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)