Heat Transfer and Energy Nick Bassill February 18 th 2009

Temperature Hotter objects are more energetic Consider a gas... the warmer the gas, the faster the molecules in that gas move Using the definition of Kinetic Energy – KE=(1/2)*mass*velocity 2 We can see that faster molecules are more energetic

Heat 1 st Law of Thermodynamics - Heat can be thought of as a form of energy (transfer) 2 nd Law of Thermodynamics – Heat always goes from hotter objects to colder objects The amount of heat lost by the warm object is equivalent to the heat gained by the second object From:

More Heat When you touch an object, heat is transferred This is why certain objects feel warm and others feel cold Note: although heat is similar to temperature, it is technically incorrect to say that we feel another object’s temperature Rather, we observe the transfer of heat from ourselves to other objects

Specific Heat This is a measure of the heat energy needed to heat 1 g of an object 1º C Different objects have different specific heat values Examples: SubstanceValue (Jg −1 K −1) Liquid Water4.183 Ice2.050 Wood0.420 Sand0.835 Air1.012

More Specific Heat Therefore it takes more energy to warm water by 1º C than it does to warm air by 1º C This is why the water temperature in a swimming pool stays fairly constant during the day, while the air might change temperature much more

Mechanisms of Heat Transfer (1)Conduction (2)Convection (3)Advection (4)Radiation

Conduction Conduction is the transfer of heat through an object molecule by molecule For example, if you were to hold a metal poker in a fire, eventually the handle will warm as well, primarily due to conduction

Heat Conductivity This is a measure of the effectiveness of different materials at conducting heat Examples: MaterialValue (W/mK) Air0.023 Water0.6 Ice2.1 Iron80

Heat Conduction Continued This is why some materials “feel” colder than others, even if they are at the same temperature For example, 40º F water “feels” colder than 40º F air, even though they are the same temperature This is because water is more effective at conducting heat (i.e. more effective at taking heat from your body)

Convection Convection is heat transfer by the movement of fluids (gases and liquids) in the vertical (along gravity) As shown in the ideal gas law, warm air is less dense than cold air This means warm air tends to rise, and cold air tends to sink

Convection Continued As air in contact with the ground gets heated during the day (through conduction), it will rise (convection) However, because the pressure decreases with altitude, as the air rises, it also cools If it becomes colder than the surrounding air, it will sink again This process is called convection

Convection Continued As dry air rises, it cools at a rate of 9.8º C per km Moist (saturated) air will cool at a slower rate due to the conversion of water vapor to liquid water (more on this later in the semester) Convection is the process that produces “fair weather” cumulus clouds and thunderstorms

Advection Advection is the transfer of heat through the horizontal movement of a fluid (gases or liquids) For example, a southerly wind might “advect” warmer air from the south to Madison

“Cold Air Advection” “Warm Air Advection”

Radiation Unlike Conduction, Convection, and Advection, Radiation does not need a medium to travel through (i.e. it can travel through empty space) Radiation carries energy in the form of electromagnetic waves This is how the Earth receives energy from the sun

Radiation Continued All objects emit and receive radiation However, the emitted wavelength of the radiation depends on the temperature of the object Hotter objects emit radiation with a shorter wavelength Shorter wavelengths are more energetic As objects emit radiation, they cool (i.e. lose energy), unless they are being warmed by some other process

Radiation Continued Radiation can be – (1)Emitted (such as from the sun) (2)Absorbed (like sunlight on your face) (3)Reflected (visible light off a mirror) (4)Scattered (visible light as seen in a rainbow)

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