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

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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