Chapter_3_heat_and_work

Chapter_3_heat_and_work - Chapter 3 Heat and Work An engine...

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Chapter 3 – Heat and Work An engine is a device that extracts energy from some source and converts that energy into useful work. In aircraft engines, the energy source is the fuel, and the useful work that is produced is used to push the aircraft forward. In order to understand how energy is converted into useful work and to understand why aircraft engines are built the way they are, we need to study a subject called thermodynamics . Thermodynamics is a branch of science concerned with the conversion of heat into work and vice versa. You are probably very familiar with the ease in which we can turn work into heat. For example, you can feel your hands warm up when you rub them together rapidly. On the other hand, converting heat into work is not usually a simple task. Heat and work are both manifestations of one thing - energy – and we use the same units to measure both. The SI unit is called the Joule (abbreviated J ), and the English unit is called the British Thermal Unit (abbreviated BTU ). To convert from one system to the other, use the conversion: 1 BTU = 1055.1 J Energy, work, and heat are terms you probably use quite often, and you probably have a qualitative idea of what they are. In our study of the thermodynamics of aircraft engines, we will assign a precise definition to these terms. Work Work can be defined in a number of ways depending on the context. In the thermodynamic context, we usually mean mechanical work which is defined as the product of force and distance moved by the point of application of the force, i.e., force times distance. W = F · d (3.1) Actually, this definition is somewhat simplified, but you will learn more about that later in your physics course. Since we are multiplying a force times a distance, we need to introduce a few more units here. In the SI system, a force has the units of Newtons (abbreviated N ), and distance has the units of meters (abbreviated m ). One newton-meter equals one Joule. 1 N-m = 1 J Example: A man pushes a block across the floor from Point A to Point B. The force he applies is a steady 100 N and Point A is 30 meters away from Point B. How much work has the man done? 22
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Solution: Work = force x distance J Nm J Nm m N 3000 3000 30 100 = = × Heat Heat is not so easily defined. If we add heat to a substance, then usually the temperature increases. If we denote the amount of heat by Q (Joules) and the temperature rise T (measured in appropriate units), then we can often say that Q is proportional to T. Mathematically, we can state that as follows: Q α T (3.2) What I have shown here is a proportion. In order to turn this into an equation, we need a constant of proportionality . We can get this constant by performing experiments in which we carefully add a known quantity of heat to a substance and measure the temperature. We can do this over a wide range of heat inputs and plot the temperature response on a graph. If the temperature really is proportional to the input, our graph will
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Chapter_3_heat_and_work - Chapter 3 Heat and Work An engine...

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