Physics and Temperature Help

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By — McGraw-Hill Professional
Updated on Sep 5, 2011


What do we mean by the term temperature? You have an intuitive idea of this; the temperature is generally higher in the summer than in the winter, for example. Temperature is a quantitative expression of the average kinetic energy contained in matter. This is the most familiar definition. In general, for any given substance, the higher the temperature, the faster the atoms and molecules dance around.

Temperature can be expressed in another way. For example, to measure the temperatures of distant stars, planets, and nebulae in outer space, astronomers look at the way they emit electromagnetic (EM) energy in the form of visible light, infrared, ultraviolet, and even radio waves and x-rays. By examining the intensity of this radiation as a function of the wavelength, astronomers come up with a value for the spectral temperature of the distant matter or object.

When energy is allowed to flow from one substance into another in the form of heat, the temperatures try to equalize. Ultimately, if the energy-transfer process is allowed to continue for a long enough time, the temperatures of the two objects will become the same, unless one of the substances is driven away (for example, steam boiling off of a kettle of water). The kinetic energy of everything in the entire universe is trying to level off to a state of equilibrium. It won’t succeed in your lifetime or mine or even during the lifetime of the Sun and solar system, but it will keep trying anyway, and gradually it is succeeding. This process is known as heat entropy .

The Celsius (or Centigrade) Scale

Up to now, we’ve been talking rather loosely about temperature and usually have expressed it in terms of the Celsius or centigrade scale (°C). This is based on the behavior of water at the surface of the Earth under normal atmospheric pressure and at sea level.

If you have a sample of ice that is extremely cold and you begin to warm it up, it will eventually start to melt as it accepts heat from the environment. The ice, and the liquid water produced as it melts, is assigned a temperature value of 0°C by convention (Fig. 11-2a). As you continue to pump energy into the chunk of ice, more and more of it will melt, and its temperature will stay at 0°C. It won’t get any hotter because it is not yet all liquid and doesn’t yet obey the rules for pure liquid water.

Once all the water has become liquid and as you keep pumping energy into it, its temperature will start to increase (see Fig. 11-2b). For awhile, the water will remain liquid and will get warmer and warmer, obeying the 1 cal/g/°C rule. Eventually, however, a point will be reached where the water starts to boil, and some of it changes to the gaseous state. The liquid water temperature, and the water vapor that comes immediately off of it, is then assigned a value of 100°C by convention (see Fig. 11-2c).

Temperature, Pressure, and Changes of State Temperature The Celsius (or Centigrade) Scale

Fig. 11-2. Ice melting into liquid water (a), liquid water warming up without boiling (b), and liquid water starting to boil (c).

Now there are two definitive points—the freezing point of water and the boiling point —at which there exist two specific numbers for temperature. We can define a scheme to express temperature based on these two points. This is the Celsius temperature scale , named after the scientist who supposedly first came up with the idea. Sometimes it is called the centigrade temperature scale because one degree of temperature in this scale is equal to 1/100 of the difference between the melting temperature of pure water at sea level and the boiling temperature of pure water at sea level. The prefix multiplier centi- means “1/100,” so centigrade literally means “graduations of 1/100.”

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