This approach idea is explored through:
contrasting scientific and student opinions
everyday experiences of students
Energy is a word students frequently encounter and use every day. Many of the experiences they have and the ideas they construct from these experiences lead students to the view that energy is not conserved.
While students commonly accept that energy is needed to make the changes they see happening in their world, they often do not believe that (new forms of) energy are still present after the change. For example, most students will agree that it takes energy to heat a cup of cold water, to roll a stationary ball, or to lift a box onto a shelf. many, however, will hold the view that the glass of hot water, the rolling ball, and the tall box have no more energy once the process of change is over.
The common meaning of the word “potential” refers to something you don’t have now but may have in the future. this creates difficulties for students with the scientific phrase “potential energy”. students see the various forms of potential energy not as energy but as the potential for energy. once again, students do not see energy as something that is conserved. for example, they see a box on a high shelf as having no more energy (ie, gravitational potential energy) than an identical box on a lower shelf. rather, they see that it could gain kinetic energy if taken off the shelf (it has the “potential” to gain energy).
Most students have a considerably narrower view of what phenomena are forms of energy than scientists. many students don’t see that various changes such as lifting things, stretching things, speeding things up, burning fuels, and producing light or sound involve the common attribute of energy.
This problem of what counts as energy is compounded by very abstract notions of chemical and nuclear energy: it’s easy to conceive of the effects when these forms of energy are transformed into others, but it’s much more difficult to construct a mindset. image of how these could be considered as forms of energy before such transformations occur.
These student ideas cause learning problems that are further explored in the energy production and consumption reactions of the central idea.
A different aspect of students’ experiences with energy conservation is the puzzle presented in the title: “is energy conserved or depleted?” that we are wasting/losing energy at an unsustainable rate. Phrases like “we need to conserve energy” use the word conserve in a very different way from its meaning in the phrase “energy is always conserved.”
research: brook, briggs, bell, & driver (1984), carr, kirkwood, newman & birdwhistel (1987), conductor & Thousand (1986), Duit & haeussler (1994), watts (1983)
The concept of energy and our understanding that it is conserved have taken a long time to develop and are invented ideas created to better understand the changes occurring in our world. scientists don’t know what energy is.
A cup that has been heated, a ball that has been accelerated, and a box that has been raised higher have more energy than they did: heat, kinetic and gravitational potential energy, respectively. each could transfer this energy to another system (for example, hot water will heat a teaspoon placed in it, the moving ball could speed up another ball). these statements about what types of energy are present in a system only have meaning when we include what might happen next. for example, a rolling wooden ball can be thought of as a store of kinetic energy in the context of collisions with other balls, or as a store of chemical energy if it is about to burn.
The various forms of energy labeled “potential energy” are forms of energy that systems currently have, not that they will gain in the future. a stretched spring has more energy than before stretching; energy in this form is called elastic potential energy. Similarly, a girl who has climbed a ladder has more energy (gravitational potential) than she had at the bottom of the ladder. changes in chemical energy arise from changes in the kinetic and electrostatic potential energy of electrons in atoms and molecules; nuclear energy arises from the (different) forces that hold protons and neutrons together in nuclei.
The total amount of energy in the universe never changes in any energy transformation. In household appliances, machines, and the human body, some of the energy is transformed into sound and low-grade thermal energy (for example, moving parts get hot). hardly any low-quality thermal energy can be converted into useful work. we can’t use power in these low-grade forms to run another appliance, so we commonly say the power is “depleted.” however, it is really that useful energy that is being transformed into non-useful energy.
The fact that we cannot convert low-grade thermal energy into useful work means that the commonly cited definition of energy as “the ability to do work” is not satisfactory: it only applies to mechanical energy.
critical teaching ideas
- The concept of energy was invented over a long period of time as a concept to explain a wide range of changes. energy has many forms. it can be measured and the units (joules and calories) apply to all shapes.
- In most changes we observe, energy changes form. energy transformation devices are fundamental to our society. it is useful to describe the energy they require as “usable” energy.
- The concept of energy can be used to describe changes and the ability to make changes.
- The phrase ‘we need to conserve energy’ uses the word ‘conserve’ differently from its use in the phrase ‘conserving energy’. when we conserve energy, we are concerned with limiting the use of “usable” forms of energy.
- The principle of conservation of energy refers to the idea that energy is neither created nor lost, only transformed from one form to another. In all changes, some energy is always converted to forms (mainly low-grade heat) that cannot be used to make any more changes.
- many students wrote that electrical energy was entering.
- some wrote that thermal energy came out.
- many wrote that electrical energy entered.
- some students got many types of energy.
- Nuclear power in and out.
- nuclear input but I don’t know what comes out.
- nuclear input, radioactive output.
- chocolate gives energy.
- the body uses energy to chew.
- movement is energy.
- food is energy.
- there is energy in the mars bar.
- The sun’s rays provide energy.
- the tree needs energy to grow.
- Light and thermal energy enter.
- fire uses energy but I don’t know what it’s called.
- thermal energy is emitted.
- Light energy is emitted.
- almost all the students wrote that energy had been used.
- very few students identified a specific form of energy.
- the students knew that the skier had used energy but could not give it a name.
- the caster uses energy and a sound is made which is energy.
- energy was used to stretch the elastic.
- energy was used to move the arm that did the stretch.
- we are creating a force by stretching it.
- the students wrote that energy was used but they did not know what to call it.
- ball a and ball b are identical balls, except ball a is moving slowly and ball b is moving very fast. Students are asked to predict what will happen when ball a and ball b collide head-on with each other.
- Carefully bend approximately 2.5 meters of curtain rail into a smooth curve and position as shown. select a metal ball that will roll (not slide) up and down the track.
- turn the track so that the upslope is much longer than the downslope and the angle of the downslope is steeper than that of the upslope.
Explore the relationships between observational and fieldwork ideas in the Concept Development Maps: Energy Transformations and Energy Resources.
The views that students have about energy and the conservation of energy make teaching the principle of conservation of energy much more complicated than is often believed. attempts to teach energy by starting with a textbook definition are not helpful because they do not apply to all situations involving energy. the common definition of energy as the “capacity to do work”, for example, ignores the big problem that low-grade heat cannot be used to do work. therefore, it is better to try to describe energy rather than look for a single definition. energy changes can be measured, and energy can be thought of as an accounting system that allows us to compare, for example, the amounts of energy required to produce different materials such as paper, plastic, and glass cups, or the efficiencies of various energy transformation devices such as different types of motors.
As noted in the focus idea, using energy, this is an area where students’ existing conceptions need to be reshaped, but are not firmly held. It is important to emphasize that while there is good evidence to suggest that energy is always conserved, at least in the vast majority of situations, this cannot be verified by experiment. this is because some of the energy is always transformed into low-grade forms that cannot be measured. teaching will involve some presentation (rather than discovery or proof by experiment) of current science, however the reasons why scientists find it useful to use the notion of energy in the way they do should be discussed regularly.
promote reflection and clarification of existing ideas
This activity is designed to highlight students’ views on whether the energy needed to make a change remains in the system. Emphasize that this is not a test, but a starting point for discussion, and ask students to select one of the three opinions for each situation. the activity should also encourage students to rethink their ideas when necessary.
Below are three pairs of objects. a change has been made to one object in each pair. Students should select the statement that best describes their opinion about the changes that have occurred:
1) a cup of cold water; the same cup but now with hot water
in my opinion:a) energy was not involved in any wayb) hot water has more energy than cold waterc) it took energy to heat the cold water, but hot water has no more energy now.
2) a stationary ball; the same ball but rolling
in my opinion: a) energy was not involved in any way b) the moving ball has more energy than the stationary ball c) it took energy to make the ball move, but the moving ball has no more energy now.
3) a box on the ground; the same box on a high shelf
in my opinion: a) energy is not involved in any way b) the tall box has more energy than the short box c) it took energy to lift the box, but the tall box has no more energy than the short box.
many students will select opinion b (the scientifically acceptable one) for each situation. After collecting the votes, call the discussion on the first situation (the glass of water). this and the ball situation are ones where students with opinion b can usually convince their colleagues: they can cite experience that you need energy-using appliances to heat water and (separately) that hot water could heat something else; Equivalent arguments apply to the rolling ball. The third situation (the box) is more difficult as it involves a less obvious form of energy, however prior agreement on opinion b for the first two situations is helpful in exploring the third situation.
highlight existing student ideas
The following activity is a useful test of students’ ideas about what forms of energy are. as can be seen from the responses of a year 9 class, students’ ideas vary and are often unclear rather than in strong conflict with accepted science.
The following table shows some situations where a change is actually taking place. introduce them to students and ask, ‘would you use the word energy when thinking about these situations? If yes, please explain how.” students should also be encouraged to consider if there is more than one type of energy involved and list them all
summaries of some year 9 students’ ideas are also provided, as a guide to possible student responses.
focus students’ attention on overlooked details
Without introducing the notion of conservation of energy, ask students to refer back to their answers to the previous question. the overall goal is to try to build a class list of forms of energy that all students agree on. the group activity means that since some students suggest a form of energy in each image that others may miss, the class is likely to expand the list of energy forms they recognize.
Divide the class into small groups to complete the following table. the table discusses the forms of energy that enter and exit in several (or all) of the situations in the probe detailed above. Within their groups, students must decide whether they agree or disagree about the forms of energy that go in and out. groups can decide that no energy goes in or out and this is acceptable. The idea behind completing this activity is to make a more complete list of ideas about forms of energy. each of the groups should report back to the rest of the class and the list should be added to as different ways are suggested and agreed upon.
electric mixer (practical)
bar amars digest
help students figure out some of the “scientific” explanations for themselves
After a discussion of the group’s views on forms of energy, present the story of the 19th century scientists who gradually invented the idea that energy must always be conserved and extended its range of forms of energy. then the class can look at what they came up with and identify where forms of energy should exist that they hadn’t included in any of the columns.
open discussion through shared experience
The following poems (predict-observe-explain) involve the transformation of energy into moving balls. a moving ball is said to have more energy than an identical static ball. this type of energy, the energy of motion, is called kinetic energy.
set up the following situations for students to consider:
Ask students to predict the height the ball will roll when the lane is set up so that the angles of the downward and upward slopes are equal. the ball rolls from a to near b.
Students should be asked to decide whether the ball will roll down the opposite slope at the same height from which it started (near position d), or roll the same distance along the track (near position d). position e). discussion of these predictions and the observed result should focus on the original gravitational potential energy which is converted to kinetic energy as the ball increases in speed and then converted back to gravitational potential energy as the ball goes up the opposite slope. /p>