In physics Physics is a natural science that involves the study of matter and its motion through space-time, as well as all applicable concepts, including energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves and thermodynamics In science, thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure, heat is the process of energy In physics, energy is a quantity that is often understood as the ability to perform work. This quantity can be assigned to any particle, object, or system of objects as a consequence of its physical state transfer from one body or system In thermodynamics, a thermodynamic system, originally called a working substance, is defined as that part of the universe that is under consideration. Anything under consideration is called a system. A hypothetical boundary separates the system from the rest of the universe, which is referred to as the environment, surroundings, or reservoir. A to another due to thermal contact In thermodynamics, a thermodynamic system is said to be in thermal contact with another system if it can exchange energy with it through the process of heat. Perfect thermal isolation is an idealization as real systems are always in thermal contact with their environment to some extent. Thermal contact does not imply direct physical contact, which in turn is defined as an energy transfer to a body in any other way than due to work In thermodynamics, work performed by a system is the quantity of energy transferred by the system to another that is accounted for in a particular way; namely, by changes in the external generalized mechanical constraints on the system performed on the body.[1]
A related term is thermal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all, loosely defined as the energy of a body that increases with its temperature Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable. Heat is also loosely referred to as thermal energy, although many definitions require this thermal energy to actually be in the process of movement between one body and another to be technically called heat (otherwise, many sources prefer to continue to refer to the static quantity as "thermal energy").
Energy transfer by heat can occur between objects by radiation Thermal radiation is electromagnetic radiation emitted from a material which is due to the heat of the material, the characteristics of which depend on its temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater. A person near a raging bonfire will feel the radiated heat of, conduction In heat transfer, conduction is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. Conduction takes place in all forms of matter, viz. solids, liquids, and convection Convection is the movement of molecules within fluids . It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids. Energy can only be transferred by heat between objects - or areas within an object - with different temperatures Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable (as given by the zeroth law of thermodynamics The zeroth law of thermodynamic is a generalization about the thermal equilibrium among bodies, or thermodynamic systems, in contact. It results from the definition and properties of temperature). This transfer happens spontaneously only in the direction of the colder body (as per the second law of thermodynamics The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium; and that the entropy change dS of a system undergoing any infinitesimal reversible process is given by δq /). The transfer of energy by heat from one object to another object with an equal or higher temperature can happen only with the aid of a heat pump A heat pump is a machine or device that moves heat from one location at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to heat a home as well as cool via mechanical work or by using mirrors or lenses to focus radiation which thereby increase its energy flux density.
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Overview
Heat Q can flow across the boundary of the system In thermodynamics, a thermodynamic system, originally called a working substance, is defined as that part of the universe that is under consideration. Anything under consideration is called a system. A hypothetical boundary separates the system from the rest of the universe, which is referred to as the environment, surroundings, or reservoir. A and thus change its internal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all U.In modern terms, heat is defined as energy in transit. Scottish physicist James Clerk Maxwell James Clerk Maxwell was a Scottish theoretical physicist and mathematician. His most important achievement was classical electromagnetic theory, synthesizing all previously unrelated observations, experiments and equations of electricity, magnetism and even optics into a consistent theory. His set of equations—Maxwell's equations—demonstrated, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of “heat”. Maxwell outlined four stipulations on the definition of heat. One, it is “something which may be transferred from one body to another”, as per the second law of thermodynamics The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium; and that the entropy change dS of a system undergoing any infinitesimal reversible process is given by δq /. Two, it is a “measurable quantity”, and thus treated mathematically. Three, it “can not be treated as a substance”; for it may be transformed into something which is not a substance, e.g. mechanical work In physics, mechanical work is the amount of energy transferred by a force acting through a distance. Like energy, it is a scalar quantity, with SI units of joules. The term work was first coined in 1826 by the French mathematician Gaspard-Gustave Coriolis. Lastly, it is “one of the forms of energy In physics, energy is a quantity that is often understood as the ability to perform work. This quantity can be assigned to any particle, object, or system of objects as a consequence of its physical state”.
Heat flows between systems that are not in thermal equilibrium with each other; it spontaneously flows from the areas of high temperature Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable to areas of low temperature. When two bodies of different temperature come into thermal contact, they will exchange internal energy until their temperatures are equalized; that is, until they reach thermal equilibrium In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states. The local state of a system at thermodynamic equilibrium is determined by the values of its.
The first law of thermodynamics The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed states that the energy of an isolated system In the natural sciences an isolated system, as contrasted with an open system, is a physical system that does not interact with its surroundings. It obeys a number of conservation laws: its total energy and mass stay constant. They cannot enter or exit, but can only move around inside. An example is in the study of spacetime, where it is assumed is conserved. Therefore, to change the energy of a system, energy must be transferred to or from the system. Heat and work are the only two mechanisms by which energy can be transferred. Work performed on a body is, by definition [1], an energy transfer to the body that is due to a change to external parameters of the body (such as the volume, magnetization, center of mass in a gravitational field, etc.). Heat is the energy transferred to the body in any other way.
In the case of bodies close to thermal equilibrium where notions such as the temperature can be defined, heat transfer can be related to temperature difference between bodies. It is an irreversible process that leads to the bodies coming closer to mutual thermal equilibrium.
Adjectives such as "hot" and "cold" are relative terms and are generally used to compare one object’s temperature to another or it's surroundings.
Definitions
Several modern definitions of heat are as follows:
- The energy going from a high-temperature object to a lower-temperature object is called heat.[2]
- The noun heat is defined only during the process of energy transfer by conduction or radiation.[3]
- Any spontaneous flow of energy from one object to another caused by a difference in temperature between the objects is called heat.[4]
As for its relationship with kinetic energy:
- The energy inside a substance is due to the kinetic energy of molecules or atoms.[5] The kinetic energy and heat may formally be equivalent, but they are not identical.
- In a thermodynamic sense, heat is never regarded as being stored within a body. Like work, it exists only as energy in transit from one body to another; in thermodynamic terminology, between a system and its surroundings. When energy in the form of heat is added to a system, it is stored not as heat, but as kinetic and potential energy of the atoms and molecules making up the system.[6]
According to some definitions, heat can flow in the absence of any temperature difference:
- The energy going between the two systems is called heat. However, an interaction between two closed systems that are initially isolated and in a stable equilibrium cannot do work on each other, so they only undergo pure heat (non-working) interactions.[7]
Notation and units
The unit for the amount of energy transferred by heat in the International System of Units SI The International System of Units specifies a set of seven base units from which all other units of measurement are formed. These other units are called SI derived units and are also considered part of the standard is the joule The joule , named after James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy expended in applying a force of one Newton through a distance of one metre (1 Newton·metre or N·m). In terms of dimensions: (J), though the British Thermal Unit The British thermal unit is a traditional unit of energy equal to about 1.06 kilojoules. It is approximately the amount of energy needed to heat one pound of water one degree Fahrenheit. It is used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of and the calorie The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy are still used in the United States. The unit for the rate of heat transfer is the watt The watt is a derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736–1819). The unit measures the rate of energy conversion. It is defined as one joule per second (W = J/s).
The total amount of energy transferred through heat transfer is conventionally abbreviated as Q. The conventional sign convention is that when a body releases heat into its surroundings, Q < 0 (-); when a body absorbs heat from its surroundings, Q > 0 (+). Heat transfer rate, or heat flow per unit time, is denoted by:
- .
It is measured in watts The watt is a derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736–1819). The unit measures the rate of energy conversion. It is defined as one joule per second. Heat flux is defined as rate of heat transfer per unit cross-sectional area, and is denoted q, resulting in units of watts per square metre, though slightly different notation conventions can be used.
Internal energy
Main article: Internal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in allHeat is related to the internal energy U of the system and work In thermodynamics, work performed by a system is the quantity of energy transferred by the system to another that is accounted for in a particular way; namely, by changes in the external generalized mechanical constraints on the system W done by the system by the first law of thermodynamics The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed:
which means that the energy of the system can change either via work or via heat flows across the boundary of the thermodynamic system In thermodynamics, a thermodynamic system, originally called a working substance, is defined as that part of the universe that is under consideration. Anything under consideration is called a system. A hypothetical boundary separates the system from the rest of the universe, which is referred to as the environment, surroundings, or reservoir. A. In more detail, Internal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all is the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules; it comprises the following types of energies:[8]
The transfer of heat to an ideal gas at constant pressure increases the internal energy and performs boundary work (i.e. allows a control volume of gas to become larger or smaller), provided the volume is not constrained. Returning to the first law equation and separating the work term into two types, "boundary work" and "other" (e.g. shaft work performed by a compressor fan), yields the following:
This combined quantity ΔU + Wboundary is enthalpy, H, one of the thermodynamic potentials. Both enthalpy, H, and internal energy, U are state functions. State functions return to their initial values upon completion of each cycle in cyclic processes such as that of a heat engine. In contrast, neither Q nor W are properties of a system and need not sum to zero over the steps of a cycle. The infinitesimal expression for heat, δQ, forms an inexact differential for processes involving work. However, for processes involving no change in volume, applied magnetic field, or other external parameters, δQ forms an exact differential. Likewise, for adiabatic processes (no heat transfer), the expression for work forms an exact differential, but for processes involving transfer of heat it forms an inexact differential.
Enthalpy and internal energy changes
See also: Heat capacityIdeal gas
Main article: Ideal gasFor a simple compressible system such as an ideal gas inside a piston, the changes in enthalpy and internal energy can be related to the heat capacity at constant pressure and volume, respectively. Constrained to have constant volume, the heat, Q, required to change its temperature from an initial temperature, T0, to a final temperature, Tf is given by:
Removing the volume constraint and allowing the system to expand or contract at constant pressure:
Incompressible substances
For incompressible substances, such as solids and liquids, the distinction between the two types of heat capacity (i.e. Cp which is based on constant pressure and Cv which is based on constant volume) disappears, as no work is performed.
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Miami welcomes the three kings of the Miami Heat as Lebron James, Dwyane Wade, and Chris Bosh join the NBA team in a spectacular show at ... youtube.com.
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Joe Baker, who now lives in Washington, has been a die-hard steelheader ever since. He spent 100 days on the streams last year, fishing and guiding others.
