heat
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What is heat in physics?
In physics, heat is energy transferred from one body to another due to a temperature difference. This transfer increases the temperature of the colder body and decreases the temperature of the hotter body, though substances can absorb heat without raising their temperature by changing physical states.
Heat is a form of internal energy linked to the random motion of molecules or radiation. It's measured in units like joules, calories, or BTUs. Unlike thermal energy, heat refers specifically to energy in transit, not energy stored within a body. Thermodynamics studies the relationship between heat, work, temperature and energy, with heat corresponding to a definite amount of mechanical work. Heat transfer occurs through conduction, convection, and radiation.
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How is heat different from temperature?
Heat and temperature are related but distinct concepts in thermodynamics. Heat refers to the energy transferred between bodies due to temperature differences. It flows from hotter to colder objects, potentially changing their temperatures or causing phase transitions like melting or boiling.
Temperature, on the other hand, is a measure of the average kinetic energy of the molecules within a substance. It indicates the amount of thermal energy present in a body. When objects reach thermal equilibrium, they are said to be at the same temperature. It's incorrect to refer to the heat within a body, because heat describes energy in transit.
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How does heat move from one object to another?
Heat moves from one object to another through conduction, convection, and thermal radiation. Conduction transfers energy between adjacent molecules and occurs when heat moves through molecular or atomic agitation within a substance. Convection is the movement of heated fluids, such as air or water. Thermal radiation transmits energy as electromagnetic radiation from a heated surface to another.
The transfer of heat continues until thermal equilibrium is reached, meaning all parts of the system are at the same temperature.
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What are conduction, convection, and radiation in relation to heat transfer?
Heat transfer involves mechanisms that convey energy and entropy from one location to another, including conduction, convection, and thermal radiation.
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Conduction moves heat through molecular or atomic agitation and contact between particles. It occurs when heat moves along a temperature difference from an area of high temperature to an area of low temperature until thermal equilibrium is achieved.
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Convection transfers heat through the movement of a heated fluid like air or water. Natural convection occurs when heated fluids expand, become less dense, and rise, while forced convection involves fluid transport using methods like a fan or pump.
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Radiation emits energy as electromagnetic radiation from a heated surface, traveling to its absorption point without needing an intervening medium.
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How do different materials affect the transfer of heat?
Different materials affect heat transfer due to their thermal conductivity, or their ability to conduct heat. How quickly heat moves through a material depends on the material's atomic or molecular structure.
Materials with high thermal conductivity, such as metals, transfer heat more efficiently. Insulators, like fiberglass and rock wool, have low thermal conductivity and resist heat flow.
In solids, heat moves through particle agitation and contact. In liquids and gases, heat transfer occurs through particle agitation and the movement of molecules. Thermal conductivity is highest in solids and lowest in gases because gas particles are more spread out, resulting in fewer collisions to transfer energy. A material's resistance to heat flow is known as thermal resistance.
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Why is understanding heat important in everyday life and technology?
Understanding heat is crucial in everyday life and technology because it is a fundamental form of energy that affects various systems.
In daily life, controlling heat is important for cooking, heating and cooling homes, and maintaining body temperature. Technologies like geothermal power plants rely on heat to generate electricity, while high-temperature materials are developed to improve the efficiency of energy conversion in engines. The principles of thermodynamics and heat transfer are therefore essential for optimizing energy use and developing new technologies.
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heat, energy that is transferred from one body to another as the result of a difference in temperature. If two bodies at different temperatures are brought together, energy is transferredâi.e., heat flowsâfrom the hotter body to the colder. The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body. A substance may absorb heat without an increase in temperature by changing from one physical state (or phase) to another, as from a solid to a liquid (melting), from a solid to a vapour (sublimation), from a liquid to a vapour (boiling), or from one solid form to another (usually called a crystalline transition). The important distinction between heat and temperature (heat being a form of energy and temperature a measure of the amount of that energy present in a body) was clarified during the 18th and 19th centuries.
Heat as a form of energy
Because all of the many forms of energy, including heat, can be converted into work, amounts of energy are expressed in units of work, such as joules, foot-pounds, kilowatt-hours, or calories. Exact relationships exist between the amounts of heat added to or removed from a body and the magnitude of the effects on the state of the body. The two units of heat most commonly used are the calorie and the British thermal unit (BTU). The calorie (or gram-calorie) is the amount of energy required to raise the temperature of one gram of water from 14.5 to 15.5 °C; the BTU is the amount of energy required to raise the temperature of one pound of water from 63 to 64 °F. One BTU is approximately 252 calories. Both definitions specify that the temperature changes are to be measured at a constant pressure of one atmosphere, because the amounts of energy involved depend in part on pressure. The calorie used in measuring the energy content of foods is the large calorie, or kilogram-calorie, equal to 1,000 gram-calories.
In general, the amount of energy required to raise a unit mass of a substance through a specified temperature interval is called the heat capacity, or the specific heat, of that substance. The quantity of energy necessary to raise the temperature of a body one degree varies depending upon the restraints imposed. If heat is added to a gas confined at constant volume, the amount of heat needed to cause a one-degree temperature rise is less than if the heat is added to the same gas free to expand (as in a cylinder fitted with a movable piston) and so do work. In the first case, all the energy goes into raising the temperature of the gas, but in the second case, the energy not only contributes to the temperature increase of the gas but also provides the energy necessary for the work done by the gas on the piston. Consequently, the specific heat of a substance depends on these conditions. The most commonly determined specific heats are the specific heat at constant volume and the specific heat at constant pressure. The heat capacities of many solid elements were shown to be closely related to their atomic weights by the French scientists Pierre-Louis Dulong and Alexis-ThérÚse Petit in 1819. The so-called law of Dulong and Petit was useful in determining the atomic weights of certain metallic elements, but there are many exceptions to it; the deviations were later found to be explainable on the basis of quantum mechanics.
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It is incorrect to speak of the heat in a body, because heat is restricted to energy being transferred. Energy stored in a body is not heat (nor is it work, as work is also energy in transit). It is customary, however, to speak of sensible and latent heat. The latent heat, also called the heat of vaporization, is the amount of energy necessary to change a liquid to a vapour at constant temperature and pressure. The energy required to melt a solid to a liquid is called the heat of fusion, and the heat of sublimation is the energy necessary to change a solid directly to a vapour, these changes also taking place under conditions of constant temperature and pressure.
Air is a mixture of gases and water vapour, and it is possible for the water present in the air to change phase; i.e., it may become liquid (rain) or solid (snow). To distinguish between the energy associated with the phase change (the latent heat) and the energy required for a temperature change, the concept of sensible heat was introduced. In a mixture of water vapour and air, the sensible heat is the energy necessary to produce a particular temperature change excluding any energy required for a phase change.
