热力学第一定律Energy and the First Law of Thermodynamics课件.ppt

Chapter 2 Energy and the First Law of Thermodynamics 本章要求 深刻认识热力学第一定律的实质——能量守恒 了解热和功是系统与外界交换能量的两种方式，定义、特性及计算方法 热力学第一定律能量方程的基本表达式 运用热力学第一定律进行工程实践分析 Work Work is done by a system on its surroundings if the sole effect on everything external to the system could have been the raising of a weight. 功是系统间相互作用而传递的能量 当系统完成功时，其对外界的作用可以用在外界举起重物的单一效果来代替。 注意：“举起重物”在热力学定义中是“过程产生的效果相当于重物举起，而不一定是真实举起。 Sign convention and Notation W0: work done by the system W0: work done on the system Work Work = Force ′ Distance W = F Dx The unit for work is the Newton-meter which is also called a Joule. Heat Interaction Heat interaction is an interaction between systems in quasi-equilibrium states. Examples of heat interaction: Two blocks of metal at different temperatures Heat interaction is “positive” if it is to the system. It is “negative” if it is from the system Adiabatic Process A process involves only work interaction is called an adiabatic process. An adiabatic boundary (or wall) is one permit only work interaction to occur. The First Law of Thermodynamics For a system undergoing an adiabatic process, the amount of work is fixed by its end states and is independent of the details of the process. Therefore, for different adiabatic processes that link the same end states of a system, the same amount of work is involved in each of these adiabatic processes. Energy Since the adiabatic work interactions involved between states 1 and 2 for the various adiabatic processes linking 1 and 2 are the same, the adiabatic work is independent of the processes linking 1 and 2. Therefore, according to the definition of property, this work can be used to define a new property - ENERGY. dWadi = dE or, W1-2 = E2 – E1 Remarks on First Law: The first law is a “postulation” based on past experiences. It is not a law of nature and can never be proven. So far as there has never been any experimental findings that contradict it, we will accept it as a statement of truth. The first law define