Lesson 13

Read the text: Modeling and study of the processes in the heat

In physics and chemistry, heat is energy in transfer between a system and its surroundings other than by work or transfer of matter. The transfer can occur in two simple ways, conduction, and radiation and in a more complicated way called convective circulation. Heat is not a property or component or constituent of a body itself.

It can occur that the surroundings of a system can be described also as a second thermodynamic system that has its own definite temperature. In this special circumstance, if the two are connected by a pathway for heat transfer, then, according to the second law of thermodynamics, heat flow occurs spontaneously from the hotter to the colder system. Consequently, in this circumstance, heat is transfer of energy due purely to temperature gradient or difference. It is accompanied by an increase in the total entropy of system and surroundings.

In a heat engine, which operates in a cyclic process, internal energy of bodies is harnessed to provide useful work, heat being supplied from a hot reservoir, always with an associated discharge of waste heat to a cold reservoir. Through an arrangement of systems and devices, which operate in a cyclic process, called a heat pump, externally supplied work can be used to transfer internal energy indirectly from a cold to a hot body, but such a transfer cannot occur directly between the bodies, without the heat pump.

Transfers of energy as heat are macroscopic processes. Kinetic theory explains them as the microscopic motions and interactions of microscopic constituents such as molecules and photons. It explains heat flow as occurring when the more rapidly moving or strongly excited molecules in a high-temperature body transfer some of their energy, other than by work or bulk transfer of matter, to the less energized molecules in a lower temperature body. Thus heat flow is said to be a diffusive, as distinct from a bulk flow, transfer of internal energy, driven purely by temperature difference.

The SI unit of heat is the joule. Heat can be measured by calorimeters, or determined by calculations based on other quantities, relying on the first law of thermodynamics. In calorimetry, the concepts of latent heat and of sensible heat are used. Latent heat produces changes of state without temperature change, while sensible heat produces temperature change without change of state.

Heat in physics is defined as energy transferred between a system and its surroundings other than by work or transfer of matter. The surroundings of a thermodynamic system are in themselves a physical system, but the surroundings that directly contact the indicated thermodynamic system may or may not themselves consist of properly defined thermodynamic systems. A physical system may for example be so turbulent that it is not feasible to define its temperature. A properly defined thermodynamic system has a temperature that indicates how hot or cold it is.

Heat flows spontaneously from a hotter to a colder closed system. When two closed systems come into thermal contact, they exchange energy through the microscopic interactions of their particles. When the systems are at different temperatures, the result is a spontaneous net flow of energy that continues until the temperatures are equal. At that point the net flow of energy is zero, and the systems are said to be in thermal equilibrium. It is also permitted to say that heat can pass spontaneously, by conduction and radiation, from a physical system that does not have a temperature, to a thermodynamic system. Spontaneous heat transfer is an irreversible process.

According to the first law of thermodynamics, the internal energy of an isolated system is conserved. To change the internal energy of a system, energy must be transferred to or from the system. For a closed system, heat and work are the mechanisms by which energy can be transferred. For an open system, internal energy can be changed also by transfer of matter. Work performed by a closed system is, by definition, an energy transfer from the system that is due to a change to its external or mechanical parameters, such as the volume, magnetization, and location of center of mass in a gravitational field.

When energy is transferred to a body purely as heat, its internal energy increases. The additional internal energy becomes indistinguishable from the general pool of internal energy of the body.

As described by kinetic theory, internal energy is present microscopically in a body as kinetic and potential energy of its microscopic particles, such as molecules, atoms and electrons. Such energy is also described by the presence of phonons.

In the kinetic theory, heat is explained in terms of the microscopic motions and interactions of constituent particles, such as electrons, atoms, and molecules. Heat transfer arises from temperature gradients or differences, through the diffuse exchange of microscopic kinetic and potential particle energy, by particle collisions and other interactions. An early and vague expression of this was made by Francis Bacon. Precise and detailed versions of it were developed in the nineteenth century.

Reading Exercises:

Exercise 1. Read and memorize using a dictionary:

Consequently, pathway; latent heat; sensible heat; feasible; irreversible process; indistinguishable;  

Exercise 2. Answer the questions:

1) What is heat in physics and chemistry?

2) How can heat be measured or determined?

3) What does latent heat produce?

4) What does sensible heat produce?

Exercise 3. Match the left part with the right:

1. The transfer can occur in two simple ways,	a) interactions of microscopic constituents such as molecules and photons.
2. Kinetic theory explains them as the microscopic motions and	b) conduction, and radiation and in a more complicated way called convective circulation.
3. In the kinetic theory, heat is explained in terms of	c) its internal energy increases.
4. When energy is transferred to a body purely as heat,	d) the microscopic motions and interactions of constituent particles, such as electrons, atoms, and molecules.

Exercise 4. Open brackets choosing the right words:

Transfers of energy as heat are macroscopic processes. Kinetic theory (explains/ interprets) them as the microscopic motions and interactions of microscopic (constituents/components) such as molecules and photons.

II. Speaking Exercises:

Exercise 1. Learn the definitions: heat; conduction; radiation.

Heatis the form of energy that flows between two samples of matter due to their difference in temperature. Usually denoted by the variable 'Q'.

Conduction -the transfer of heat between two parts of a stationary system, caused by a temperature difference between the parts.  

Radiation -the process in which energy is emitted as particles or waves.

Exercise 2. Ask questions to the given answers:

1) Question: ___________________________________________?

Answer: Heat flows spontaneously from a hotter to a colder closed system.

2) Question: ___________________________________________?

Answer: Latent heat produces changes of state without temperature change.

3) Question: ___________________________________________?

Answer: Sensible heat produces temperature change without change of state.

III. Writing exercises:

Exercise 1. Complete the sentences with the suggested words: in; between; of; system;

Heat 1 physics is defined as energy transferred 2 a system and its surroundings other than by work or transfer 3 matter. The surroundings of a thermodynamic 4 are in themselves a physical system, but the surroundings that directly contact the indicated thermodynamic system may or may not themselves consist of properly defined thermodynamic systems.

Exercise 2. Compose a story on one of the topics (up to 100 words):

“ Heat is energy in transfer between a system and its surroundings ”

Дата добавления: 2015-08-03