Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Dianne Gordon on 19/08/2009 14:30:03
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Dianne Gordon asked the Naked Scientists:
Hi Chris,
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Why is so much more static electricity during winter?
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Kind regards,
Dianne Gordon
What do you think?
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I suspect that we sense more static electricity in winter because things are more dry and so more insulating. With dampness there are paths for electrons to move between materials and so they do not build up as much on insulators such as dry skin and clothing. There may be other things that I haven't thought of.
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Indeed it's to do with dry air being highly insulating (thus enabling static charge to accumulate) whereas humid air is weakly conducting so discharges the static so you don't get a shock.
The reason for the air often being very dry in winter has to do with the concept of relative humidity - and more specifically that at low temperatures the air can hold much less moisture. When you cool down air the moisture comes out as fog or dew.
Air close to freezing or colder (eg outdoors in the winter) holds very little moisture so is dry almost by definition. When that air circulates indoors it can remain very dry, and you get static shocks.
Of course if you're cooking, showering, or just well-sealed and you're breathing inside then you'll put humidity into the indoor air. And you won't get the shocks.
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I wonder if it might also have something to do with winter clothing. We wear more during the winter, including wind and water proofs. Many of these are man made fibres, which do generate more static.
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I think that we sense more static electricity in winter because things are more dry and so more insulating. The reason for the air often being very dry in winter has to do with the concept of relative humidity - and more specifically that at low temperatures the air can hold much less moisture. When you cool down air the moisture comes out as fog or dew.
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In winter the temperatures are lower and air is unable to retain as much water vapour. Outside, at these temperatures, most of the time the change in air conductivity may not be very noticeable, except in extreme climatic conditions like Antarctica or parts of Arizona, which are both exceptionally dry places. The problem with static in winter is when the cold, outside air is warmed in a typical house or office etc. The air contains the same amount of moisture but the relative humidity then drops hugely and the air becomes much less conductive resulting in static problems.
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As this is a Science Forum, I think we should be doing better than saying that "Air holds water".
It's all just a matter of the vapour pressure of water at different temperatures. As the temperature rises, the vapour pressure rises and more water molecules turn up in the atmosphere.
It may 'feel like' the air is 'holding' the water but the water is there in spite of the air. Air is not ' sponge'. If you take a bell jar and try to evacuate it with some water in the bottom, you will arrive at a situation where the pressure in the bell jar is the same as the vapour pressure of water at the ambient temperature. You won't get lower until the water has all evaporated. This pressure is the partial pressure of the water in the atmosphere when it's 'saturated'. The other gases in the air have their own partial pressures and they all add up to atmospheric pressure. The reason that it may not be saturated will be because 1. There's not enough water nearby or, 2. The temperature has risen in that particular volume of air.
I wouldn't be fussy if I was talking to my Granny (sexism and agism - sorry; let's say to my young Grandson) but we should get these things right if we want to improve our understanding - which is what were here for - isn't it?
I have a feeling, to return to the original question, that the reason for static build up in dry air may be more to do with the presence or absence of water droplets on the surfaces of insulators, affecting the leakage of charges. This will be related to the humidity.
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Air is a mix of gases including water vapour. The amount of water vapour in the air depends on the atmospheric pressure, temperature and the availability of water. Warm air that has passed over expanses of water will be likely to have a high water vapour content for example. For a particular pressure there is a temperature at which the water vapour will condense out of the air mixture; this temperature is known as the "dew point". For a particular temperature (at sea level, say) the higher the dew point, the more humid is the air. If the dew point is the same as the air temperature it means the air is "saturated" and water will condense out as dew or fog. In winter the temperatures are lower which means that even on a wet day, where the dew point is the same as the air temperature, the air will have a lower water content than if it were warm. That is to say the total mass of water in a saturated volume of air will be less; for example the mass of water at 0 degrees centigrade would be about half that at 12 degrees centigrade. In fact the mass of water in a mass of saturated air doubles about every 11 to 12 degrees within normally experienced air temperatures.
A measure of humidity is "relative humidity" which is the percentage ratio of "partial pressures" of the water vapour to that of the air. Suffice it to say that this is a useful measure of humidity that we can relate to. 100% RH means fog (as humid as it can get) and anything over 60% is uncomfortable as it reduces the ability to cool by perspiration because the water will not evaporate from your skin so rapidly. A way of restating the effect of warming air is to say that the warmer air has a lower relative humidity. RH is an important measure also as, for example, many electronic components are only guaranteed within a range of RH values (typically 5% to 95%). Too low and there is a risk of static build up and too high and there can be moisture ingress into plastic integrated circuit packages which can damage the devices over a long period. (A particular problem is absorption prior to soldering when the subsequent rapid application of heat causes water vapour to delaminate the package).
In winter the air can also be dry, especially so if the temperature is well below freezing. Outside, at these temperatures, most of the time the change in air conductivity may not be very noticeable, except in extreme climatic conditions like Antarctica or parts of Arizona, which are both exceptionally dry places. The problem with static in winter is when the cold, outside air is warmed in a typical house or office etc. The air contains the same amount of moisture but the relative humidity then drops hugely and the air becomes much less conductive resulting in static problems. The conductivity of air is roughly linear with RH unless the RH gets to about 80% (from memory, it will be on the web). There are usually other conducting paths too which may also be affected by moisture (or lack of it) in the surrounding atmosphere, but usually the needed leakage, to stop you getting zapped, is via the air; it is usual to get the charge from walking on an artificial fibre carpet and the leakage via your rubber soled shoes is probably minimal, especially as it is this contact generating the charge.
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graham.d
All correct. BUT air is merely a mixture of gases. Water vapour just happens to be one of them.
Would one say that air 'can hold' so much Carbon Dioxide? Around -57 Celcius the same thing would apply to CO2 as to water at room temperature. There would be an equilibrium between the solid form of CO2 and the gaseous form air would be 'saturated' at a temperature where the CO2 starts to condense. Water is a bit special in that its vapour pressure doesn't vary a lot around room temperature and it happens to be very relevant to our lives. I think this is why the, frankly, non-scientific model tends to be used; it is such a familiar phenomenon that the home made description is the generally accepted one.
But should we not try to do better here?
Repeat after me - " Air is not a sponge."
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Care to check on this number?
"Around -57 Celsius the same thing would apply to CO2"
If you get CO2 down to about -78C it freezes- provided that it's there at one atmosphere pressure. At only 300 ppm the effective freezing point would be much lower.
Anyway, while we are debunking myths, wet air is just about as good an insulator as dry air.
What changes is the conductivity of the solids in coontact with the air. If there's not much water about then things like cotton and timber are much better insulators than when there is more water.
Dry air isn't the issue- it's dry everything else that makes shocks commoner in Winter.
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If CO2 were the only gas around, there would be a million parts per million. The vapour (partial / only) pressure would be, as always, according to the temperature / pressure characteristic.
The -57C I was referring to would be its triple point (n'est ce pas?). Of course the conditions would have to be different from those for water but you could still talk in terms of relative 'saturation' for air, rather than the 'concentration of CO2'.
Anyway, while we are debunking myths, wet air is just about as good an insulator as dry air.
What changes is the conductivity of the solids in coontact with the air. If there's not much water about then things like cotton and timber are much better insulators than when there is more water.
Just the point I made in post 6039.
btw, is "coontact" the sensitive way that raccoons deal with each other's personal problems?
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Air is a mix of gases including water vapour.
This is the first line. Then SC you say...
graham.d
All correct. BUT air is merely a mixture of gases. Water vapour just happens to be one of them. [...]
But should we not try to do better here?
Repeat after me - " Air is not a sponge."
Thank you for marking my essay!!! I think you should try to be a little less condescending and pay attention to what is written. Perhaps also you should see if the original questioner got more from answers that explained things in common terms than my lengthier answer. Your banging on about the misconception of using the phrase "air holding water" (not exactly used by me by the way) is straight out of wikipedia I note and, in terms of use of English (the water being part of air) is not really so bad and does not necessarily imply that air is a sponge.
Really SC, it is a long time since I was treated like a schoolboy :-)