Energy and the Confused Student III: Language - Loreto-Unican

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Potential EnergyIn discussions of potential energy, a commonmisleading practice is to use a phrase such as the “potentialenergy of the ball,” in which potential energyis associated with an object rather than with a systemof two or more interacting objects. This is misleadingconceptually and ignores the importance of the system,as discussed in the second article in this series. 1 Itis important to stress that potential energy is a propertyof a system, not an object. It is associated with aforce that acts between members of the system so itcannot be associated with one member only—a singleobject cannot possess potential energy. 2 Therefore, thebetter phrase for gravitational potential energy is the“potential energy of the ball-Earth system.”Another common misleading statement is to saythat potential energy is related to the positions ofobjects that are interacting within a system. This isa claim that causes little or no trouble in simple mechanicsproblems but must be revised later on whenthe student studies electromagnetism. Imagine anelectric dipole in an electric field or a magnetic dipolein a magnetic field, either of which can be rotatedwithout a change in position. Even though the positionof the center of the rotating dipole remainsunchanged, the potential energy of the dipole–fieldsystem changes. This change is caused by a change inthe orientation of the dipole, not a change in position.Consequently, it sets the stage for future possibilitiesto discuss in mechanics that potential energy is associatedwith the configuration of interacting objects in asystem. This allows for changes in both position andorientation.In a related issue, a common statement in mechanicsis that the “potential energy is zero at the bottom ofthe ramp.” This suggests that potential energy is likea field, having a unique value at all points in space. Itis more proper to say that the “potential energy is zerofor the configuration of the system in which the car isat the bottom of the ramp.”HeatMoving to thermodynamics, we find the mostmisused physics word in popular language—heat. Itis important to stress that physicists use this word torefer to (1) a process by which energy is transferred and(2) the amount of energy transferred by this process,normally denoted as Q. It is not the entity that is beingtransferred (heat is not transferred; it is energy that istransferred) and, even worse, it is not the energy contentof a system with a temperature (correctly describedas internal energy). Romer 3 claims that heat shouldnot be used as a noun. While I agree with the spirit ofusing the word heat correctly, I disagree that heat is nota noun. Heat is indeed a noun, but it is the name of aprocess, not the name of what is transferred. Bauman 4has also discussed the use of the word heat and its interpretationas temperature and internal energy.Consider some phrases used in common language:“heat transfer,” “flow of heat,” and “the heat radiatedoutward.” These phrases refer to a transfer of energybut represent incorrect uses of the word heat. Thephrases can be tested by substituting the words “energytransfer” for “heat.” Each phrase sounds awkwardor redundant when this is done. For example, “heattransfer” becomes “energy transfer transfer.” Othercommon phrases include “the heat of the day” and“too much heat in the air.” In these uses, heat is beingused to represent temperature. Another commonstatement is “heat rises.” In this case, heat is used tomean warm air!Barrow 5 makes a radical suggestion that the wordswork and heat should “vanish from the thermodynamicsscene.” He stresses that thermodynamics shouldfocus on energy, and not on work and heat. I wholeheartedlyagree that thermodynamics should focus onenergy; in fact, all branches of physics should focuson energy as the entity that is being transferred. Idisagree, however, that we should rid ourselves of thewords work and heat. I think students clearly see a differencebetween (1) applying a force on a system and(2) placing a cold system in a warm environment, andwe need words to differentiate these two very differentsituations.While it is not likely that we can turn societyaround in its use of the word heat, we can strive tohave a fraction of the population, our students, usingthe word correctly. For the case in which energymoves between a system at one temperature and itssurroundings at a different temperature, the processthat occurs is “energy transfer by heat.” I dislike theslightly different phrasing “energy transfer in the formof heat” because this suggests that heat is a form of energystorage rather than a method of energy transfer.150 The Physics Teacher ◆ Vol. 46, March 2008
Energy Transfer and TransformationFollowing on the discussion of the importance ofthe system in the second article 1 in this series, let usnow discuss the important distinction between transferof energy and transformation of energy. One populartextbook 6 states that (1) energy is transferred fromkinetic energy to gravitational potential energy. Thisis an example of a statement that confuses students,especially when compared to an earlier statement inthe same textbook that states that (2) energy can betransformed from one type to another and transferredfrom one object to another. Statement (2) is correct,albeit incomplete (the emphasis is on an object ratherthan a system), but statement (1) is incorrect.It should be made clear to students that transformationsof energy occur within a system and result in oneform of energy storage changing to another. For example,in a ball-Earth system, the kinetic energy of thesystem as the thrown ball rises upward is transformedto gravitational potential energy of the system. In thiscommon situation, the kinetic energy of the system isassociated with only one moving object (in the referenceframe of the Earth), but in general, it is the energyof the entire system that is transformed. In a stickof dynamite, chemical potential energy transforms tointernal energy and kinetic energy of shattered pieceswhen the stick explodes. When a block slides across afloor and stops, kinetic energy of the block-floor systemis transformed to internal energy in the block andfloor. If the system is isolated, only transformationsof energy can occur and the total energy of the systemremains constant.In contrast, transfers of energy occur across thesystem boundary and can result in a change in thetotal energy of the system. These transfer mechanismsinclude work, heat, and electromagnetic radiation, aswell as others that will be discussed in the fourth articlein this series. 7Additional careless language includes statementssuch as “energy enters the light bulb as electricity andturns into light and heat.” This statement has threeweaknesses in terms of clarity of language. First, energydoes not enter any electrical device “as electricity.”This suggests that electricity is a form of energy.Similarly, the second and third weaknesses are relatedto the statement that energy “turns into” light andheat: because “turns into” is equivalent to “transformsinto,” this statement suggests that light and heat arealso forms of energy rather than mechanisms forenergy transfer. A stronger, more correct statementis that “energy enters the system of the light bulb byelectrical transmission and transfers out of the systemby electromagnetic radiation and heat.” Notice thatchanging “as” to “by” is an important step in identifyingelectricity, light, and heat as transfer mechanismsrather than forms of energy storage.Energy Dissipation and LossThe study of electric circuits has an example of anunfortunate but popular word choice—energy is “dissipatedin a resistor.” While physics teachers know thatthis means that the resistor warms up and energy istransferred to the environment by heat and radiation,students may interpret this word to mean that energyis disappearing. One dictionary definition of dissipateis “to cause to spread out to the point of vanishing.”The student can apply this notion and come up withthe idea that the energy is vanishing in some way.It is better to say that energy is delivered to the resistor,which reinforces the notion of energy transfer toa system. Because of this transfer, the internal energyof the resistor increases. In turn, there is typically asubsequent transfer of energy by heat and electromagneticradiation from the warm resistor to the coolerenvironment.A similar wording that appears in textbooks is“loss” of energy. This is dangerous territory for studentconfusion, especially when energy has been correctlyintroduced earlier in these textbooks by sayingthat energy is neither created nor destroyed. If energycannot be created nor destroyed, how can it be “lost”?It is far better to be consistent and talk about energytransfers and transformations and to avoid the word“loss” completely.Conditions for ValidityAnother important issue related to language usedwhen discussing energy, as well as any topic in physics,is to present physical principles along with theconditions for which they are true. For example, inmechanics, it would be meaningless to state “an objectin motion remains in that state of motion” withoutadding the condition “for the case in which no forcesact on the object.”The Physics Teacher ◆ Vol. 46, March 2008 151
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Energy and the Confused Student III: Language - Loreto-Unican

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