Crystal P. LaVoie
Students will be asked:
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- What is energy? Where does your energy come from? How is energy different from power?
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- What does it mean to do work?
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- What is heat?
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- What is an engine?
Energy Work and Power
Energy can be a confusing topic because it seems familiar to us, and our students, but as we begin to delve into it, what
IS
energy, really? If you look up the definition of energy in a dictionary you will find “potential forces; inherent power; capacity for vigorous action” (5) and if you look in the glossary of your college physics text you will find “the ability to do work”. Work is similarly difficult to talk about because we are also familiar with the concept of work, but again have some difficulty in explaining what work is from a scientific perspective.
I ask the question, as you read this document, are you doing any work? The answer is no, not because you are reading in an effortless process, but because in a strictly physical sense you are not doing any work. In order to be doing work you must be moving an object some distance by applying a force on that object. Work is done anytime there is force on and motion of an object. If there is force, but no motion (as when a table is holding up a bag of groceries) then there is no work. If there is motion, but no force (as with a moving hockey puck after it has been struck in the absence of friction
),
then no work is done. Additionally, the force must be applied in the direction of the motion in order for there to be work done. Consider another example: moving a barbell from the floor over your head requires work, however, once the barbell is overhead and is no longer moving, you are no longer doing any work.
So then, is work being done as an object falls freely? Your instinctive answer might be no, and you could be right, but it depends on how you define your system. If the system that you define includes something attached to the freely falling object, say through a pulley and the falling object actually pulls something else up at the end of their tether. Imagine an anvil falling as a piano attached to the end of a connecting rope is being lifted, using a fulcrum pulley. In this case, work is certainly being done as the piano is raised.
Defining the system then becomes an important aspect of our discussion of energy and work. Defining the system is simply indicating precisely which objects or “bodies” you are talking about and what is the environment those bodies are confined within. While I say that it is simple to make this indication, it may not be as easy as you would think. It is important that you give an unambiguous description of what you include in the system. When defining a system we talk about quantities that are in either the whole system or some well-defined portion thereof. Depending on how you define it, the quantities will change. In thermodynamics the quantities that we use are energy, heat, work and entropy.
Energy is the ability to do work. Work is any interaction between bodies that is not heat and is represented by the equation:
W = FL
where W = work, F = force and L = length, and is measure in Newton-meters, also called a Joule. Again, within the system we define, an object must be moved by a force if we are to conclude that work has been done. Doing work transfers energy. An example would be when you hammer a nail into a piece of wood. The force that you apply on the hammer is transferred first to the nail, and then to the piece of wood. Some energy is transferred to the environment in a form that cannot be harnessed. This is discussed further later.
It is necessary to distinguish between the concepts of energy and power, as students will often think of these as the same thing. Power is defined as the rate at which energy is converted from one form to another or transferred from one place to another and is given by the formula
P = W/t
where W is work and t is time (8). The
watt
is a unit of power and is defined as one Joule/second.
Activities for the above discussions:
Students will conduct activities that will connect with the above discussions from class. I would expect to spend about 3 days discussing the above topics and integrating hands-on and technological resources into those lessons. Students will be asked to write short essays nightly on what was done in class and how it relates back to the discussion and notes from the beginning of the class. An activity will be necessary to work on the units of energy and work. Students should conduct the matter and energy activity and the online energy activity detailed at the end of this unit. Other activities done during class include numbers 1-3 and 7 from the detailed activities section at the end of this unit. A field trip opportunity would be to visit the local power company.
Forms of energy
Most types of energy fall into two categories: potential energy (the energy from location) and kinetic energy (the energy from movement). Potential energy (specifically gravitational potential energy) is given to an object by moving it against an opposing force, like lifting a book against gravity to the top of a shelf. The book energy has energy that is “stored” as a result of position. The book has no kinetic energy as it sits on top of the shelf, but when you push it off the shelf, it accelerates to the floor, having a maximum kinetic energy just before it hits the floor. The stored energy the book had just before it was pushed was the potential energy given to it by the work you did in lifting it to the shelf. The equation used to calculate the amount of potential energy an object has is given:
PE = mgh
where m=the object mass, g=acceleration due to gravity (9.8 m/sec2), and h=the height above earth at which the object lay.
Kinetic energy is energy of motion and depends on an object’s mass and speed. The faster the object moves and the more mass it has, the more kinetic energy it will have. Kinetic energy is given by the formula
KE = 1/2mv2
where m=mass and v=speed. Kinetic energy can either increase or decrease by transformation either from or to potential energy, friction or heat. A moving object can transfer its kinetic energy to a stationary one, causing it to move. However, what happens when a moving object hits something soft, like sand. Where does that energy go? Is it lost in the sand? The energy of that motion is not, in fact, lost, but is transformed into a different form of energy, like heat. There are many forms of energy, but all of them can be classified as either potential or kinetic.
Internal energy is due to the kinetic energy associated with the movement of particles in matter. Thermal energy, also called heat, occurs as objects warm up and is related to an increase in the kinetic energy of a material’s atoms and molecules. We said that work is an interaction that is not HEAT. Consider dragging a heavy box across a wooden floor. Where does your work go? Or when you rub your hands briskly together, where does your work go? The answer is thermal energy and is a part of where most energy ends up. Any form of energy can be changed entirely into heat, but when heat is changed into other forms (like in the automobile engine when you convert the chemical energy of fuel into heat which is then converted into mechanical energy to make the car move), the change is not complete, and some always remains as heat. The energy converted to heat is no longer available as potential or kinetic energy, but it was not destroyed. Thus, the energy exchange of a system satisfies the following:
/\E = Q-W
where /\E is the change in energy, Q is heat (the energy exchanged between the system and the environment due to temperature changes) and W is work (the energy exchanged between the system and the environment other than heat). The total energy of a system includes the potential energy, kinetic energy and internal energy. This total energy remains constant so long as there is no exchange with the environment of heat or work.
Heat flows spontaneously from hot objects to cold objects in contact with each other (3). This transfer of heat between objects of matter is called heat conduction. In order to test how hot or cold something is, you might use a thermometer to measure its temperature. Temperature is a function of the energy in an object as a result of the random motions of its particles (3). Atoms and molecules of all matter are in motion; changes in their average energy of motion result in changes in the temperature of the matter.
It becomes clear from this discussion that energy can change forms. In fact, as long as there is no exchange with the environment of heat or work, the total energy of a system always remains constant. However, the forms of energy that are present in the system will change. This is the law of conservation of energy (the 1st Law of Thermodynamics) which states that energy cannot be created or destroyed (it does not just appear and disappear), but it CAN be converted from one form to another. This process of energy conversion is taking place all of the time, and allows for everything to balance. An example is in a wind-up toy, which stores potential energy in a compressed spring. That potential energy is converted into kinetic energy when the spring unwinds. If we could ignore the energy that is lost to the environment (due to the force of friction, for example), then the total energy of the system would remain unchanged, only the
form
of energy present would change. Some different forms of energy include: thermal energy, chemical energy, mechanical energy, electromagnetic (radiant) energy, nuclear energy, and electric energy.
Consider the following examples of the 1st law of thermodynamics:
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- In a pendulum, kinetic energy and potential energy are constantly converted back and forth as the pendulum swings. At the highest point of the swing, the pendulum has maximum potential energy and no kinetic energy. As the pendulum swings down the potential energy is converted to kinetic energy. At the bottom of the swing the pendulum has maximum kinetic energy and zero potential energy. This energy can also be transferred if you have two pendulums next to each other, their energy will be transferred back and forth as they collide.
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- In a roller coaster, the coaster climbs to the first hill where it has maximum potential energy. That is converted into kinetic energy, the energy of motion. That kinetic energy is later converted back into potential energy as the speed of the coaster decreases as it climbs the next hill. Over the course of the ride the roller coaster’s total energy decreases due to friction from the rails and the air, until all of the potential energy from the original lift is completely lost.
The 2nd Law of Thermodynamics says that some energy always degrades into forms that cannot be exploited (heat), for example, frictional effects. Friction is a force that results form relative motion between objects, like the wheel an axle of a car (4). These frictional forces work against the motion that made them. Friction comes from two surfaces that move against each other. The smoother the surfaces are, the less will be the frictional forces between them. Even seemingly very smooth surfaces have microscopic “hills” and “valleys” that contribute to friction. Air also will cause friction. In machines, axles and ball bearings are inventions that help to reduce friction.
Activities for the above discussions:
Students will conduct activities to connect the above discussions with real-life scenarios. I would expect to spend at least 8 days discussing the above topics and integrating hands-on and technological resources into those lessons. Students will be asked to write short essays nightly on what was done in class and how it relates back to the discussion and notes from the beginning of the class. The heat activities detailed at the end of this unit will be used during this section. Other activities conducted during class include numbers 4-6 and 8-9 from the detailed activities section at the end of this unit. Students must also complete the CAPT performance task
“Craters”
during this section and are required to write up their lab using the standard CAPT format (see rubric Appendix B).
Engines
We will now turn to an examination of one way that energy is transformed. A motor is one type of machine that produces motion or power for doing work at the expense of some other form of energy. An engine is a work machine that converts energy in a chemical form to mechanical energy for the performance of work. A combustion engine takes the potential energy in fuel (chemical energy) and converts it into motion so that something can move, your car for instance. The efficiency of an engine is the amount of work you get out of it versus the amount of energy that was put into it. Efficiency can never be 100% because the conversion of heat into mechanical energy is involved. Heat can never be converted completely into mechanical or electrical energy, as discussed above in the second law of thermodynamics.
One example of an engine that is capable of producing work was the first practical steam engine, designed by Thomas Newcomen. This engine worked on the principle that as steam condenses into water, it occupies a much smaller space. If you put a piston between the condensing steam and the air, that piston will move as the air pushes down to fill space. By attaching a pump to the piston, you have created a machine capable of doing work. James Watt later improved the steam engine by separating the heating and cooling units, making the engines more efficient and able to drive many other machines than pumps. This is from the fundamental concept from Carnot that you can take heat from a high temperature reservoir and put it into a cool temperature reservoir, and get work out of that temperature difference. The higher the heat difference, the more work you can get, and the more efficient your engine is.
The four-stroke spark ignition engine, also known as the Otto Engine (named for is inventor Nikolaus Otto) works on a cycle originally proposed by Alphonse Beau de Rochas. This sequence of events is what runs most of our cars today (2):
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1) An
intake stroke
, when the piston lowers in the chamber to allow fuel mix to enter through open intake valve.
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2) A
compression stroke
, when both intake and exhaust valves are closed and the piston rises to compress the fuel mixture. The temperature and the pressure both go up.
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3) A
power stroke
, where the hot high-pressure gasresulting from the combustion by spark that occurs right at the end of the compression strokepushes the piston down and does work on the crankshaft.t.
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4) An
exhaust stroke
when the piston pushes the spent gas out through the exhaust valve.
In combustion, a substance (wood, natural gas, e.g.) combines with oxygen to release energy in the form of light or heat (4). The general formula for combustion is given:
Fuel (carbon compound) + O2 (Oxygen) -> CO2 (Carbon Dioxide) + H2O (Water).
The reaction rate increase as the temperature rises, so in that way combustion is a self accelerating process, at least until one of the reactants runs out.
There are two common modes of combustion: 1) that with a premixed combustion source, where the fuel and the oxygen are mixed prior to the combustion initiation (e.g. a Bunsen burner) and 2) that which results as the fuel and the oxidizer diffuse toward each other and flame results at the front at which they meet (like in a candle). Our four-stroke engines are premixed combustion, while the diesel engine is non-mixed or diffusion regulated.
The energy sources that we primary use in our society are hydrocarbons, also known as, fossil fuels. They are called fossil fuels because they were formed underground from the remains of once-living organisms. Fossil fuels are a non-renewable resource, which means that they exists in limited quantities and cannot be replaced. It is necessary to conserve use of these non-renewable resources by finding ways to use less energy or by using energy more efficiently. This is known as energy conservation and is discussed in the unit enclosed in this publication titled, “Fossil Fuel Sources, Usage and Alternatives: What Are the Options?” by S. Van Biersel (Resource Teacher at NHA and Institute Fellow, 2004).
Also at issue with combustion sources of energy, is the pollution that results from that process. Primary products of the combustion reaction include: CO2 (a greenhouse gas), CO, NOx, particulate (soot), ozone (O3), PAN, and photo oxidants. Adverse health effects of this pollution include but are not limited to bronchitis, vomiting, nausea, headache, coma, and death. These also contribute to acid rain, which damaging buildings, plants and animals. All of this is beyond the discussion of this particular unit, but will be followed up during the earth science section.
Activities for the above discussions:
Students will conduct activities that will connect with the above discussions had in class. I would expect to spend approximately 5 days discussing the above topics and integrating hands-on and technological resources into those lessons. Students will be asked to write short essays nightly on what was done in class and how it relates back to the discussion and notes from the beginning of the class. Students will need to discuss the relationship between volume and pressure. Then they can examine the Vipratech website (http://techni.tachemie.uni-leipzig.de/otto/index_e.html) where they can view the cycles of the four-stroke combustion engine with the corresponding Pressure: Volume graphic. The hyper physics website is also a must see for this section (http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html). Students will also view Matt Keveney’s personal web page (http://www.keveney.com). Students will diagram the four-stroke engine and will graph the changes in pressure versus volume as the engine progresses through the cycle. Students will conduct an in-depth research project on an engine of their choice following the given rubric (Appendix C). A definite field trip opportunity would be the Eli Whitney Museum on 915 Whitney Avenue in Hamden, CT (http://www.eliwhitney.org/index.html).
