equations, we find that work on a system is equal to the pressure applied to it divided by its volume (). Notice that this equation provides the area under the curve of a PV diagram, which is why calculating this is our method for determining the work done on a system.

Isobaric Processes

An isobaric process is a process that occurs under constant pressure. For example, consider a piston with gas molecules inside. If heat flows into the system, then the gas molecules inside the system are moving faster and exerting a greater pressure on the container walls. The greater pressure inside will push the piston up, increasing the volume of the system. This process on a PV diagram is represented by a horizontal line.

Isochoric Processes

Isochoric processes are those that occur with a constant volume. If volume is unable to change, no work can be performed on the system. By process of elimination, any change in the energy of a system must be due to heat added to or released from the system. On a PV curve, since volume is a constant, these processes are represented by a vertical line with no area under the curve.

Isothermal Processes

Isothermal processes are those which occur at a constant temperature. Temperature is directly proportional to the energy of the system, so the energy of the system must not change (ΔE = 0 and Q + W = 0). In other words, any heat added to the system must be compensated for by the system doing work.

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