How much do grade 10 students know about our energy consumption?

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Do students understand how similar we are to the machines we make?

Big Ideas: 
  • While the internal workings are different, the human body and the internal combustion engine are, thermodynamically, quite similar.
  • Most students, at least partially, understand this.

One of the questions in this year's Michael Smith Science Challenge, a Canada-wide grade 10 contest put out by the University of British Columbia, was the following:

Some scientists have pointed out that it may be better for the environment for a person to commute in a car than to cycle to and from work, particularly if that person is an exclusive carnivore. How can this possibly be, given that a car is about 100 times heavier than a bicycle?

We hoped to discover whether students understand that humans require fuel (food) in order to cycle to work just as a car requires fuel (fossil fuels) to move.

An analysis of the responses yeilded the following conclusions:

  • About 55% of all student responses included some understanding that a human who cycled would need additional "fuel" in order to make up for the energy consumed in cycling.
  • About 16% of these answers also realized that food, which often goes through multiple trophic levels before humans consume it, is a very inefficent source of energy due to approximately 90% loss of energy for each trophic level it passes through

A breakdown of student responses.

Food production, especially meat, consumes large amounts of energy in agriculture, transportation and retail. In comparison, fossil fuels require much smaller resources to extract, refine and transport; with a high energy density it is cheap to transport, it does not need feeding and housing, it doesn't die or go bad. 

To put some rough numbers on the problem:

  • Carnivores are on the third trophic level (plants->meat->human) of the food chain, which gives a total efficiency of about (10%)2 i.e., 10% loss per trophic level passed through.
  • The human body and internal combustion engine have approximately the same mechanical efficency (~ 20%):

$ \textnormal{Human Energy Efficiency} = (10\textnormal{\%})^2(20\textnormal{\%}) = 0.2\textnormal{\% efficency} $

$ \textnormal{Internal Combustion Engine Energy Efficiency} = 20\textnormal{\% efficiency} $

Hence it is easy to see that while a car+driver is about least 20 times heavier than a bicycle+rider (not forgetting that the rider is much heavier than the bicycle, meaning that the comparison in the question is misleading!) the carnivore cyclist, viewed as a heat engine, may be 100 times less efficient than the car (10 times less for a vegan). Hence the demand in meat products from hungry cyclists may hurt the environment more than a short drive.

Of the other 45% of the papers turned in some rejected the possibility that the statement may be true. Some suggested as a reason the impact of cycling over a pristine environment, while others mentioned the possibility of human injury in cycling accidents.

One student said driving is better than cycling because cycle paths need to be built, whereas we already have roads. However, as this student is from Newfoundland, home to Canada's sharpest satirists and comedians, one can not be sure how firmly this student's tongue was planted in his/her cheek.

For more information about the student responses to this question and the Michael Smith Challenge in general please click here.



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