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A PHYSICIST WRITES . . .

(March 2014)

I am a simple physicist. What I mean is that I often think in terms of simple physics Ė school physics, if you like Ė when trying to understand (for myself) or explain (to you) some phenomenon or other. University-level physics seems much less helpful for this purpose, but fortunately much of my school physics and maths has stayed in my head, for half a century and more.

Hereís an example of a physics topic in which nothing has changed over the years (how could it have?) to disturb my simple view of how it works: evaporation. Any pool or layer of liquid, such as water, evaporates continuously from its surface, at a rate that increases with its temperature (until it actually boils). At the same time the invisible water vapour in the air can condense out to form visible clouds, or to appear on cool surfaces (or to return to the liquid, of course). Thereís a limit to how much vapour the air can hold, and if it is exceeded then the vapour must condense out. The limit goes up with the temperature of the air, however.

Donít worry if you havenít fully grasped this, but itís consistent with the fact that in all my previous cars (the last being a Toyota Corolla) I could direct warm air from the heater on to the windscreen in order to remove condensation (or to defrost the outside) whenever needed: it is fairly obvious from the last paragraph that air that is warm will demist a windscreen fastest, especially if the glass is heating up too.

If you own a modern car, you might be able to guess whatís coming. The first time (last autumn) that I needed to clear the windscreen of my recently acquired VW Golf, I turned the knob to the screen postion Ė and the air-conditioning automatically came on! Nor could I switch it off. The next surprise was that the glass demisted almost immediately. The driverís manual, so detailed in other respects, didnít even hint at this apparent change to basic motoring physics, which presumably occurred sometime between 1997 (the year of the Corolla) and 2010 (that of the Golf). In short, cold air now clears a misted-up windscreen faster.

I needed to investigate this. It turned out that my simple understanding of moisture in the air was still correct: I just wasnít applying it far enough back into the ventilation system. On a cold morning the windscreen is foggy because the day before, the air inside was warm and held much more vapour, which mostly condensed out overnight (and you donít improve things yourself after getting in, by breathing out moist air). What I appreciate now is that because the incoming air has cooled down even further in passing through the Golf air-con unit, it has had to give up virtually all of its moisture.

Thatís what makes the difference: when this air blows across the windscreen, it is extra dry. It has also lost its chill a bit in coming through the ducts, so it not only absorbs condensation from the screen effectively, but also doesnít deposit any at the same time. And itís having a further beneficial effect in drying the air throughout the car, discouraging misting on all the glass.

Naturally, the fogging-up problem should go away altogether once the interior has warmed up (as long as a reasonable flow of air is coming in). But here I find myself wanting to complain mildly about my Golf. Firstly, why does it take some miles for the engine to reach full temperature, according to the gauge? Secondly, if the windscreen needs defrosting before I can drive off then thereís a double annoyance: a low tick-over speed (hence a long time to wait for warm air), and also my having to remember not to turn the knob exactly to the screen position, as this brings on the air-conditioning!

Which reminds me of another thing: as well as this knob there are the usual other two alongside, for heat level and fan speed. But each knob is round, not pointed, so youíre obliged to look down at the little red marker-dot on the edge of the knob: you canít tell where itís set to by feel. This matters most with the first knob (direction of flow), so Iíve contrived to give it a Ďpointí by clamping a thin cable-tie round it and aligning the joint with the dot. But how is it that a car can irritate like this, still, after being developed for decades and manufactured in millions?

Iím sorry Ė I shouldnít be attempting to entertain you with my Golf problems (unless you own one yourself?). But let me just mention one other Ďadvanceí in the heating system, in comparison to all my other cars: in those, the heat-level knob (or slider or whatever) directly controlled the temperature of the air coming out of the vents: perfect! Whereas in the Golf, the knob controls a thermostat.

So if I turn it a notch in order to be a bit warmer, a blast of hot air comes from the vents until the interior temperature has risen. Conversely, if I want to lower this a little ... well, you can guess. Actually, what I miss most from the Corolla is being able to toast my feet and cool my face simultaneously: one of the face-vents always delivered air direct from the outside. In the Golf, perversely, if my feet are sitting comfortably then my head overheats, and if instead I have cold air coming from the face-vents it chills the footwell. (This is simple physics again, of course: warm air rises and cool air sinks.)

Even so, I wouldnít swap this car for another. Because of the slightly smaller engine, both the insurance and the tax are less than for the old Corolla, even though the Golf has far more power and acceleration Ė not to mention a superb 7-forward-gear automatic box. Stop me if I start on about this item, next time!

Peter Soul

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