Energy transfer

28th February 2003 at 00:00
I find energy transfer diagrams (Sankey diagrams), can pose problems in science, especially when there needs to be a quantitative element regarding the conservation of energy. To get around this I now use graph paper. With the paper in landscape, cut a strip of graph paper 100 squares high across the whole width of the sheet.

Using the example of, say, 100 joules of electricity going into a hairdryer and being converted into 60J of heat, 35J of kinetic energy and 5J of sound: draw the hairdryer in the middle of the strip. On the left-hand side write "100J electrical energy". On the right, cut the paper into strips 5, 35 and 60 squares high, a third of the way across the big strip. Label the strips "sound", "kinetic", and "heat". Higher-ability groups could use narrower strips of paper and adjust the scale.

Groups can compare efficiencies of different appliances by laying the strips on each other and comparing the widths of each energy type. For example, a traditional light bulb would have strips of 95 squares high "heat" and five squares high "light". An energy-saving bulb would be 75 heat and 25 light.

Alternatively, the paper for the traditional bulb could be 100 squares across with 5 squares (5J) marked off as light. The energy-saving bulb would only require a strip 20 squares (20J) across with five squares as (5J) light. The difference in energy input of both devices can be compared for the same level of useful output.

Points can be cut into the ends of the strips to show the direction of the energy transfer. I have used this with high-level GCSE groups and middle-ability KS3 groups.

Carl Smith, science teacher, Turves Green Girls' School and Technology College, Birmingham

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