The Transfer of Heat Chapter 13

Convection Heat energy is transferred by moving the fluid that contains the heat energy. convection currents

Natural Convection Density differences produce convection currents that move fluids from hotter places to cooler places because hotter, less dense fluids rise. Natural convection currents Gliders use “thermals” to gain altitude.

Conceptual example: baseboard heaters and refrigerators Rising heated air and sinking cooler air produce natural convection currents that transfer heat energy around a room or around in a refrigerator.

Forced Convection Hotter Fans and pumps move fluids from hotter places to cooler places. Cooler Radiator Hotter

Conduction Heat energy is transferred directly through a solid material by molecular collisions, but the solid material doesn’t move. Solid material Collisions transfer kinetic energy from the hotter, faster particles to the cooler, slower particles. Materials that conduct heat well are called thermal conductors. Materials that conduct heat poorly are called thermal insulators.

Conduction What determines the amount of heat transfer? more time → more heat transfer only a limited amount of heat is transferred each second more area → more heat transfer more area →more heat flow paths more temperature difference → more heat transfer larger temperature change per meter longer bar → less heat transfer smaller temperature change per meter kind of material metals transfer heat energy better than insulators

Conduction Q heat energy transferred k thermal conductivity of the material A area heat is passing through ΔT temperature difference t time L length of heat transfer path Thermal conductivity units:

Aluminum conducts heat 17 times more rapidly than stainless steel. (very low) (high) ($$$$) Aluminum conducts heat 17 times more rapidly than stainless steel.

Conduction Air has a low thermal conductivity. Materials with trapped air spaces are excellent thermal insulators. And convection currents are blocked from transferring heat energy.

Conduction Q is smaller when R is bigger.

Iced-up refrigerator Heat energy is transferred from the warmer air into the colder refrigerant fluid that flows inside the aluminum tubes. Ice has 100 times less thermal conductivity than aluminum so ice reduces conduction heat energy transfer. As the ice gets thicker, the conduction heat energy transfer rate gets smaller. Thermal conductivity Aluminum 240 J/(s m C°) Ice 2.2 J/(s m C°)

Radiation Radiate means move outward from a source. Energy is transferred by electromagnetic radiation such as visible light and infrared light. Black objects absorb and emit energy best. An ideal energy absorber and emitter are perfectly black and are called a perfect blackbody. The radiation they emit is called blackbody radiation. An object's emissivity e is the ratio that it absorbs or emits compared to a perfect blackbody. Emissivity is a dimensionless ratio.

Stefan-Boltzmann radiation law Q radiation heat energy transfer e emissivity σ Stefan-Boltzmann constant T absolute temperature A emitting or absorbing area t time Stefan-Boltzmann constant Since Q depends on T4, a surface that is twice as hot emits 16 times more energy each second compared to an equal size cooler surface.

Radiation How much energy do you radiate in one second? Assume A = 2.2 m2, T = 303 K, and e = 0.9 . How much energy do you absorb in one second? Assume A = 2.2 m2, T = 293 K, and e = 0.9 .

Minimizes conduction, convection, and radiation. Thermos bottle Minimizes conduction, convection, and radiation. No material between the walls no convection no conduction Silvered surfaces don't emit very well low radiation nothing between the two glass walls

Cooking with infrared heat A halogen stovetop radiates infrared electromagnetic energy. The ceramic stovetop is transparent to the infrared energy so the infrared energy passes through the ceramic top, but is absorbed by the bottom of the cooking pot.

The End