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Unformatted text preview: stance, such as a strip of metal, or a gas at fixed pressure. Or we could use an
electrical property, such as the resistance, of some standard object.
A thermometer based on expansion of a gas is especially interesting, though, because if you
extrapolate the scale down to very low temperatures, you are led to predict for any low-density gas at
constant pressure, the volume should go to zero at approximately -273°C. Alternately, if you hold the
volume of the gas fixed, then its pressure will approach zero as the temperature approaches -273°C. This
special temperature is called absolute zero, and defines the zero-point of the absolute temperature
scale, first proposed by William Thomson in 1848. Thomson was later named Baron Kelvin of Largs, so
the SI unit for temperature is Kelvin which is, measured from absolute zero instead from the freezing
point of water. In round numbers, room temperature is approximately 300 K.
Many equations of thermodynamics are correct ONLY when you measure temperature in the
Kelvin scale. For this reason, it’s usually wise to convert temperatures to Kelvins before plugging them
into any formula. Celsius is ok, though, when your talking about the DIFFERENCE between two
The transferred energy is called heat and is symbolized Q. Heat is positive when energy is
transferred to a system’s thermal energy from its environment (we say that heat is absorbed by the
system). Heat is negative when energy is transferred from a system’s thermal energy to its environment
(we say that heat is released or lost by the system)
Thus we can claim:
Heat is the energy transferred between a system and its environment because of a
temperature difference that exists between them.
Recall that energy can also be transferred between a system and its environment as work W via a
force acting on a system. Heat and work, unlike temperature, pressure, and volume, are not intrinsic
properties of the system. They have meaning only as to describe the transfer of energy into or out of a
system. Similarly, the phrase “a $600 transfer” has meaning if it describes the transfer to or from an
account, not what is in the account, because the account holds money, not a transfer. Here, it is proper to
say: “During the last 3 min, 15 J of heat was transferred to the system from its environment.” It is
meaningless to say “This system contains 450 J of heat” or “This system contains 385 J or work”.
Before scientists realized that heat is transferred energy, heat was measured in terms of its ability
to raise the temperature of water. Thus, the calorie (cal) was defined as the amount of heat that would
raise the temperature of 1 g of water from 14.5°C to 15.5°C.
In 1948, the scientific community decided that since heat (like work) is transferred energy, the SI
unit for heat should be the one we use for energy—namely, the joule. The calorie is now defined to be
4.1868 J (exactly), with no reference to the heating of water. (The “calorie” used in nutrition s...
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This document was uploaded on 03/20/2014 for the course PHYS 215 at Lafayette.
- Fall '09