How is heat conducted through a solid

Black is the most effective, and white the least. People living in hot climates generally avoid wearing black clothing, for instance. Similarly, black asphalt in a parking lot will be hotter than the adjacent gray sidewalk on a summer day, because black absorbs better than gray.

The reverse is also true—black radiates better than gray. Thus, on a clear summer night the asphalt will be colder than the gray sidewalk because black radiates energy more rapidly than gray. An ideal radiator, often called a blackbody, is the same color as an ideal absorber, and captures all the radiation that falls on it.

In contrast, white is a poor absorber and also a poor radiator. A white object reflects all radiation, like a mirror. A perfect, polished white surface is mirror-like in appearance, and a crushed mirror looks white. There is a clever relation between the temperature of an ideal radiator and the wavelength at which it emits the most radiation.

Gray objects have a uniform ability to absorb all parts of the electromagnetic spectrum. Colored objects behave in similar but more complex ways, which gives them a particular color in the visible range and may make them special in other ranges of the nonvisible spectrum. Take, for example, the strong absorption of infrared radiation by the skin, which allows us to be very sensitive to it.

Good and Poor Radiators : A black object is a good absorber and a good radiator, while a white or silver object is a poor absorber and a poor radiator. The rate of heat transfer by emitted radiation is determined by the Stefan-Boltzmann law of radiation:.

The symbol e stands for the emissivity of the object, which is a measure of how well it radiates. Real objects fall between these two values. The radiation rate is directly proportional to the fourth power of the absolute temperature—a remarkably strong temperature dependence. Furthermore, the radiated heat is proportional to the surface area of the object. If you knock apart the coals of a fire, there is a noticeable increase in radiation due to an increase in radiating surface area.

The net rate of heat transfer by radiation absorption minus emission is related to both the temperature of the object and that of its surroundings. In other words, it does not matter whether the surroundings are white, gray, or black; the balance of radiation into and out of the object depends on how well it emits and absorbs radiation. Privacy Policy. Skip to main content. Heat and Heat Transfer. Search for:. Methods of Heat Transfer. Conduction is the transfer of heat through physical contact.

Learning Objectives Assess why particular characteristics are necessary for effective conduction. Conduction is the most significant form of heat transfer within a solid object or between solids in thermal contact. Conduction is most significant in solids, and less though in liquids and gases, due to the space between molecules. The rate of heat transfer by conduction is dependent on the temperature difference, the size of the area in contact, the thickness of the material, and the thermal properties of the material s in contact.

Learning Objectives Illustrate the mechanisms of convection with phase change. Solids cannot transport heat through convection. Convection can transport heat much more efficiently than conduction. Air is a poor conductor and a good insulator if the space is small enough to prevent convection. Convection often accompanies phase changes, such as when sweat evaporates from your body. Solution: 1 Determine the mass of air from its density and the given volume of the house.

This rate of heat transfer is equal to the power consumed by about forty-six W light bulbs. Convection Cells : Convection cells in a gravity field. Learning Objectives Explain how the energy of electromagnetic radiation corresponds with wavelength. All objects emit and absorb electromagnetic energy.

The color of an object is related emissivity, or its efficiency of radiating away energy. An ideal radiator, often called a blackbody, is the same color as an ideal absorber and captures all the radiation that falls on it. The net rate of heat transfer is related to the temperature of the object and the temperature of its surroundings.

The larger the difference, the higher the net heat flux. The temperature of an object is very significant, because the radiation emitted is proportional to this quantity to the fourth power. KEY TERMS blackbody : A theoretical body, approximated by a hole in a hollow black sphere, that absorbs all incident electromagnetic radiation and reflects none; it has a characteristic emission spectrum. Licenses and Attributions. As the pan gets hot, some of that heat transfers to the molecules of water sitting on the bottom of the pan via conduction.

That speeds up the motion of those water molecules — they are warming. As the water warms, it now begins to expand. That makes it less dense. It rises above denser water, carrying away heat from the bottom of the pan. Cooler water flows down to take its place next to the hot bottom of the pan. As this water warms, it expands and rises, ferrying its newly-gained energy with it.

In short order, a circular flow of rising warm water and falling cooler water sets up. This circular pattern of heat transfer is known as convection.

Large birds such as frigate birds and human flyers riding engineless gliders often ride these thermals — rising blobs of air — to gain altitude without using any energy of their own.

In the ocean, convection caused by heating and cooling helps to drive ocean currents. These currents move water around the globe. The third type of energy transfer is in some ways the most unusual.

It can move through materials — or in the absence of them. This is radiation. Consider visible light, a form of radiation. It passes through some types of glass and plastic. X-rays, another form of radiation, readily pass through flesh but are largely blocked by bone.

The atoms of a solid are held together by chemical bonds. The atoms are fixed in place but are free to vibrate. When part of a solid absorbs heat energy the atoms vibrate faster and with bigger amplitude.

These vibrations pass from atom to atom transferring heat energy as they do so. This process happens in all solids when heated but is a slow process.

Conduction in metals. Some of the electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions.

When the free electrons absorb heat energy, they move much faster.