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Chapter Review

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This is the first of three chapters on thermodynamics which can be loosely described as the study of heat and the physical processes associated with heat transfer. In this chapter we focus on the concepts of temperature and heat as well as a few physical consequences of heat transfer.

16-1   Temperature and the Zeroth Law of Thermodynamics

In this section we seek to develop working definitions of temperature and heat. The two concepts are intimately related and must be defined together. Heat is a form of energy that flows between two systems. Systems are said to be in thermal contact when it is possible for heat to flow between them. However, heat does not always flow between systems that are in thermal contact. The property of systems that determines whether or not heat flow will occur is called the temperature. If there is a temperature difference between two systems that are in thermal contact heat will flow from the system with higher temperature to the system with lower temperature. Two systems are said to be in thermal equilibrium if, when brought into thermal contact, no heat transfer occurs. In this latter case the systems must have equal temperatures. These statements are embodied in the zeroth law of thermodynamics

If system A is in thermal equilibrium with system B, and system C is also in thermal equilibrium with system B, then systems A and C will be in thermal equilibrium if brought into thermal contact.

The result of all this then is the following working definitions:

• Heat is the energy that is transferred between systems because of a temperature difference.
  


• Temperature is the property of systems that determines the existence and direction of the heat flow betwen them when they are in thermal contact.

Temperature is a new fundamental quantity, not defined in terms of length, mass, and time. It is assigned the dimensional symbol [K].

Practice Quiz

Which of the following statements is most accurate? If there is a temperature difference between systems A and B, heat will flow from system A to system B. If there is a temperature difference between systems A and B, heat will flow from system B to system A. If there is a temperature difference betwwen two systems, heat will flow from one system to the other. If there is a temperature difference between two systems, heat will flow from the system with higher temperature to the system with lower temperature. If there is a temperature difference between two systems, heat will flow from the system with higher temperature to the system with lower temperature if they are brought into thermal contact.

16-2   Temperature Scales

The primary temperature scales in common use are the Celsius, Fahrenheit, and Kelvin scales. Each scale is based on different choices for setting values for two convenient fixed points. The Celsius scale takes the freezing temperature of water to be at 0 ^oC and the boiling temperature of water to be 100 ^oC. On the Fahrenheit scale, water freezes at 32 ^oF and boils at 212 ^oF. In addition to having different settings for the freezing and boiling of water, the scale of these two systems are different in that a temperature change of one degree on the Fahrenheit scale corresponds only to a change of 5/9 degrees on the Celsius scale. Converting between the Celsius and Fahrenheit scales can be accomplished using the following formulas:

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Exercise 16.1   A Change in Temperature: On a Winter day in Michigan the temperature rose from a morning low of -8.0 ^oF to an afternoon high of 22 ^oF. By how many Celsius degrees did the temperature rise?

Solution: We are given the following information:

Given: T_i = -8.0 ^oF, T_f = ^oF     Find: DT in C^o

Since we know how to convert from Fahrenheit to Celsius let's find an expression for DT_C.

.

Therefore, we have

.

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The Kelvin temperature scale is based on the existence of a lowest temperature below which (even to which) it is physically impossible to cool any system. This temperature is called absolute zero. The Kelvin scale is essentially identical to the Celsius scale except that the zero mark is shifted to correspond to absolute zero (-273.15 ^oC). The conversion between temperatires on the Kelvin scale and the Celsius scale is the following:

T = T_C + 273.15

Quoting Temperatures

You should be aware of the following with regard to quoting temperatures and temperature differences.

• When quoting a temperature in Celsius or Fahrenheit the degree symbol is both written and spoken first. That is, 10 ^oC ("10 degrees Celsius") for example.
  


• When quoting temperature differences in Celsius or Fahrenheit the degree symbol is both written and spoken last. That is, 10 C^o ("10 Celsius degrees") for example.
  


• The Celsius scale is no longer alternatively referred to as the Centigrade scale.
  


• No degree symbol is written or spoken with the Kelvin scale. That is, both temperatures and temperature differences of 10 are denoted by 10 K ("10 Kelvin").

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Exercise 16.2 A Temperature Conversion: The normal body temperature of 98.6 ^oF corresponds to what temperatuer on the Kelvin scale?

Solution: We are given the following information:

Given: T_F = 98.6 ^oF     Find: T

Since we know how to convert from Fahrenheit to Celsius and from Celsius to Kelvin we can put the two together to get a conversion from Fahrenheit to Kelvin. To get T_C we use,

,

and substitute this into the conversion for T to get

.

Therefore, we get that

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Practice Quiz

A temperature change of one Kelvin corresponds to how much of a change in Fahrenheit degrees? 0.55 1.8 5/9 32 273.15

What temperature corresponds to absolute zero on the Fahrenheit scale? 0 oF -273.15 oF -459.67 oF -212 oF -100 oF

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