{"id":20019,"date":"2018-10-14T05:30:06","date_gmt":"2018-10-14T05:30:06","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=20019"},"modified":"2023-02-10T12:11:40","modified_gmt":"2023-02-10T12:11:40","slug":"heat-in-physics-definition-of-heat","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/heat-transfer\/introduction-to-heat-transfer\/heat-in-physics-definition-of-heat\/","title":{"rendered":"Heat in Physics – Definition of Heat"},"content":{"rendered":"
<\/div>\n

Definition of Heat<\/h2>\n

\"zeroth-law-of-thermodynamics-heat\"<\/a>While internal energy <\/strong><\/a>refers to the total energy of all the molecules within the object, heat<\/strong> is the amount of energy flowing<\/strong> spontaneously from one body to another due to their temperature difference. Heat<\/strong> is a form of energy, but it is energy in transit<\/strong>. Heat is not a property of a system. However, the transfer of energy as heat occurs at the molecular level due to a temperature difference<\/strong>.<\/p>\n

Consider a block of metal<\/strong> at high temperature that consists of atoms oscillating intensely around their average positions. At low temperatures<\/strong>, the atoms continue to oscillate but with less intensity<\/strong>. If a hotter block of metal is put in contact with a cooler block, the intensely oscillating atoms at the edge of the hotter block give off their kinetic energy to the less oscillating atoms at the edge of the cool block. In this case, there is energy transfer<\/strong> between these two blocks, and heat flows<\/strong> from the hotter to the cooler block by these random vibrations.<\/p><\/div><\/div>

\u00a0
<\/span>Thermal Energy in Microscopic Scale<\/div>
Internal energy<\/strong>\u00a0involves energy on the\u00a0microscopic scale<\/strong>. It may be divided into microscopic potential energy,\u00a0U<\/em>pot<\/sub>, and microscopic kinetic energy,\u00a0U<\/em>kin<\/sub>, components:\n

U = Upot<\/sub>\u00a0+ Ukin<\/sub><\/strong><\/p>\n

\"Microscopic<\/a>where the microscopic kinetic energy, Ukin<\/sub>, involves the\u00a0motions<\/strong> of all the system\u2019s particles for the center-of-mass frame. For an ideal\u00a0monatomic gas<\/strong>, this is just the\u00a0translational kinetic energy<\/strong> of the linear motion of the atoms. Monoatomic particles do not rotate or vibrate. The behavior of the system is well described by the kinetic theory of gases. Kinetic theory is based on the fact that during an\u00a0elastic collision<\/a> between a molecule with high kinetic energy and one with low kinetic energy, part of the energy will transfer to the molecule of lower kinetic energy. However, there is\u00a0rotational<\/strong>\u00a0and\u00a0vibrational kinetic energy<\/strong> for\u00a0polyatomic gases<\/strong>.<\/p>\n

The microscopic potential energy,\u00a0Upot<\/sub><\/strong>, involves the\u00a0chemical bonds<\/strong> between the atoms that make up the molecules, binding forces in the nucleus, and the physical force fields within the system (e.g., electric or magnetic fields).<\/p>\n

There is a significant component of potential energy associated with the intermolecular attractive forces<\/strong> in liquids and solids.<\/p><\/div><\/div><\/div>

<\/div>In general, when two objects are brought into thermal contact<\/strong>, heat will flow<\/strong> between them until<\/strong> they come into equilibrium<\/strong> with each other. \u00a0When a temperature difference<\/strong> does exist, heat flows spontaneously from the warmer system to the colder system<\/strong>. Heat transfer occurs by conduction<\/strong> or by thermal radiation<\/strong>. When the flow of heat stops<\/strong>, they are said to be at the same temperature<\/strong>. They are then said to be in thermal equilibrium<\/strong><\/a>.\n

As with work, the amount of heat transferred depends upon the path<\/strong> and not simply on the initial and final conditions of the system. There are many ways to take the gas from state i to state f.<\/p>\n

Also, as with work, it is important to distinguish between heat added<\/strong> to a system from its surroundings and heat removed<\/strong> from a system to its surroundings. Q is positive for heat added to the system, so Q is negative if heat leaves the system. Because W in the equation is the work done by the system, then if work is done on the system, W will be negative, and Eint <\/sub>will increase.<\/p>\n

The symbol q<\/strong> is sometimes used to indicate the heat added to or removed from a system per unit mass<\/strong>. It equals the total heat (Q) added or removed divided by the mass (m).<\/p><\/div><\/div>

<\/div>\n

Distinguishing Temperature, Heat, and Internal Energy<\/h2>\n

Using the kinetic theory, a clear distinction between these three properties can be made.<\/p>\n