Chapter 23 The first law of thermodynamics
Chapter 23 The first law of thermodynamics
23-1Heat: energy in transit(传输) 1. Heat is energy that flows between a system and its environment because of a temperature difference between them We choose our sign convention so that Q is positive in the case that the internal energy of the system tends to be increased Like other forms of energy heat can be expressed in the sl unit of oules
23-1 Heat: energy in transit(传输) 1. “Heat is energy that flows between a system and its environment because of a temperature difference between them” We choose our sign convention so that Q is positive in the case that the internal energy of the system tends to be increased. Like other forms of energy, heat can be expressed in the SI unit of Joules (J)
2. Misconceptions(误解) about heat Neither heat nor work is an intrinsic property of a system We cannot say that a system"contains"a certain amount of heat or work. They are not state functions Both heat and work associated with a thermodynamic process 3. The understanding of heat in history See动画库\力学夹\5-03作功与传热
2. Misconceptions(误解) about heat Neither heat nor work is an intrinsic property of a system. We cannot say that a system “contains” a certain amount of heat or work. They are not state functions. Both heat and work associated with a “thermodynamic process” 3. The understanding of heat in history See动画库\力学夹\5-03作功与传热
23-2 The transfer of heat 1.Thermal conduction Fig 23-2 Consider a thin slab of a homogeneous material of A thickness Ar and area A(Fig 23-2). One face is held at T+△T Q T and the other at a somewhat higher constant temperature T+△T
23-2 The transfer of heat 1.Thermal conduction Consider a thin slab of a homogeneous material of thickness and area A (Fig 23-2). One face is held at T and the other at a somewhat higher constant temperature . Q A T x x T + T T + T Fig 23-2
H is the“ rate of heat transfer, r的 Experiment shows that H(g) ∠1T △t h=kA (23-1 k is the thermal conductivity" of the material a is the area of the slab Ax is the thickness of the slab Considering the direction of h and infinitesimal thickness of the slab. we have H=-kA (23-4)
Experiment shows that is (23-1) H is the “rate of heat transfer” k is the “thermal conductivity” of the material A is the area of the slab is the thickness of the slab ( ) t Q H = Δx ΔT H = kA x Considering the direction of H and infinitesimal thickness of the slab, we have: dx dT H = −kA (23-4)
Sample problem 23-2 A thin, cylindrical metal pipe is carrying steam at a temperature of Ts=100c. The pipe has a diameter of 5.4 cm and is wrapped with a thickness of 5.2 cm of fiberglass((玻璃丝) insulation. A length D=62mof the pipe passes through a room in which the temperature iS TR=11c. At what rate does heat energy pass through the insulation?
Sample Problem 23-2 A thin, cylindrical metal pipe is carrying steam at a temperature of Ts=1000c. The pipe has a diameter of 5.4 cm and is wrapped with a thickness of 5.2 cm of fiberglass(玻璃丝) insulation. A length D=6.2 m of the pipe passes through a room in which the temperature is TR=110c. At what rate does heat energy pass through the insulation?
2. Convection(对流) 3 Radiation
2. Convection(对流) 3. Radiation
Problem TWO identical rectangular rods of metal are welded end to end as shown in Fig. a and 10J of heat flows through the rods in 2.0 min. How long would it take for 30 J to flow throught the rods if they are welded as shown in Fig. b? a) 00C 100C 00C 100C
Problem Two identical rectangular rods of metal are welded end to end as shown in Fig. a), and 10 J of heat flows through the rods in 2.0 min. How long would it take for 30 J to flow throught the rods if they are welded as shown in Fig. b? a) b) 0 0C 1000C 100 0 0C 0C
23-3 The first law of thermodynamics 1. For a thermodynamic system, internal energy is the only type of energy the system may have the law of conservation of energy of the system can be expressed as First law of thermodynamics) Q+M=△Emt (23-6) 0Q is the energy transferred (as heat) between the system and its environment because of a temperature different
23-3 The first law of thermodynamics 1.For a thermodynamic system, internal energy is the only type of energy the system may have. The law of conservation of energy of the system can be expressed as (First law of thermodynamics) (23-6) (i) Q is the energy transferred (as heat) between the system and its environment because of a temperature different. Q +W = Eint
(ii)W is the work done on ( or by the system by forces that act through the system boundary Fig 23-9 / initial final state process state Boundary Environment Eint int, f △E.=E Int/ Q+w
(ii) W is the work done on ( or by) the system by forces that act through the system boundary. (a) (b) (c) initial state process final state Fig 23-9 Environment Q W Eint,i Eint,f Eint = Eint,f − Eint,i = Q +W Boundary
