Contents
1. One-way processes
I am the undertow Washing tides of power Battering the pillars Under your things of high law. I am a sleepless Slowfaring eater, Maker of rust and rot In your bastioned fastenings, Caissons deep. I am the Law Older than you And your builders proud. I am deaf In all days Whether you Say 'Yes' or 'No'. I am the crumbler: tomorrow.
Carl Sandburg, Under (1916).
In Collected poems (1950) Harcourt Brace Jovanovich, Inc.
Timing
It would be best if this Part could be kept within two periods, even at the price of omitting some examples. The purpose is to start the thinking off, not to complete it, and the answers that emerge are too indefinite to carry the weight of several periods of teaching. If more than three periods are taken, some other Part, probably Part Two, will have to be taken more quickly.
On getting older
Every man or woman is born, grows older, and dies. Cars, bicycles, and washing machines wear out and have to be thrown away. Bridges and ships rust and corrode. Houses wear out; timber rots or cracks, bricks lose their sharp new edges, the mortar crumbles. Sand is what the sea, in time, makes of rocks and pebbles. A cup of tea becomes cold if forgotten - but not freezing cold. An abandoned ice cream melts away.
These may be some of the reasons why we all have such a strong sense of time going inexorably onward. People have always tried to understand what the flow of time means, but few would claim to have a full grasp even of the problem, let alone the answer. Philosophers continue to puzzle over it. Here all we want to point out is the simple fact that all by themselves things tend to get old and worn out; they don't spontaneously become new, clean, and tidy.
Time
In a sense, time is irrelevant here. The important thing is that these things happen inevitably. How quickly or slowly they happen is beside the main point. The discussion is not one about time machines, or the fourth dimension, and to present it as such would be irrelevant or misleading. The intention is simply to give a general point of view from which to think about the films that follow, which show events going forwards and backwards.
Scientists have been able to reach a little understanding of some aspects of these problems. They have found ways of explaining why change is such a one-way journey from new to old, fresh to rotten, clean and tidy to dirty and muddled, very hot or very cold to lukewarm. And the answers have thrown new light on some very practical problems - how to make blast furnaces yield more steel, for example, by finding out how to encourage the reaction to go the way the steelmaker wants. A good way to start thinking about such problems is to ask what several processes would look like if they went the wrong way.
Questions
There follows the first of many sets of questions incorporated into the text. Discussion of these questions and thinking about them are intended to be integral parts of the reading of this book. Answers are provided in the Answer part. It may sometimes be best to devote the discussion to arguments about the adequacy of these answers.
Q1 If a film of a tall chimney falling over and smashing to pieces were to be taken and projected, but in reverse, so that you saw the film backwards, what would it look like? Why would it be obvious that the film was being run backwards?
Q2 The sketches in figures 1 to 4 show instants from sequences of events. The times between instants in a, b, c, and e are all equal. Some sequences have been printed in reverse order. Decide whether each sequence is in the normal order or is in reverse order, or explain why you cannot tell.
a An oscillating simple pendulum.
Figure 1
b Marbles being shaken in a container. The marbles are of two different colours, but they are otherwise identical.
Figure 2
c Same as b but there are only two of each sort of marble.
Figure 3
d Two beakers of water in thermal contact.
Figure 4
e A ball rolling on a horizontal table.
Figure 5
Although questions 1 and 2 above have answers, in a deeper sense this whole Unit provides their answer. Their job is to illustrate the direction in which the thinking will go, and to start that thinking off.
Film loops, Forwards or backwards?
Three 8 mm loop films, specially made for this Unit, are available. See the film list for details.
Forwards or backwards? Loop 1
- Event A: A brick falling to the floor, shown first forwards and then in reverse.
- Event B: A piece of red hot steel being dipped into water, shown first forwards, and then in reverse.
- Event C: A piece of paper burning, shown first in reverse, and then forwards.
Forwards or backwards? Loop 2
- Event A: A ball bouncing on a flat, horizontal surface, first shown forwards and then in reverse.
- Event B: A vehicle travelling to and fro along an air track, shown only in reverse.
- Event C: A collision between a pair of nearly frictionless pucks, shown first forwards and then in reverse.
Forwards or backwards? Loop 3
- Event A: Ink mixing with water, shown only in reverse.
- Event B: Shaking a box of coloured balls, starting with them sorted into two layers. Initially, there are many balls. The shaking is shown only in reverse. Then the shaking is repeated with just four balls, also shown only in reverse.
- Event C: The film shows a row of heavy pendula, which are not coupled but have various lengths. First, just one pendulum is shown swinging on its own, in reverse only. Then uncoupled pendula of various lengths, in a row, are shown swinging together, both forwards and in reverse.
The purpose of these films is to focus attention, through discussion, on the problem of why some events look absurd in reverse while others look sensible. In Part Two the implications of the special case of the one-way burning of fuels are considered.
Energy changes
What energy transformations occur when a brick falls from a height onto the floor? Initially, the brick has gravitational potential energy. As it falls this energy is transformed to kinetic energy of the brick. When the brick hits the floor this energy passes to extra energy shared among the molecules of the brick and the floor - the temperatures of the brick and of part of the floor increase. The energy transfer of gravitational potential energy to energy shared among molecules is quite sensible - bricks often fall. The reverse event, with the brick jumping up off the floor, does not seem sensible.
Figure 6: Energy boxes showing energy transfers
Figure 6 suggests a way of drawing a diagram to represent energy transformations. Energy is conserved, so the energy in one form at one stage will appear in another form at another stage. This is represented by putting the name of the form of the energy at each stage into a box. Only the first and last stages are shown.
Figure 6 shows how the energy boxes could be drawn for the impossible event of the brick jumping upwards all by itself. So far as energy is concerned, this reverse event is not impossible. Energy can still be conserved; the reverse process does not break the law of energy conservation.
The First Law of Thermodynamics
Appendix A discusses the difference between the First Law of Thermodynamics and the Law of Conservation of Energy.
The Law of Conservation of Energy is thus no help in determining the direction of an event. The law would allow a hot brick and the floor to cool, and so to provide the energy for the brick to leap into the air. For this to happen the molecules would have to move in unison, so that they all hit the brick at the right moment, and lose energy as they push it into the air. As bricks do not generally leap off floors, such organized movement must be very rare. It does not appear likely that the molecules will move in just the right way to hit the brick upwards.
Q3 Draw energy boxes for a a piece of paper burning, b an air track vehicle travelling forwards and backwards along a horizontal track.
Q4The piece of paper burning does not look sensible when reversed; the vehicle moving along the air track does look sensible. Do your answers to question 3 give any clue to why one event looks sensible when reversed and the other does not?
If there is one word which is the key to the difference between one-way and two-way processes, it is spreading. Whenever motion or energy becomes spread out among many particles, something two-way has happened. The trick for making the motion of a body as nearly one-way as possible is to prevent any of its energy from being shared with the atoms of matter nearby.
Brownian motion
See Nuffield O-level Physics Guide to experiments I, experiment 52. Any student who has not seen Brownian motion should see it now.
In the Brownian motion experiment smoke particles are seen being knocked around as a result of gas molecules hitting them. Some of the kinetic energy of the gas molecules has been given to the smoke particles. This is similar to the hot brick and floor getting cooler and the brick jumping up from the floor; a sufficient number of gas molecules have moved together to hit the smoke particle (in one direction) at the right moment, lose energy, and give the particle a push. There is, however, one significant difference between the smoke particle and the brick - the smoke particle is considerably smaller. Not so many molecules have, by chance, to hit it in unison in order to produce an observable motion.
Q5 Which is more likely, that everyone in a class of only three students might choose by chance to go to the cinema at the same time, or that everyone in a class of forty students might choose to do so, again by chance?
Q6 Would a film of Brownian motion look sensible if run in reverse?
A process in which energy is shared out among many molecules looks implausible in reverse. The sharing must be among only a few molecules to be plausible in reverse. Two-way processes usually involve large numbers of objects.
9.1 Demonstration: Energy conversions
9A motor/generator unit
9C switch unit
9D lamp unit
9E flywheel unit
9L storage battery unit
9M driving belt
176 12 volt battery
or
1033 cell holder with three U2 cells
44/2 G-clamp, small 2
1056 dilute sulphuric acid
1000 leads
a A motor driving a flywheel; then the flywheel driving the motor as a dynamo.
See Nuffield O-level Physics Guide to experiments II, experiment 61 (21).
b A pair of lead plates dipping in sulphuric acid charged from a battery, then used to light a lamp.
Figure 7
The storage battery unit, filled with dilute sulphuric acid, is connected via the switch to the battery (4 or 4.5 V) and to the lamp unit (using one lamp only). When the switch is thrown over to connect the lead plates to the lamp, the lamp will light for a few seconds.
One-way and two-way machines
Some machines are used to change energy from one form to another. The transformation could be of electrical energy to mechanical energy, or of mechanical energy to electrical energy, for example. Films of some machines would look sensible in reverse.
Q7 Suggest a machine which, if shown running in reverse on a film, would still look reasonably sensible.
Q8 Draw energy boxes for a lead-acid battery driving a motor which lifts a load. Could this process work backwards?
Q8 Draw energy boxes for a lead-acid battery lighting a lamp. Could this process work backwards?
Clearly, the distinction between one-way and two-way processes is a matter of practical importance, and is not merely an intellectual puzzle.
Summary: What kinds of events are one-way?
The Law of Conservation of Energy is no help - every change it allows one way it allows the other.
One-way processes are spreading processes. In a one-way process the energy becomes spread out among more objects.
One-way processes are associated with thermal changes of energy, particularly the thermal flow of energy from hot to cold.
Mixing processes
In Part Three, we look at mixing, jumbling, and diffusion as other examples of one-way processes. If students raise such ideas now, the teacher may want to use material which occurs at the beginning of Part Three.
Difficulty over the meaning of one-way processes
There may be a difficulty over the meaning of one-way processes. As they have been described, the melting of a lump of ice is one-way, but students may object that the water can, after all, be frozen again. To freeze it, a cooling device is needed, driven by some source of energy. Perhaps the energy is electrical, from a hydro-electric power station; then some water ends up at the bottom of a hill instead of being at the top. How might it get back? The Sun could evaporate it, and it could then fall as rain, having spread the energy liberated on condensation out into the atmosphere. So the water can be frozen again, but not without other one-way processes going on. It is not possible to go back to the starting point with everything in the world exactly as it was before. Bent's The Second Law, Chapter 4, discusses this problem.
Reversible and irreversible processes
Teachers will recognize that our versions of reversible and irreversible processes are crude. The aim has been to get the class thinking in a fruitful direction, not to anticipate all the answers with careful definitions. All natural, spontaneous processes are irreversible. Such processes move from a state of non-equilibrium to one of equilibrium. A completely reversible process is not possible, if all the side effects are included. Such a process would imply a move from one equilibrium state to another and there would be no impetus for such a move. Nearly reversible processes are those which involve a move from a state near to equilibrium to another state at or near equilibrium.