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Abnormal Psychology

The Second Law of Thermodynamics and Heat Engines

There are a number of ways to express the Second Law of Thermodynamics. One expression of the Second Law by Lord Kelvin is that it is impossible to engineer a transformation whose only result would be to convert heat from a source at constant temperature into work. Another expression of the Second Law is by Rudolf Clausius, that heat cannot of itself pass from a colder to a hotter body. What can unify these expressions expressions conceptually is to see that they are both ways of saying that the entropy of an isolated system will increase over time, approaching a maximum value at equilibrium. Entropy is a function which can only increase for a system and its surroundings. Entropy is time's arrow. A function which is always going to be greater in the universe. While it is useful to think of entropy as 'disorder', make sure you learn to see this in an abstract statistical sense. Entropy is the number of possible microscopic configurations of a system. Think about that. As spontaneous change occurs, and a system approaches equilibrium, as entropy increases, differences in temperature or chemical potential smooth out. If a direction of change leads to a state that is statistically more likely, it occurs spontaneously.

The Second Law appears in direct fashion with moderate frequency on the MCAT. For example, a physical sciences passage on a chemical reaction or thermodynamic process may pose a question as to whether a particular change corresponds to an increase or decrease of entropy. Bear in mind, though, that the direct importance for the test, with regard to this topic, is dwarfed by the significance of the Second Law of Thermodynamics for understanding the physical and biological sciences in a rich way.

Focus please! This is an important conceptual bridge which only a minority of premedical students will ever cross, where you can not only sieze an advantage over the competition, but also gain a foothold into a much deeper understanding of science, which will make the universe more coherent to you for the rest of your life. Most of your peers don't have a good conceptual feel for the Second Law. Believe me. I have worked with many premedical students. I hate to say it but this means that they don't really understand chemistry or biology, in my opinion, no matter how good their grades have been in college. Start thinking about the 2nd law as a walking around habit. Walk around with it. Be patient. Always look for another step into a deeper understanding. It will be worth it!

Heat and Temperature

The First Law of Thermodynamics

The Second Law of Thermodynamics and Heat Engines

As you recall, the First Law of Thermodynamics is based on conservation of energy. The energy that a system exchanges with its surroundings, i.e. heat flow and work, must be offset by a change in internal energy in the system. Conversely, the internal energy of a system has changed, the amount of change must be complete accountable in terms of heat flow and work.

Okay, now it is time for a big transition in the main sequence. We are moving to the Second Law of Thermodynamics, a 'deeper' law. Our study will be asking why some thermodynamic changes are more likely to occur than others. Through a cyclic process, work can be converted completely into heat, but it is impossible for a thermodynamic cycle to convert an amount of heat flow completely into work without changing the surroundings. Why is the evolution of heat by the system favored?

We will learn that in a thermodynamic process involving the conversion of heat into work, some heat must be released at the cold sink. Otherwise, the entropy of the universe would decrease with each cycle. The universe would be winding up instead of winding down, but this doesn't happen. The universe winds down. In other words, entropy is a function that always increases in the physical world.

There are many ways to express the second law. All basically say that overall change produces disorder because disorder is the more probable state of things. Therefore, increasing disorder is a component of every spontaneous, real energy conversion.

Heat and Temperature

The Ideal Gas and Kinetic Theory

The Second Law of Thermodynamics and Heat Engines

In our discussion of the Ideal Gas and Kinetic Theory we approached temperature as a measure of the concentration of particle level kinetic energy in a substance. Concentration is a somewhat difficult idea here, requiring reference to the basis of molar heat capacity, with reference to how many modes for kinetic energy at the particle level, translational, vibrational, rotational, a substance can absorb. A substance with many vibrational and rotational modes can absorb a large amount of internal energy with only a small change in temperature. Okay, so that is the Kinetic Theory approach to temperature which we have been using very fruitfully thus far.

Now that we are moving into the Second Law of Thermodynamics, let us expand our conception of the temperature. Let us begin to look at temperature within a different conceptual framework and start thinking of temperature as a sort of potential function for the escaping tendency of heat.

If the temperature is high in a region or area, it will probably not be so for very long. As heat flows from the warmer to the cooler object in thermal contact, the increase in entropy in the cold object is greater than the decrease in entropy for the warm object. Think about that! The heat brings more disorder to the cold object than is lost by the hot object. The total entropy in the universe increases. The temperature difference predicts the direction of spontaneous heat flow.

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