Energy: predicting what is and is not possible
Physics Narrative for 11-14
Predicting what is possible
The most important feature of the energy concept is that it can tell us if a particular change is possible. So:
- Define carefully the part of the world to be studied.
- Decide on a possible change.
- Compare the energy before and after that change.
This comparison will not tell you if or how the change will happen, but if these energy considerations tell you that the change cannot happen, then it absolutely will not.
An example to think about is climbing Ben Nevis, Scotland's highest peak. We can set up a very simple model of this process. The tourist route up Ben Nevis rises from about 40 metre to 1344 metre, a gain of 1304 metre. A simple calculation shows that to raise a female walker (55 kilogram) by this amount takes about 704,000 joule.
Plain chocolate, according to the figures on the wrapper, provides 21,420 joule for each gram consumed. So if supplied with only 2 g the walker would not make it to the top, as the energy available from the chemical store is less than the energy that must be shifted to the gravitational store to allow the climb to happen. In practice, of course, the chemical store does not start completely empty, and many of us can climb Ben Nevis with no immediate food intake at all. The quantity of energy shifted from the chemical store shows how unwise this attempt is, however. It is not a recommended activity!
This example emphasises an important fact. The essence of working with energy ideas is that they ultimately involve quantitative manipulation; you have to work with numbers. The first question for energy is
how much? Energy is concerned with the
how much, and not with the
To describe processes from an energy perspective is to paint a relatively simple picture. In the Ben Nevis example, you can give a precise limit to the possible height gain for a given amount of chocolate (by working out how much energy the climber needs to shift) without needing to worry about how the climber's physiology and anatomy work together to enable her to climb the hill.
Some jobs simply cannot be done because there is not enough energy available. So how do we find out the amount of energy available?
For many fuels, from petrol to breakfast cereals, we can simply look up the energy shifted from the chemical store when these fuels are burnt. If you burn them in oxygen, then they shift a known number of joules from a chemical store for each kilogram of fuel burnt.
Explaining the causes of changes, such as the collision of a car with a wall, often involves modelling the situation with forces. Descriptions in terms of energy do not involve mechanisms.
Teacher: Tracking the quantities of energy through changes provides an account of:
What is and what is not possible.
How much energy is available?
The quantity of methylated spirits burnt limits how much water can be warmed for the hot drink.