Solutions for QUIZ 2 - October 17, 2002

A. 1d, 2b, 3a, 4i, 5k, 6c, 7l, 8m, 9e, 10j, 11f, 12g, 13h.

B.
B.1. Watts (or Kilowatts or Megawatts)
B.2. decrease
B.3. anemometer
B.4. direction
B.5. hygrometer, humidity
B.6. ozone.

C. 1d, 2b, 3a, 4c, 5a, 6b, 7a, 8c.

D. 1T, 2F, 3F, 4F, 5T, 6F.

E.1. Mercury Barometer

Mercury barometers work because of balance of pressures. The diagram below shows a schematic layout of such an instrument


The instrument comprises a thin glass tube-like chamber, open at the bottom and closed at the top, which is placed in a bowl of Mercury, with all air in the top part removed. Because the chamber is a vacuum in the upper part, there is no air pressure from above acting on the mercury in the tube. However, pressure due to the outside air does act upon the surface of the mercury in the bowl, and therefore throughout the bowl, and the pressure in a liquid acts equally in all directions. Hence the pressure due to the air acts to push UPWARD on the mercury in the evacuated tube. This forces the mercury up the tube. However, as the mercury is forced higher into the tube, its own weight starts to cause a pressure to act DOWNWARDS at the base of the tube. When the pressure due to the atmosphere balances the pressure due to the weight of mercury at the bottom of the tube, movement stops, and the weight of the mercury and the pressure of the air are in equilibrium. The height of the mercury is then a measure of the atmospheric pressure. If the atmospheric pressure increases. the mercury rises up the tube; if it decreases, the mercury moves down the tube.
We talk about the atmospheric pressure as being some number of "millimetres of mercury", e.g. 755. Sometimes we give the height in inches. However, a HEIGHT is not really a measure of pressure, and we use it this way only as a convenience. Real units of pressure are Newtons per square metre, or PASCALS. Other units in common usage are "millibars", "hectopascals", "kilopascals" and "atmospheres".


E.2. A psychrometer comprises two thermometers placed side-by-side. The bulb of one of the thermometers is covered with a muslin wick, which is moistened with distilled water. Then a flow of air is caused to pass over both thermometers. This is achieved by a variety of methods - one common one is to use a fan. In the "sling-phsychrometer" the instrument is whirled around above the head at high speed in order to generate air flow. In either case, this wet-bulb thermometer is cooled by evaporation (due to the air stream passing over it) to a value below the temperature shown by the dry-bulb thermometer. The computation of the humidity is carried out by comparing the two readings of the thermometers, since the difference between them depends on humidity and pressure (the pressure is measured independently using a barometer). A "look-up" table is usually used to convert the two temperature readings to a humidity.

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