Chapter 2

Ex.1 Read and translate the international words.

Transition, thermal energy, object, temperature, conduction, convection , radiation, atom vibration, constant, contact, fluids, motion, mass, vertically, horizontally, natural, electromagnetic, perfect vacuum.

Ex.2 Remember the meaning of the following words.

equilibrium	равновесие
fluid	жидкость
surroundings	окружение, окружающие предметы
in proximity	вблизи
adjacent	прилегающий
density	плотность
buoyancy	плавучесть
scatter	разбросанный, редкий
boundary	граница
artificially induced	искусственно индуцированный
current	ток, поток
without encountering	(зд.) не сталкиваясь

Ex.3 Read the text “Heat transfer” and translate it.

Heat transfer

Heat transfer is the transition of thermal energy from a hotter object to a cooler object. When an object or fluid is at a different temperature than its surroundings or another object, heat transfer occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed. Heat can be transferred by different types.

Conduction is the transfer of heat by direct contact of particles of matter, when adjacent atoms vibrate against one another, or electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. Metals (e.g. copper, platinum, gold, iron, etc.) are usually the best conductors of thermal energy. As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas. Conductivity of gases increases with temperature.

Convection is the transfer of heat energy between a solid surface and the nearby liquid or gas in motion. As fluid motion goes more quickly the convective heat transfer increases. There are two types of convective heat transfer.

Natural convection occurs when the fluid motion is caused by buoyancy forces that result from the density variations due to variations of temperature in the fluid. For example in the absence of an external source, when the mass of the fluid is in contact with a hot surface its molecules separate and scatter causing the mass of fluid to become less dense. When this happens, the fluid is displaced vertically or horizontally while the cooler fluid gets denser and the fluid sinks. Thus the hotter volume transfers heat towards the cooler volume of that fluid.

Forced convection is when the fluid is forced to flow over the surface by external source such as fans and pumps. It creates an artificially induced convection current.

Internal and external flow can also classify convection. Internal flow occurs when the fluid is enclosed by a solid boundary such as a flow through a pipe. An external flow occurs when the fluid extends indefinitely without encountering a solid surface. Both these convections, either natural or forced, can be internal or external as they are independent of each other.

Radiation is the transfer of heat energy through empty space. All objects with a temperature above absolute zero radiate energy. No medium is necessary for radiation to occur, for it is transferred through electromagnetic waves; radiation works even in and through a perfect vacuum. The energy from the Sun travels through the vacuum of space before warming the earth.

Ex.4 Read the words and say what parts of speech they are.

radiation	transfer	conduction	convection	proximity	increase
radiate	transferring	conductivity	convective	approximately	increased
radiant	transferee	conductive	convected	proximal	increasing
radii	transferor	conductor	convector	proximate	increasingly
radial	transfered	conducted	convect	proximally

Ex.5 Give Russian equivalents to the following words and expressions.

Transition of thermal energy, a temperature difference, a higher-temperature object, buoyancy forces, to result from, internal and external flow, to be displaced vertically or horizontally, to get denser, to become less dense, to be enclosed by a solid boundary, an artificially induced convection current, to be transferred through electromagnetic waves, without encountering a solid surface, a temperature above absolute zero, to work in and through a perfect vacuum.

Ex.6 Answer the following questions about the text.

1. What process do we call ‘heat transfer’? 2. What does the phrase “the body and the surroundings reach thermal equilibrium” mean? 3. Can heat be transferred by different types? 4. What substances are the best conductors of heat? 5. Are fluids and gases good conductors of thermal energy? 6. Can you explain the difference between natural convection and forced convection? 7. Do all objects with a temperature above absolute zero radiate energy? 8. Does radiation need any special medium?

Ex.7 Read and translate the sentences. Find out if a sentence relates to the text “Heat transfer”.

1. Heat transfer occurs when bodies are at the same temperature. 2. Visible light is simply another form of electromagnetic radiation with a shorter wavelength than infrared radiation. 3. A body with higher temperature transfers heat to a cooler-temperature one. 4. Copper, platinum, gold, iron are usually the best conductors of thermal energy. 5. In steady state conduction, temperature plays the role of voltage and heat transferred is the analog of electrical current.6. Radiant barriers are materials which reflect radiation and therefore reduce the flow of heat from radiation sources. 7. Radiation is transferred through electromagnetic waves, that is why no medium is necessary for radiation to occur.

Ex.8 Read the text and answer the following questions.

1. When does steady state conduction occur?

2. In which cases can we talk about thermal resistances as an analogue to electrical resistances?

3. What mode of heat conduction can be referred to as transient conduction?

Transient conduction vs. steady-state conduction

Steady state conduction is the form of conduction which happens when the temperature difference driving the conduction is constant so that after an equilibration time, the spatial distribution of temperatures (temperature field) in the conducting object does not change any further. For example, a bar may be cold at one end and hot at the other, but the gradient of temperatures along the bar do not change with time. The temperature at any given section of the rod remains constant, and this temperature varies linearly along the direction of heat transfer.

In steady state conduction, the amount of heat entering a section is equal to amount of heat coming out. In steady state conduction, all the laws of direct current electrical conduction can be applied to "heat currents". In such cases, it is possible to take "thermal resistances" as the analog to electrical resistances. Temperature plays the role of voltage and heat transferred is the analog of electrical current.

There also exist non-steady-state situations, in which the temperature drop or increase occurs more drastically, such as when a hot copper ball is dropped into oil at a low temperature. Here the temperature field within the object changes as a function of time, and the interest lies in analyzing this spatial change of temperature within the object over time. This mode of heat conduction can be referred to as transient conduction. Analysis of these systems is more complex and (except for simple shapes) calls for the application of approximation theories, and/or numerical analysis by computer.

Ex.9 Translate the following text into English using the vocabulary of this Chapter.

Теплопередача — физический процесс передачи тепловой энергии от более горячего тела к более холодному либо непосредственно (при контакте), либо через разделяющую перегородку из какого-либо материала. Когда физические тела одной системы находятся при разной температуре, то происходит передача тепловой энергии, или теплопередача от одного тела к другому до наступления термодинамического равновесия. Самопроизвольная передача тепла всегда происходит от более горячего тела к более холодному, что является следствием второго закона термодинамики (однако возможно передать тепло от холодного тела с помощью вспомогательных устройств, таких как холодильник). Теплопередачу невозможно остановить, возможно только замедлить её.

Ex.10 Retell the text “Heat transfer”.