28.3 Mechanical Systems

Transcription

1 Most mechanical systems such as automobiles, simple and complex machines, and power generators contain many moving parts that are in contact with each other. In addition, some of these mechanical systems are designed to perform tasks such as heavy lifting or harsh movements. All of this work is accomplished by the input of energy. In the process of doing work, mechanical systems also generate thermal energy in the form of heat. The heat is wasted energy that dissipates into the atmosphere. Where does this heat come from? In this section, you will learn about the heat generated in mechanical systems, and some of the technology developed to solve the problems associated with heat loss. You will also learn how heat can be used to generate mechanical work. Where does the heat in mechanical systems come from? Most of the heat comes from friction The deformation of objects generates heat Fluid or air friction is another cause of heat in mechanical systems If you rub your hands together for a minute they will soon feel very warm. This warmth is due to friction. In most mechanical systems, the major loss of energy, in the form of heat, is due to friction. Friction not only occurs with sliding objects but also with rotating objects. When a pulley turns on an axle, the axle rubs against the pulley, generating friction. Heat can also be generated from the deformation of an object. When you drop a basketball, as shown in figure 28.15, it does not return to the same height after a few bounces. Every time the basketball hits the ground, it is compressed. The change in shape of the ball causes friction between the individual molecules in the ball, thus generating heat. Another source of heat can be fluid resistance such as air resistance. When the space shuttle returns from orbit as shown in figure 28.16, it enters the atmosphere at a very high speed. The air molecules are moving so fast over the bottom of the shuttle, that if there were no heat resistant tiles, the space shuttle would burn up. Whenever meteorites enter the atmosphere they burn up. Most meteorites are no larger than a grain of sand, but if you have ever seen one in the sky, they radiate enough heat and light that we can see them from miles away. Figure 28.15: Heat can be generated from the deformation of an object. Figure 28.16: As the space shuttle enters the atmosphere, heat is generated by air resistance. 483

2 Reducing losses due to friction 484 Why do we lubricate machines? Ball bearings are used to reduce friction Reducing friction by floating on air Is friction always harmful? Two dry surfaces sliding against each other can quickly generate a lot of friction and heat. To reduce friction we use lubricants, like oil. You add oil to a car engine so the pistons can slide back and forth with less friction. Even water can be used as a lubricant under conditions where there is not too much heat. Powdered graphite is another lubricant. A common use of graphite is to spray it into locks so a key will slide better. In systems where there are axles, pulleys, and rotating objects, ball bearings are used to reduce friction. A rotating shaft in a plain hole would rub and generate a great amount of heat from friction (figure 28.17, top picture). Oil helps, but can leak out the sides of the hole. Ball bearings are small, hard balls of steel that go in between the shaft and the hole it turns in. The shaft rolls on the bearings instead of rubbing against the walls of the hole. The bearings rotate easily and greatly reduce friction in the system (figure 28.18, bottom picture). Some oil (or grease) is still required to keep the bearings rolling smoothly. Another method of reducing friction is to separate two surfaces with a cushion of air. A hover-craft, floats on a cushion of air created by a large fan. Electromagnetic forces can also be used to separate surfaces. Working prototypes of a magnetically levitated train, or maglev, have been built from several designs. A maglev train floats on a cushion of force created by strong electromagnets (figure 28.19). Once it gets going, the train does not actually touch the rails. Because there is no contact, there is far less friction than in a normal train. The ride is also smoother, allowing much faster speeds. Despite efforts to get rid of friction in machines, there are many applications where friction is very useful. In a bicycle, when you apply the brakes, two rubber pads apply pressure to the rim. Friction between the brake pads and the rim slows down the bicycle. The kinetic energy of the bicycle becomes heat in the brake pads. Without friction, the bicycle would not be able to slow down. Figure 28.17: The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat. Friction can be reduced by placing ball bearings between the shaft and the outer part. Figure 28.18: In a maglev train, there is no contact between the moving train and the rail. This means that there is very little friction.

3 Using heat to do mechanical work: external combustion engine Steam engines How a steam engine works One of the most important practical inventions to harness the power of heat was the steam engine. The steam engine is an example of an external combustion engine, because the action of heating takes place outside the engine. Originally, wood and coal were burned to create steam. In modern steam engines, coal is still burned, along with oil and even garbage! In a nuclear power plant, nuclear reactions in uranium generate heat to boil water into steam to turn a turbine and make electricity. In a simple steam engine, heat boils water to create steam. The hot steam is created at high pressure and passes through a valve into the cylinder, pushing back the piston. When the piston reaches the bottom it opens a valve to exhaust the expanded steam. The inertia of the flywheel then carries the piston back up the cylinder. When it gets to the top, the piston opens the intake valve and a fresh charge of hot steam pushes it back down again. The exhausted steam is condensed back to liquid water and pumped back into the boiler to repeat the cycle. James Watt James Watt was a Scottish engineer who perfected the steam engine. Watt s first contribution to the steam engine, in 1769, involved building separate chambers for the condensing of water. This allowed the cylinder to always remain at the temperature of steam, while the cooling of the steam took place in a separate chamber. Early steam engines were used in trains and boats. Before the invention of the gasoline-powered internal combustion engine, even early cars used steam engines. In parts of the world, steam locomotives are still used today. 485

4 Modern steam engines use a turbine instead of a piston and cylinder In the modern steam engine, the hot steam passes through fins in a giant turbine. As the steam expands, it turns the turbine. The turbine turns an electric generator which produces electricity. Turbines are much more efficient than pistons. More of the heat of the steam is converted to useful energy. When the steam leaves the turbine it is still warm and must be cooled to condense back into liquid water. The liquid water can be pumped back into the boiler to be reheated into steam. In some cases water from a flowing river is used to cool and condense the steam. In other cases cooling towers are built where water passes down the inner walls of a giant tube and is cooled by the air as it falls (figure 28.19). Most of the wasted heat goes to the atmosphere or nearby rivers. What are some the environmental consequences to these strategies for releasing wasted heat? Figure 28.19: Giant cooling towers are built to cool hot water before it leaves the power plant. The efficiency of a turbine 486 Because some heat is always rejected at the end of the cycle, not all the original heat energy is converted to mechanical energy by the turbine. The efficiency of the turbine is the ratio of how much useful energy is extracted compared with how much energy is available. Typically, the best turbines are only 40 percent efficient, meaning almost two-thirds of the heat energy from fuels gets wasted. What do you think? Other methods of turning the turbine in an electric power plant include using gravitational potential energy (hydroelectric power plants), harnessing the wind (windmills), and splitting uranium atoms (nuclear energy). What are the pros and cons of each of these methods?

5 Using heat to do mechanical work: internal combustion engine Internal combustion engines Intake stroke Compression stroke Power stroke Exhaust stroke The internal combustion engine was developed in Germany in the late nineteenth century. In an internal combustion engine, the burning process takes place inside the cylinder (see graphic at the bottom of page 485). The most common engine is the four-stroke engine. Almost every car and motorcycle is powered by this kind of engine. Credit for inventing the internal combustion engine is given to Nikolaus Otto, who constructed the first practical engines in First, the vapors from a fuel such as gasoline are mixed with air to create a highly explosive mixture. This mixing is done in a carburetor. In modern automobiles, a precise mixture of fuel and air is injected directly into each cylinder. During the intake stroke, the intake valve opens, allowing fuel and air into the cylinder. As the piston goes down, it draws the fuel and air into the cylinder. Once the piston reaches the bottom, the intake valve closes. On the way back up for the compression stroke, the piston compresses the fuel-air mixture. At the top of the stroke, the mixture is compressed almost 10 times and is ready to ignite. A spark plug creates a spark in the mixture, igniting the compressed fuel and air in a small but powerful explosion. After ignition comes the power stroke and the exploding fuel expands quickly due to the heat released by the chemical reaction of burning. The piston is pushed with great force back down, turning the shaft of the engine and making your car go forward. The bigger the cylinder and piston, the more forceful the explosion stroke and the more powerful the engine. When the piston gets to the bottom, the fuel is all burned. As it moves up again for the exhaust stroke, a second valve opens, and the piston pushes the burned fuel and air out. The flywheel in the engine keeps the piston moving to begin the cycle again. The intake valve opens and the piston draws fresh fuel and air into the cylinder. Modern car engines routinely turn at speeds of 3,000 revolutions per minute or more. Since the spark plug fires every second revolution, each cylinder in your engine experiences 1,500 explosions every minute you are driving! Figure 28.20: The four strokes of an internal combustion engine. 487