Question

Problem 9 [15] 1) Moisture wicking fabric (such as athletic clothing made by UnderArmour) keeps the body cool by drawing the sweat to the fabric-air interface to a greater extent than the average clothing (or so it claims) a. How, if at all, would this keep your body cool? Is this false advertising or is it true? 2) During winter one year, they came out with a moisture-wicking winter wear, where they claimed that it keeps the body cool and warm, a seemingly paradoxical statement. Is this a paradox (and false advertisement)? a. Explain how this moisture-wicking fabric can keep the body cool and warm as Under Armour claims. 3) A convection oven uses a fan to blow hot air (that was heated in one region of the oven) to the other parts of the oven, which heats the rest of the oven. A chemist was using a convection oven to 'break-in' leather by baking it at high temperatures for a very short period of time. Upon finishing the baking process, the chemist notices that the metal racks in the oven burned the leather. His conclusion was because the metal was hotter than the rest of the oven. Is the chemist correct? Explain why or why not. If not, explain why the metal burned the leather. 4) Don Joe often complains about humidity during summer times when humidity reaches about 60%. Don looks up the weather forecast for tomorrow and sees the following information: Description: Clear/cloudy Temperature High/Low: 45F/40F Precipitation: 0% Humidity: 50-75% Wind: N 5mph Does Don Joe have a reason to be worried about the humidity tomorrow?

Answer 1

(a) This kind of fibres are made by mixing hydrophilic and hydrophobic fibres (polyamide/spandex blend). Generally it is made in three layers.The reason behind moisture wicking fibre is its permeability (as it has nanopores whose size is greater than molecule of water and capillary pressure) Due to high permeability it can transport moisture easily. permeability depends upon the weaving process, size of pores and how different size of pores are interconnected.

capillary action takes out the moisture trapped between skin and fibre. As the moisture gets into pores, the walls of fibre exert pressure (due to surface tension) on it alongwith the atmospheric pressure. As the pores are acting as capillary and their size is small so the pressure applied on moisture is high. It leads to rise in its height in capillary which means the the moisture tries to come out of pores till the net force becomes zero. capillary action also depends upon angle of contact between fibre and moisture.

when the liquid gets out it spreads on apparel(as you the nanopores has high surface area) and is evaporated quickly keeping the fibre dry.

Hence the advertisement is not false.

(b) In such apparel alongwith wicking,breathability is counted. An extra insulated layer is added into apparel which doesnot allow air and moisture from atmosphere to enter into fibre and hence the heat of body gets trapped keeping the person warm. So due to wicking property the moisture /sweat comes out while due to insulating layer body heat remains intact keeping the body warm.

Hence the advertisement is not false.

(c) The rest of oven is heated by convection but heating of metal is done due to conduction. Since metal has high number of electron which when collide at a rapid rate cause large amount of heat loss.If specific heat capacity is low the temperature of metal rises faster. The energy lost from metal in form of radiation heats up air column and due to convection rest of oven is heated. The temperature of metal rises fastly and the rate of emitted radiation increases with it. So it can be said metal is hotter than surrounding which led to burning of leather.

(d) The temperature signifies that this is either winter season or somehow temperature is really low(may be rain/snow) and moisture content is high. In case of apparel the sweat will not dry soon. In case of house the moisture content inside is also high but temperature is higher than outside.It can lead to condensation on windows and other part of house resulting in growth of mould.