What is heat?
Heat is the form of energy that is transferred between two substances at different temperatures.
Introduction of heat
Heat is the transfer of kinetic
energy from one medium or object to another, or from an energy source to a
medium or object. Such energy transfer can occur in three ways: radiation,
conduction, and convection.
The standard unit of heat in the International System of Units (SI)
is the calorie (cal), which is the amount of energy
transfer required to raise the temperature of one gram of pure liquid water by
one degree Celsius, provided the water temperature is higher than the freezing
point and lower than the boiling point. Sometimes the kilocalorie (kcal) is
specified as a unit of heat; 1 kcal = 1000 cal. (This is the so-called diet
calorie.) Less often, the British thermal unit (Btu) is used. This is the amount
of heat required to raise the temperature of one pound of pure liquid water by
one degree Fahrenheit.
An example of heat by radiation is the effect of infrared (IR) energy
as it strikes a surface. IR is an electromagnetic
field capable of transfering
energy from a source, such as a fireplace, to a destination, such as the surfaces
within a room. Radiation does not require an intervening medium; it can occur
through a vacuum. It is responsible for the warming of the Earth by the sun.
Heat by conduction takes place when two material media or objects
are in direct contact, and the temperature of one is higher than the
temperature of the other. The temperatures tend to equalize; thus the heat
conduction consists of a transfer of kinetic energy from the warmer medium to
the cooler one. An example is the immersion of a chilled human body in a hot
bath.
Heat by convection occurs when the motion of a liquid or gas
carries energy from a warmer region to a cooler region. A good example of
convection is the tendency of warm air to rise and cool air to fall, equalizing
the air temperature inside a room containing a hot stove. Heat convection
(along with conduction) is believed to take place inside the Earth, transfering
kinetic energy from the inner core through the outer core and mantle to the
crust. In this situation, the outer core and the mantle behave like liquids
over long periods of time.
How does heat travel?
One thing you've probably noticed about heat is that it doesn't generally stay where you put it. Hot things get colder, cold things get hotter, and—given enough time—most things eventually end up the same temperature. How come?
There's a basic law of physics called the second law of thermodynamics and it says, essentially, that cups of coffee always go cold and ice creams always melt: heat flows from hot things toward cold ones and never the other way around. You never see coffee boiling all by itself or ice creams getting colder on sunny days! The second law of thermodynamics is also responsible for the painful fuel bills that drop through your letterbox several times a year. In short: the hotter you make your home and the colder it is outside, the more heat you're going to lose. To reduce that problem, you need to understand the three different ways in which heat can travel: called conduction, convection, and radiation. Sometimes you'll see these referred to as three forms of heat transfer.
Conduction
Photo: Conduction carries heat from the furnace in the center of this picture into all the pieces of metal that are touching it—making them red hot too. Photo by L.W. Wisenburg taken at Rock Island Arsenal courtesy of Defense Imagery.
Conduction is how heat flows between two solid objects that are at different temperatures and touching one another (or between two parts of the same solid object if they're at different temperatures). Walk on a stone floor in your bare feet and it feels cold because heat flows rapidly out of your body into the floor by conduction. Stir a saucepan of soup with a metal spoon and you'll soon have to find a wooden one instead: heat travels rapidly along the spoon by conduction from the hot soup into your fingers.Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water.
Convection
Convection is the main way heat flows through liquids and gases. Put a pan of cold, liquid soup on your stove and switch on the heat. The soup in the bottom of the pan, closest to the heat, warms up quickly and becomes less dense (lighter) than the cold soup above. The warmer soup rises upward and colder soup up above it falls down to take its place. Pretty soon you've got a circulation of heat running through the pan, a bit like an invisible heat conveyor, with warming, rising soup and cooling, falling soup. Gradually, the whole pan heats up. Convection is also one of the ways our homes heat up when we turn on the heating. Air warms up above the heaters and rises into the air, pushing cold air down from the ceiling. Before long, there's a circulation going on that gradually warms up the entire room.In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.Radiation
Picture: Infrared thermal images (sometimes called thermographs or thermograms) show that all objects give off some heat energy by radiation. In these two photos, you can see a rocket on a launch pad photographed with a normal camera (above) and an infrared thermal camera (below). The coldest parts are purple, blue, and black; the hottest areas are red, yellow, and white. Photo by R. Hurt, NASA/JPL-Caltech, courtesy of NASA.
Radiation is the third major way in which heat travels. Conduction carries heat through solids; convection carries heat through liquids and gases; but radiation can carry heat through empty space—even through a vacuum. We know that much simply because we're alive: almost everything we do on Earth is powered by solar radiation beamed toward our planet from the Sun through the howling empty darkness of space. But there's plenty of heat radiation on Earth too. Sit near a crackling log fire and you'll feel heat radiating outward and burning your cheeks. You're not in contact with the fire, so the heat's not coming to you by conduction and, if you're outside, convection probably isn't carrying much toward you either. Instead, all the heat you feel travels by radiation—in straight lines, at the speed of light—carried by a type ofelectromagnetism called infrared radiation.Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required.
Objects
emit radiation when high energy electrons in a higher atomic level fall down to
lower energy levels. The energy lost is emitted as light or electromagnetic
radiation. Energy that is absorbed by an atom causes its electrons to
"jump" up to higher energy levels. All objects absorb and emit
radiation. ( Here
is a java applet showing how an atom absorbs and emits radiation) When the
absorption of energy balances the emission of energy, the temperature of an
object stays constant. If the absorption of energy is greater than the emission
of energy, the temperature of an object rises. If the absorption of energy is
less than the emission of energy, the temperature of an object falls.
heat
Measures of Heat
Temperature is a measure of the average translational kinetic energy of the molecules of a system. Heat is commonly expressed in either of two units: the calorie, an older metric unit, and the British thermal unit (Btu), an English unit commonly used in the United States. Scientists express heat in terms of the joule, a unit used for all forms of energy.
Specific Heat
As heat is added to a substance in the solid state, the molecules of the substance gain kinetic energy and the temperature of the substance rises. The amount of heat needed to raise a unit of mass of the substance one degree of temperature is called the specific heat of the substance. Because of the way in which the calorie and the Btu are defined, the specific heat of any substance is the same in either system of measurement. For example, the specific heat of water is 1 calorie per gram per degree Celsius; i.e., 1 calorie of heat is needed to raise the temperature of 1 gram of water by 1 degree Celsius; it is also 1 Btu per pound per degree Fahrenheit.
When a solid reaches a certain temperature, it changes to a liquid. This temperature is a particular property of the substance and is called its melting point. While the solid-liquid transition is taking place, there is no change in temperature. All of the heat being added is being converted to the internal potential energy associated with the liquid state. The amount of heat needed to convert one unit of mass of a substance from a solid to liquid is called the heat of fusion, or latent heat of fusion, of the substance. Like specific heat, latent heat is also a property of the particular substance. The latent heat of fusion for the ice-to-water transition is 80 calories per gram.
After a substance is completely changed from a solid to a liquid, further addition of heat again causes the temperature to rise until it reaches the boiling point, the particular temperature at which the given substance changes from a liquid to a gas. During the liquid-gas transition, the temperature remains constant until the change is completed. The heat of vaporization, or latent heat of vaporization, is the heat that must be added to convert one unit of mass of the substance from a liquid to a gas.
Heat Transfer Rate formula:
V = K ΔT/d
where:
V: Heat Transfer Rate, in Calorie/cm^2.s
K : Thermal Conductivity Constant, Calorie/cm.C.s, Check followed table for common material Thermal Conductivity Constant
ΔT: Temperature Difference, in C or K
d: Distance, in meter
V = K ΔT/d
where:
V: Heat Transfer Rate, in Calorie/cm^2.s
K : Thermal Conductivity Constant, Calorie/cm.C.s, Check followed table for common material Thermal Conductivity Constant
ΔT: Temperature Difference, in C or K
d: Distance, in meter
Thermal Conductivity Constant Table of Common Materials (Click to add)
No comments:
Post a Comment