Unit 8 Theme:Distinction of thermal energy and heat Grammar: Conjunctions

Objectives: By the end of this unit, students should be able to use active vocabulary of this theme in different forms of speech exercises. Students should be better atdiscussing thermal energy and heating. Students should know the rule of conjunctions and fulfill grammar exercises.

Methodical instructions: This theme must be worked out during two lessons a week according to timetable.

Lexical material:Introduce and fix new vocabulary on theme “thermal energy and heat”.

Define thebasic of thermal energy and its role in our country. Discuss in groups‘thermal energy and heating our society’.

Grammar: Introduce and practice the conjunctions.

Ex.1.Read the text and translate

Distinction of thermal energy and heat

In engineering and technology, and particularly in fields that deal with civil energy use and conservation in building construction, heating systems, and power generation, heat and thermal energy are often indiscriminately used interchangeably.

In thermodynamics, heat must always be defined as energy in exchange between two systems, or a single system and its surroundings. According to the zerothe law of thermodynamics, heat is exchanged between thermodynamic systems in thermal contact only if their temperatures are different, as this is the condition when the net exchange of thermal energy is non-zero. For the purpose of distinction, a system is defined to be enclosed by a well-characterized boundary. If heat traverses the boundary in direction into the system, the internal energy change is considered to be a positive quantity, while exiting the system, it is negative. As a process variable, heat is never a property of the system, nor is it contained within the boundary of the system.

In contrast to heat, thermal energy exists on both sides of a boundary. It is the statistical mean of the microscopic fluctuations of the kinetic energy of the systems' particles, and it is the source and the effect of the transfer of heat across a system boundary. Statistically, thermal energy is always exchanged between systems, even when the temperatures on both sides is the same, i.e. the systems are in thermal equilibrium. However, at equilibrium, the net exchange of thermal energy is zero, and therefore there is no heat.

Thermal energy may be increased in a system by other means than heat, for example when mechanical or electrical work is performed on the system. No qualitative difference exists between the thermal energy added by other means. Thermal energy is a state function. There is also no need in classical thermodynamics to characterize the thermal energy in terms of atomic or molecular behavior. A change in thermal energy induced in a system is the product of the change in entropy and the temperature of the system.

Heat exchanged across a boundary may cause changes other than a change in thermal energy. For example, it may cause phase transitions, such as melting or evaporation, which are changes in the configuration of a material. Since such an energy exchange is not observable by a change in temperature, it is called a latent heat and represents a change in the potential energy of the system.

Ex.2 Topical vocabulary

Traverse a) used about prices and numbers, or people`s feelings, to change frequently from one thing to another

Variable b) used about a liquid, to change into steam or gas and disappear

Contrast c) to come cross the sea, river, lake by ship or boat

Fluctuation d) comparison between two people or things that shows the differences between them

Evaporation e) not staying the same, often changing

Ex.3.Read and summarize the text