物化第二章

第二章The First Law: the concepts

1. The basic concepts

System

Open system: can exchange matter and energy with its surrounding.

Closed system: can exchange energy with its surroundings, but it cannot exchange matter

Isolated system: can exchange neither energy nor matter with its surroundings.

Surroundings: comprise the region outside the system and are where we make our measurements.

Work, heat, and energy

（1）内能（U）内能是体系内部所储存的总能量。它包括体系内部分子运动的动能、分子与分子相互作用的势能、

分子内部原子与电子运动的能量以及原子核内能量等。

内能是体系的一种性质，因而是状态函数，具有状态函

数的特点。体系在一定状态下，内能有一定值，体系状

态改变时内能发生变化，只决定于初态和末态，与途经

无关。

（2）热（q）热是体系与环境交换能量的一种形式。由于体系与环境存在温度差而发生的能量交换叫作热。热有显

热、潜热和反应热等。显热是没有化学变化和聚积状态

变化，单纯生温或降温时，体系所吸收或放出的热。（3）功（w）功是体系与环境交换能量的另一种形式。又可以分为若干种。热力学中常见的一种功，是体系反抗外

压，体积膨胀所作的机械功，称为膨胀功。

The disorderly motion of molecules is called thermal motion. The distinction between work and heat is made in the surroundings. Work is identified as energy transfer making use of the organized motion of atoms in the surroundings, and heat is identified as energy transfer making use of thermal motion in the surroundings.

Energy is quantized（量子化的）, that is, a particle can possess only certain energies, called its ‘energy levels’. Broadly speaking, the heavier the particle and the greater its spatial freedom, the closer together are its energy levels.

The number of particles, N i, in a sample of N particles, that will be found in a state with an energy E i when it is part of a system in thermal equilibrium at a temperature T is expressed by the Boltzmann distribution:

N i =Ne-Ei/kT/q q=∑e-E i/kT

i

Where k is Boltzmann’s constant, k=1.381×10-23Jk-1. Boltzmann’s constant related to the gas constant by R=N A K, where N A is Avogadro’s constant. The function q is called the

partition function(配分函数)。It plays a central role in the calculation of thermodynamic properties.

The First Law

The internal energy U is the total kinetic (动能) and potential (势能) energy of the molecules in the system. We denote by△U the change in internal energy when a system changes from an initial state i with internal energy U i to a final state f of internal energy U f:

△U= U f－U i

The internal energy is a state function in the sense that its value depends only on the current state of the system and is independent of how that state has been prepared.

内能是一个广度性质

Internal energy, heat, and work are all measured in the same units, the joule (J).

1 J =1 kg m

2 s-2

Changes in molar internal energy, △U m are typically expressed more common in fields other than thermodynamics.

1 eV ≈0.16 aJ (where 1 aJ =10-28 J)

1 cal = 4.184 J exactly

The internal energy of a gas

The key to relating the internal energy to the properties of