Tracing Two Common Misconceptions about Energy

Ian Sefton
SUPER Group, School of Physics, The University of Sydney

Abstract

The misconceptions
The two misconceptions considered here are that (a) energy is stored in fuels and (b) energy is carried around circuits by electrons.

Energy and fuels
The idea that fuels contain energy is an attractive one which appears in many elementary science texts but is uncommon in university-level physics books. When asked to agree or disagree with the bald statement that "Energy is stored in fuels", 31 (~80%) out of a sample of 38 science teachers agreed and 22 of those (~60% of the total) were in strong agreement (response 5 on a five-point scale). Even more people, 34 of the 38 (~90%), agreed with the next statement in the quiz: "Energy is stored in chemical bonds."

A better statement than the one about fuels would be that energy is released when fuel combines with oxygen and it would be more accurate to say that it takes energy input to break bonds. In discussions which followed the quiz the teachers were confronted with the improved statement about fuel and its implication that the energy released comes from the system of fuel plus oxygen. Many still clung to the original idea, sticking to their claim that energy is stored in the chemical bonds. They said that re-arrangement of bonds during combustion just releases energy from the fuel.

Energy in circuits
Another statement in the same true-false quiz was as follows. "Consider a simple circuit consisting of a battery and a torch globe. Energy is carried from the battery to the globe by electrons." This proposition produced less agreement than the questions about fuels and bonds but the commonest response was strong agreement (15 out of 38) while another 7 people agreed less strongly (response 4 on the five-point scale).

Informal discussions with physicists about a better model, using the Poynting vector, in which energy is transferred by the electromagnetic field outside the wires, indicate that many of them are surprised that the field, not the electrons, can be viewed as carrying the energy from source to load.

Searching for the sources
I will summarise the results of a search for the origins of these misconceptions in a selection of resources which includes both modern and older school and university texts. The misconceptions are often stated directly in elementary school texts but there are also more subtle faults in the higher-level texts which may be planting the ideas in the minds of those who write the elementary stuff.

Faults in standard presentations
Both misconceptions may be seen as examples of a conception of energy as a kind of fluid which is a view promoted by some treatments of thermodynamics. Some other aspects of standard presentations of physics which may be responsible for the misconceptions include the following.

Preference for action-at-a-distance models over field descriptions encourages students to think of objects in isolation from their environment.

Failure to include the notion of field as one of the most basic concepts precludes valid explanations of common real examples.

Potential energy is introduced as something which belongs exclusively to a single object, such as an electron, rather than to a system such as a circuit or a mixture of fuel and oxygen.

In PhysicsSpeak some concepts are given misleading names such as "binding energy" which would be more appropriately called "unbinding energy".

The standard practice of dealing with oversimplified cases before the most general examples appears to generate understandings which persist beyond the contexts in which they were initially valid. Initial achievements in understanding morph into misconceptions.

A path to enlightenment
Remedies are easy to propose but they need testing. My proposed remedies include an approach which develops the big-picture concepts of system and field to replace oversimplified cases and examples. A strong emphasis on the nature and practice of modelling should also help.

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