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Because individual collisions are unchanged by reversing the direction of time, heat can flow just as well in one direction as the other. Thus, from the point of view of fundamental interactions, there is nothing to prevent a chance event in which a number of slow-moving cold molecules happen to collect together in one place and form ice, while the surrounding water becomes hotter.

Such chance events could be expected to occur from time to time in a vessel containing only a few water molecules. However, the same chance events are never observed in a full glass of water, not because they are impossible but because they are exceedingly improbable. This is because even a small glass of water contains an enormous number of interacting molecules about 10 24 , making it highly unlikely that, in the course of their random thermal motion, a significant fraction of cold molecules will collect together in one place.

Although such a spontaneous violation of the second law of thermodynamics is not impossible, an extremely patient physicist would have to wait many times the age of the universe to see it happen. The foregoing demonstrates an important point: the second law of thermodynamics is statistical in nature.

What is the second law of thermodynamics?

It has no meaning at the level of individual molecules, whereas the law becomes essentially exact for the description of large numbers of interacting molecules. In contrast, the first law of thermodynamics , which expresses conservation of energy, remains exactly true even at the molecular level. The example of ice melting in a glass of hot water also demonstrates the other sense of the term entropy , as an increase in randomness and a parallel loss of information. Initially, the total thermal energy is partitioned in such a way that all of the slow-moving cold molecules are located in the ice and all of the fast-moving hot molecules are located in the water or water vapour.

After the ice has melted and the system has come to thermal equilibrium, the thermal energy is uniformly distributed throughout the system. Using the entropy of formation data and the enthalpy of formation data, one can determine that the entropy of the reaction is Because both enthalpy and entropy are negative, the spontaneous nature varies with the temperature of the reaction.

The temperature would also determine the spontaneous nature of a reaction if both enthalpy and entropy were positive. When the reaction occurs at a low temperature the free energy change is also negative, which means the reaction is spontaneous. However, if the reaction occurs at high temperature the reaction becomes nonspontaneous, for the free energy change becomes positive when the high temperature is multiplied with a negative entropy as the enthalpy is not as large as the product.

Only after calculating the enthalpy and entropy of the reaction is it possible for one can answer the question. The enthalpy of the reaction is calculated to be Unlike the previous two examples, the temperature has no affect on the spontaneous nature of the reaction. Looking at the formula for spontaneous change one can easily come to the same conclusion, for there is no possible way for the free energy change to be positive.

Hence, the reaction is spontaneous at all temperatures. The second law occurs all around us all of the time, existing as the biggest, most powerful, general idea in all of science. When scientists were trying to determine the age of the Earth during s they failed to even come close to the value accepted today.

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They also were incapable of understanding how the earth transformed. Lord Kelvin, who was mentioned earlier, first hypothesized that the earth's surface was extremely hot, similar to the surface of the sun. He believed that the earth was cooling at a slow pace. Using this information, Kelvin used thermodynamics to come to the conclusion that the earth was at least twenty million years, for it would take about that long for the earth to cool to its current state.

Twenty million years was not even close to the actual age of the Earth, but this is because scientists during Kelvin's time were not aware of radioactivity. Even though Kelvin was incorrect about the age of the planet, his use of the second law allowed him to predict a more accurate value than the other scientists at the time. Some critics claim that evolution violates the Second Law of Thermodynamics, because organization and complexity increases in evolution. However, this law is referring to isolated systems only, and the earth is not an isolated system or closed system.

This is evident for constant energy increases on earth due to the heat coming from the sun. So, order may be becoming more organized, the universe as a whole becomes more disorganized for the sun releases energy and becomes disordered. This connects to how the second law and cosmology are related, which is explained well in the video below.

Introduction Why is it that when you leave an ice cube at room temperature, it begins to melt? The current form of the second law uses entropy rather than caloric, which is what Sadi Carnot used to describe the law. The first law of thermodynamics can be captured in the following equation, which states that the energy of the universe is constant.

Energy can be transferred from the system to its surroundings, or vice versa, but it can't be created or destroyed. A more useful form of the first law describes how energy is conserved. It says that the change in the internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done by or on the system.

The sign convention for the relationship between the internal energy of a system and the heat gained or lost by the system can be understood by thinking about a concrete example, such as a beaker of water on a hot plate. When the hot plate is turned on, the system gains heat from its surroundings. As a result, both the temperature and the internal energy of the system increase, and E is positive. When the hot plate is turned off, the water loses heat to its surroundings as it cools to room temperature, and E is negative.

The relationship between internal energy and work can be understood by considering another concrete example: the tungsten filament inside a light bulb. When work is done on this system by driving an electric current through the tungsten wire, the system becomes hotter and E is therefore positive. Eventually, the wire becomes hot enough to glow. Conversely, E is negative when the system does work on its surroundings.

The sign conventions for heat, work, and internal energy are summarized in the figure below. The system is usually defined as the chemical reaction and the boundary is the container in which the reaction is run. In the course of the reaction, heat is either given off or absorbed by the system. Furthermore, the system either does work on it surroundings or has work done on it by its surroundings.

Either of these interactions can affect the internal energy of the system.

2. The Problem of the Direction of Time I

Two kinds of work are normally associated with a chemical reaction: electrical work and work of expansion. Chemical reactions can do work on their surroundings by driving an electric current through an external wire. Reactions also do work on their surroundings when the volume of the system expands during the course of the reaction The amount of work of expansion done by the reaction is equal to the product of the pressure against which the system expands times the change in the volume of the system.

The sign convention for this equation reflects the fact that the internal energy of the system decreases when the system does work on its surroundings. What would happen if we created a set of conditions under which no work is done by the system on its surroundings, or vice versa, during a chemical reaction?

Thermodynamics | Physics For Idiots

Under these conditions, the heat given off or absorbed by the reaction would be equal to the change in the internal energy of the system. The easiest way to achieve these conditions is to run the reaction at constant volume, where no work of expansion is possible. At constant volume, the heat given off or absorbed by the reaction is equal to the change in the internal energy that occurs during the reaction. The figure below shows a calorimeter in which reactions can be run at constant volume.

Thermodynamics and the End of the Universe: Energy, Entropy, and the fundamental laws of physics.