• Blowing Hot and Cold

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    March 24th, 2011ImogenUncategorized

    They say that there is no such thing as bad weather, only inappropriate clothing. However, as a carefree young girl-about-town (read: impoverished student with small handbag) I happened to be tragically ill-prepared one day when the weather took a turn for the worse. As luck would have it on this drizzly day I had cycled into university. In order to retain my right to scoff at fair-weather-cyclists I felt obliged to brave the precipitation, mac or no mac and so I set off on my merry way. At least, it might have been merry if I had worn my gloves.

    For some time I was perplexed as to why my hands were getting so nippy. They were disproportionately chilly compared to the rest of my extremities but I couldn’t work out why. It was only when I got back to my house and was fumbling with the bike lock I realised that there was a very simple reason: it all comes down to evaporation.


    Mastering evaporation. (safely)

    There are two main reasons for this chilling effect (chilling where things get colder, that is. Physics is nothing scary). These are firstly, the varying energies of different molecular states and secondly, the law of diffusion. Here’s how they work.

    Molecules (with a few exceptions) are found bound in one of three states: solids, liquids and gases. A gas molecule has more energy than a liquid molecule, which in turn has more energy than a solid. This energy takes the form of tiny molecular vibrations – generally measured as heat. As temperature increases the individual molecules gain more energy and so start to vibrate more vivaciously. As vibrations grow the molecules will gradually break bonds to their neighbours. This is why as you go from solid to liquid to gas the substance becomes less rigid and more free-flowing.

    When I was learning about molecular states at school we did a practical in the playground, swinging about hanging onto each others’ elbows. As our class’s “heat” increased we were ordered to progressively let go of each other and observed how much further we could rampage as a result. If you feel a desire to experience this yourself I suggest experimenting by grasping on to a friend while you are a) standing on plastic bags, b) standing on skateboards or c) at the mercy of a small footloose child. The less random movement occurring, the greater your chances of remaining bound to your fellow molecule.

    Particles always diffuse from an area of high concentration to low concentration. This is a fundamental rule in physics, and explains why the smell of burnt toast manages to permeate every room in the house. Water on a surface is of a higher concentration than water vapour suspended in the air, so eventually the surface will “dry out” as all the water evaporates in an attempt to even out the concentrations. The surface will also be left cooler, as we have seen before. Finally, moving air increases the rate of evaporation because the concentration in the air is kept much lower than at the surface as evaporated molecules are consistently removed.

    In order to escape from a liquid and become an unconstrained gas particle, a water molecule needs to gain some energy from the liquid first. So how does this relate to my cold hands? As the tiny water droplets left my skin they took some energy with them, making my skin feel cool (because it’s just lost some heat energy). My skin then had to burn stored energy (from food) to generate more heat to bring it back up to temperature – I wouldn’t recommend hypothermia as a great weight-loss method though.

    Instinctively, we already know all this, that’s why everyone blows on soup. By huffing and puffing you increase the rate of evaporation. The greater number of particles escaping the surface means the liquid loses heat energy. This makes your food (/drink, I’m never quite sure with soup) less likely to burn your tongue.

    So what’s a similar effect? A fan in a closed room will not actually cool the room since it’s only moving air around (if anything, it’s heating the room up by creating heat via friction in its moving parts). However, if it’s blowing air at you, you will feel less hot as the moisture on your fevered brow evaporates.

    And this is why my hands were so very chilly in the wind and rain. What I really needed was a good pair of waterproof gloves to keep me warm. In fact, I have some such gloves at my disposal, but I forgot them that fateful day. Maybe what I really need is a better memory, but unfortunately neurology isn’t my speciality. In fact, if anyone would like to break down the science of remembering things for me, I would always love to learn more.

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