C H A P T E R 13 The Transfer of Heat

C H A P T E R 13 The Transfer of Heat

The Transfer of Heat Q: Name three methods of heat transfer?

The Transfer of Heat Q: Name three methods of heat transfer? A: Convection, Conduction, and Radiation.

13.1 Convection Convection is the process in which heat is carried from place to place by the bulk movement of a fluid.

Boiling Water Convection currents are set up when a pan of water is heated.

During a volcanic eruption, smoke at the top of the plume rises thousands of meters because of convection.

Q: In the living room, the heating unit is placed in the floor but the the refrigerator has a top-mounted cooling coil. Why?

Q: In the living room, the heating unit is placed in the floor but the the refrigerator has a top-mounted cooling coil. Why? A: Air warmed by the baseboard heating unit is pushed to the top of the room by the cooler and denser air. Air cooled by the cooling coil sinks to the bottom of the refrigerator.

13.2 Conduction Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer. Those materials that conduct heat well are called thermal conductors, while those that conduct heat poorly are known as thermal insulators. Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal insulators. The free electrons in metals are responsible for the excellent thermal conductivity of metals.

Conduction Of Heat Through A Material

Rate of heat transfer by conduction, Q/t through the length, L across the cross-sectional area, A is given by the following equation, where k is the thermal conductivity and ΔT is the temperature difference between the two ends. Q kA T . t L SI Unit of Thermal Conductivity: J/(s · m · C )

TABLE 13.1 Thermal Conductivitiesaa of Selected Materials Substance Metals Aluminum Brass Copper Iron Lead Silver Steel (stainless) Gases Air Hydrogen (H2) Thermal Conductivity, k [J/(s · m · C )] 240 110 390 79 35 420 14 0.0256 0.180

Styrofoam

Fruit growers sometimes spray water to protect their crops against freezing. After a subzero night, these berries are visible in their insulating jackets of ice.

13.3 Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. Heat transfer by radiation can take place through vacuum. This is because electromagnetic waves are involved in radiation and they can propagate through empty space.

Suntans Suntans are produced by radiation, ultraviolet rays.

Black Body

Black Body A material that is a good absorber, like lampblack, is also a good emitter, and a material that is a poor absorber, like polished silver, is also a poor emitter.

Summer Clothing Q: People are uncomfortable wearing dark clothes during the summer. Why?

Summer Clothing Q: People are uncomfortable wearing dark clothes during the summer. Why? A: Dark clothes absorb a large fraction of the sun's radiation and then reemit it in all directions. About one-half of the emitted radiation is directed inward toward the body and creates the sensation of warmth. Light-colored clothes, in contrast, are cooler to wear, since they absorb and reemit relatively little of the incident radiation.

A White sifaka Lemur To warm up in the morning, they turn their dark bellies toward the sun.

The Stefan–BOLTZMANN Law Of Radiation The rate at which an object emits radiant energy is proportional to the fourth power of its absolute temperature. This is known as Stefan’s law and is expressed as follows, where σ is the Stefan-Boltzmann constant, σ 5.67 10-8 W/m2.K4. Q 4 P eAT . t The factor e is called the emissivity, which is a number between 0 and 1. Perfect radiators have a value of 1 for e. A is the surface area and T is the temperature of the radiator in Kelvin.

The net power is the power the object emits minus the power it absorbs. Applying the Stefan-Boltzmann law as in Example 6 leads to the following expression for Pnet when the temperature of the object is T and the temperature of the environment is T0:

13.4 Applications 1. Rating thermal insulation by R values. 2. Regulating the temperature of an orbiting satellite. 3. A thermos bottle. 4. A halogen cooktop stove.

R Value The term L/k in the denominator is called the R value of the insulation. In the US R-values are expressed in British units, ft2.h.F0/Btu. For example, the 6.0 inch thick fiber-glass batting has an Rvalue of 19 ft2.h.F0/Btu.

Owens Corning

Owens Corning Rafters and Collar Beams Recommended Insulation Products: R-21 Fiber Glass Insulation 5 1/2" Thick R-19 Fiber Glass Insulation 6 1/4" Thick R-38C 2 x 12 Cathedral Ceiling Fiber Glass Insulation 10 1/4" Thick R-30C 2 x 10 Cathedral Ceiling Fiber Glass Insulation 8 1/4" Thick

Highly reflective metal foil covering this satellite minimizes heat transfer by radiation.

Thermos Bottle A thermos bottle minimizes energy transfer due to convection, conduction, and radiation. Stopper- minimize conduction. Double-walled glass vessel with the space between the walls is evacuated to minimize energy losses due to conduction and convection. The silvered surfaces reflect most of the radiant energy that would otherwise enter or leave the liquid in the thermos.

Halogen Cooktop In a halogen cooktop, quartz-iodine lamps emit a large amount of electromagnetic energy that is absorbed directly by a pot or pan.