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

(Steven Felgate) #1

13.2 Conservation of Energy –First Law of Thermodynamics 379


Thermal Energy Units


In Chapter 11, we explained that 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 is calledheat. Remember the fact that heat always flows from a high-
temperature region to the low-temperature region. Moreover, we discussed the three differ-
ent modes of heat transfer: conduction, convection, and radiation. We also discussed three
units that are commonly used to quantify thermal energy (1) the British thermal unit,
(2) the calorie, and (3) the joule.
As we explained the Btu (British thermal unit) in Chapter 11, one Btu is formally
defined as the amount of thermal energy needed to raise the temperature of 1 lbmof water
by 1F. The calorie is defined as the amount of heat required to raise the temperature of 1 g
of water by 1C. And as you may also recall from our discussion in Chapter 11, in SI units
no distinction is made between the units of thermal energy and mechanical energy, and
therefore the units of thermal energy are defined in terms of fundamental dimensions of
mass, length, and time. In the SI system of units, the joule is the unit of energy and is
defined as

The U.S. Customary unit of thermal energy is related to mechanical energy through


Finally, internal energy is a measure of the molecular activity of a substance and is related to the
temperature of a substance. As we explained in Chapter 11, the higher the temperature of an
object, the higher its molecular activity and thus its internal energy.

13.2 Conservation of Energy–First Law of Thermodynamics


Earlier in this chapter, we discussed the conservation of mechanical energy. We stated that in the
absence of heat transfer, and assuming negligible losses and no work, the conservation of mechan-
ical energy states that the total mechanical energy of the system is constant. In this section, we will
discuss the effects of heat and work in conservation of energy. There are a number of different ways
that we can describe the general form of the conservation of energy, or the first law of thermody-
namics. Expressed simply, the first law of thermodynamicsstates that energy is conserved. It
cannot be created or destroyed; energy can only change forms. Another more elaborate statement
of the first law says that for a system having a fixed mass, the net heat transfer to the system minus
the work done by the system is equal to the change in total energy of the system (see Figure 13.8)
according to

(13.8)


where


QW¢E


1 Btu1055 J


1 Btu778 lb#ft


1 joule1 N#m1 kg#m
2
/s
2

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