Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.3 Temperature Difference and Heat Transfer 315

And simplifying this relationship leads to

Also note that you could have converted the temperature difference in degrees Rankine to

Kelvin directly in the following manner:

11.3 Temperature Difference and Heat Transfer

Thermal energy transfer occurs whenever there exists a temperature difference within an object,

or whenever there is a temperature difference between two bodies, or a temperature difference

between a body and its surroundings. This form of energy transfer that occurs between bodies

of different temperatures is called heat transfer. Additionally, heat always flows from a high-

temperature region to a low-temperature region. This statement can be confirmed by

observation of our surroundings. When hot coffee in a cup is left in a surrounding such as a room

with a lower temperature, the coffee cools down. Thermal energy transfer takes place from the

hot coffee through the cup and from its open surface to the surrounding room air. The thermal

energy transfer occurs as long as there is a temperature difference between the coffee and its sur-

roundings. At this point, make sure you understand the difference betweentemperatureand

heat. Heat is a form of energy that is transferred from one region to the next region as a result

of a temperature difference between the regions, whereas temperature represents on a macro-

scopic level, by a single number, the level of microscopic molecular movement in a region.

There are three units that are commonly used to quantify thermal energy: (1) the British

thermal unit (Btu), (2) the calorie, and (3) the joule. TheBritish thermal unit(Btu)is defined

as the amount of heat required to raise the temperature of 1 pound mass (1 lbm)ofwaterby

1 degree Fahrenheit (1F). Thecalorieis defined as the amount of heat required to raise the tem-

perature of 1 gram of water by 1C. Note, however, that the energy content of food is typically

expressed inCalories, which is equal to 1000 calories. In SI units, no distinction is made between

the units of thermal energy and mechanical energy and therefore energy is defined in terms of

the fundamental dimensions of mass, length, and time. We will discuss this in more detail in

Chapter 13. In the SI System of Units, thejouleis the unit of energy and is defined as

The conversion factors among various units of heat are given in Table 11.2.

1 joule1 N#m1 kg#m

2

/s

2

¢T 1 K 2 

5

9

¢T 1 °R 2 a

5

9

b1 422 23.3 K

T 1 1 °R 2 T 2 1 °R 2 T 1 1 °F 2 T 2 1 °F 2

TABLE 11.2 Conversion Factors for Thermal Energy and Thermal Energy

per Unit Time (Power)

Relationship Between the Relationship Between the Units of

Units of Thermal Energy Thermal Energy per Unit Time (Power)

1 Btu 1055 J 1 W 1 J/s

1 Btu 252 cal 1 W 3.4123 Btu /h

1 cal 4.186 J 1 cal /s 4.186 W

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