Some Effects of Temperature Help
Temperature can affect the volume of or the pressure exerted by a sample of matter. You are familiar with the fact that most metals expand when they are heated; some expand more than others.
Temperature, Volume, And Pressure
A sample of gas confined to a rigid container will exert more and more pressure on the walls of the container as the temperature goes up. If the container is flexible, such as a balloon, the volume of the gas will increase. Similarly, if you take a container with a certain amount of gas in it and suddenly make the container bigger without adding any more gas, the drop in pressure will produce a decrease in temperature. If you have a rigid container with gas in it and then some of the gas is allowed to escape (or is pumped out), the drop in pressure will chill the container. This is why, for example, a compressed-air canister gets cold when you use it to blow dust out of your computer keyboard.
Liquids behave a little more strangely. The volume of the liquid water in a kettle and the pressure it exerts on the kettle walls don’t change when the temperature goes up and down unless the water freezes or boils. Some liquids, however, unlike water, expand when they heat up. Mercury is an example. This is how an old-fashioned thermometer works.
Solids, in general, expand when the temperature rises and contract when the temperature falls. In many cases you don’t notice this expansion and contraction. Does your desk look bigger when the room is 30° C than it does when the room is only 20°C? Of course not. But it is! You don’t see the difference because it is microscopic. However, the bimetallic strip in the thermostat, which controls the furnace or air conditioner, bends considerably when one of its metals expands or contracts just a tiny bit more than the other. If you hold such a strip near a hot flame, you actually can watch it curl up or straighten out.
Standard Temperature And Pressure (stp)
To set a reference for temperature and pressure against which measurements can be made and experiments conducted, scientists have defined standard temperature and pressure (STP). This is a more or less typical state of affairs at sea level on the Earth’s surface when the air is dry.
The standard temperature is 0°C (32°F), which is the freezing point or melting point of pure liquid water. Standard pressure is the air pressure that will support a column of mercury 0.760 m (just a little less than 30 in) high. This is the proverbial 14.7 pounds per inch squared (lb/in 2 ), which translates to approximately 1.01 × 10 5 newtons per meter squared (N/m 2 ).
Air is surprisingly massive. We don’t think of air as having significant mass, but this is because we’re immersed in it. When you dive only a couple of meters down in a swimming pool, you don’t feel a lot of pressure and the water does not feel massive, but if you calculate the huge amount of mass above you, it might scare you out of the water! The density of dry air at STP is approximately 1.29 kg/m 3 . A parcel of air measuring 4.00 m high by 4.00 m deep by 4.00 m wide, the size of a large bedroom, masses 82.6 kg. In Earth’s gravitational field, that translates to 182 pounds, the weight of a good-sized, full-grown man.
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