Engineering Thermodynamics/Introduction

Introduction to Classical Thermodynamics Thermodynamics is the study of energies. More specifically, introductory thermodynamics is the study of energy transfer in systems. Classical thermodynamics consists of methods and constructs that are used to “account” for macroscopic energy transfer. In fact, energy accounting is an appropriate synonym for classical thermodynamics. In much the same way that accountants balance money in and money out of a bank account, rocket scientists simply balance the energy in and out of a rocket engine. Of course just as a bank account’s balance is obfuscated by arcane devices such as interest rates and currency exchange, so too is thermodynamics clouded with seemingly difficult concepts such as irreversibility and enthalpy. But, also just like accounting, a careful review of the rules suggests a coherent strategy for maintaining tabs on a particular account. If a statement about the simplicity of thermodynamics failed to convert would-be students, they may be captured with a few words on the importance of understanding energy transfer in our society. Up until about 150 years ago or so, the earth’s economy was primarily fueled by carbohydrates. That is to say, humans got stuff done by converting food, through a biological process, to fuel we could spend to do work (e.g. raise barns). This was a hindrance to getting things accomplished, because it turned out that most of that energy went to growing and cultivating more carbohydrates (e.g. crops and livestock). We won’t even talk about how much food the horses ate! Today, we have the luxury, primarily through an understanding of energy, to concentrate our energy production into efficient low maintenance operations. Massive power plants transfer energy to power tools for raising barns. Extremely efficient rocket engines tame and direct massive amounts of energy to blast TV satellites into orbit. This improvement in energy mastery frees humanity’s time to engage in more worthwhile activities such as watching cable TV. Although most are content to blissfully ignore the intricacies that command their way of life, I challenge you to embrace the contrary. By no means is the energy battle over. Understanding energy transfer and energy systems is the second step to overcoming the limits to what humanity can accomplish. The first step is commanding an interest in doing so from an inclined portion of the population. Given the reader (and editor) has read this far through this aggrandizing rhetoric, I welcome your interest and hope to see it continue until the end. The main macroscopic forms of energy It will be in the best interest of the reader to have defined the means by which energy is contained, and transferred, before it is discussed further. There are three primary forms of energy that are discussed in macroscopic thermodynamics. Several other forms of energy exist, but they generally exist on a microscopic level and should be deferred to more advanced study. Kinetic energy The first form (probably most easily understood idea of energy) is defined by the motion of an object. Kinetic energy is the energy of a moving mass. For instance, a moving car will have more kinetic energy than a stationary car. The same car traveling at 60 km/h has more kinetic energy than it does traveling at 30 km/h. Kinetic Energy = (1/2) x (mass) x (velocity)2 For example, imagine two vehicle moving at constant speeds, the first at 60 km/hr, and the second at 30 km/hr. How much more kinetic energy does the first vehicle, compared to the second? By applying the previously introduced equation for kinetic energy, and making a ratio of the two, it is possible to easily compare them: Ratio of v602 / v302 =((60)(1,000 meters/sec)(3600 sec/hour))2 / ((30)(1,000 meters/sec)(3,600 sec/hour))2 So, once the algebra is completed properly we find the vehicle traveling at 60 km/h has four times as much kinetic energy as when it is traveling at 30 km/h. Now imagine that the second vehicle moving at 30 km/hr, now has a mass, that is double the first's. Even though the vehicle's mass doubled, the energy only doubled. Hopefully it is apparent that while kinetic energy is proportional to mass, it is proportional to the square of velocity. In addition, given the same reference plane, the vehicle has zero kinetic energy when it is stationary because the velocity = zero(0) results in 1/2mv2(1/2*m*0=0) being zero. Potential energy The second type of energy is called potential energy. Gravitational potential energy describes the energy due to elevation, or the potential difference in a gravitational field. A car at a height of 50 meters has more potential energy than a car at a height of 25 meters. This may be understood more easily if the car is allowed to drop from its height. The moment before impact with the earth at 0 m, the car that initially rested at 50 m will have more kinetic energy because it was moving faster (allowed more time to accelerate). The idea that potential energy can convert to kinetic energy is the first idea of energy transfer. Transfers between kinetic and potential energy represent one type of account balance rocket scientists need be aware of. Potential Energy = (mass)*(acceleration due to gravity)*(elevation with respect to reference line). Internal Energy of Matter The third and most important concept of energy is reflected by temperature. The internal energy of matter is measured by its temperature. Hot water has more internal energy than the same amount of cold water. Internal energy is a measure of kinetic energy of the molecules and atoms that make up the substance. Since each atom or molecule is acting on its own accord, this internal energy is different from the bulk kinetic energy associated with the movement of the entire solid. The internal energy of matter is exhibited by molecular motion. The molecules of a gas at high temperature zip around their container constantly colliding with walls and other molecules. The molecules of a high temperature solid also move around a lot; however, since they are stuck together with other molecules, the most they can do is vibrate in place. In a nutshell, the above forms of energy are studied in classical thermodynamics. Those forms of energy are allowed to transfer among each other as well as in to or out of a system. Thermodynamics essentially provides some definitions for interpreting thermodynamic systems. It then goes on to define an important rule about fairly balancing energy and one rule about the quality of energy. (some energy is more valuable) Understanding the framework and the few rules that govern macroscopic thermodynamics proves to be an incredibly powerful set of tools for analyzing a myriad of not only engineering problems, but issues of practical concern.