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Thermodynamics Study Guide

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Introduction

The purpose of this lesson is to take this knowledge one step forward and put into law some of the "natural" behaviors of fluids when they exchange heat with their surroundings–the law of thermodynamics. Thermodynamics is a compound word meaning change of heat, in Greek. We will relate heat, work, and internal energy, and then study the laws of thermodynamics. We will exemplify these laws through simple processes that are the foundation of applications such as engines and refrigerators.

Thermodynamic Work

From a microscopic point of view, we have learned that particle—atoms, molecules, and ions—are in a continuous motion. They also interact with their neighbors. These facts are measured by two types of energies that can characterize the atomic components of each object, be it in a solid, a liquid, or a gas phase. The two types of energies are called kinetic and potential energies. Kinetic energy measures the translational, rotational, and vibrational motion, whereas the potential energy deals with the interaction between particles. The two energies together constitute the internal energy of the system regardless of its phase (solid, liquid, or gas).

The question we will be asking now is how to change this state of motion and interaction? The answer is through energy exchange. Then, the next step is to realize what forms of energy are appropriate for this goal. One evident answer, from the past lesson, is heat. Heat transferred to or from an object will change the state of the object, its temperature, and its internal energy. Particles will move faster or slower with the input or release of heat (respectively).

But is heat the only way to accomplish the change in internal energy? The answer: With objects, you can always use mechanical interaction, or work. The mechanical parameters characterizing the object are the pressure, P, and the volume, V. Both of them will affect the internal state of the system.

The mechanical parameters depend on each other through an equation called the equation of state. In the special case of the ideal gas, where particles are considered identical and independent of each other, these two parameters are connected through the expression called the ideal gas law:

P · V = n · R· T

Where T is the temperature in Kelvin, n is the number of moles of substance studied, and R is the universal gas constant and is equal to:

R = 8.314472[15] J. K1 · mol–1

Consider now a thermodynamic system on which we apply mechanical work (for instance, the piston in a car), and assume that there is a gas in the piston. Next, assume that we compress the volume by applying an external force. What we are doing is performing work on the gas; so the gas receives energy, this time in the form of mechanical work. We can use the definition of work we studied a few lessons back and calculate this work by replacing the force with the corresponding constant pressure (force exerted on an area A, as shown in Figure 12.1).

W = F · d = (P · A) · d

But A · d = ΔV is the change of volume for the gas. You can see that because the piston is pushed in, the volume decreases, and then the change in volume is negative. In order to maintain a positive work performed on the system, the previous work equation has to be modified by inlcluding a minus sign.

W = –P · ΔV

The work done on a gas at constant pressure is equal to the product of pressure and the change in volume.

As we can see from the above definition, work is dependent on a mechanical change. Therefore, a certain state has no characteristic work; rather the change in state can be triggered by work being exerted on the system or by the system exerting work on its surroundings.

Positive or Negative Work

The work performed by a thermodynamic system on its surroundings is considered negative (as in the case of an exploding pop bottle). The work performed by the surroundings on the system is considered positive (as in the previous example where the gas is compressed from outside).

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