Vocabulary 3 Kinds of Energy Internal Energy Chapter 11 Energy in Thermal Processes U Energy of microscopic motion and intermolucular forces Work W F x P V is work done by compression next chapter Heat Q Energy transfer from microscopic contact U Q P V next chapter Temperature and Specific Heat Add energy T rises Q mc T Example 11 1 Bobby Joe drinks a 130 calorie can of soda If the efficiency for turning energy into work is 20 how many 4 meter floors must Bobby Joe ascend in order to work off the soda and maintain her 55 kg mass Mass Property of material cH20 1 0 cal g C 1 calorie 4 186 J Example 11 2 Aluminum has a specific heat of 0924 cal g C If 110 g of hot water at 90 C is added to an aluminum cup of mass 50 g which is originally at a temperature of 23 C what is the final temperature of the equilibrated water cup combo T 87 3 C Nfloors 50 4 Phase Changes and Latent Heat T does not rise when phases change at constant P Examples solid liquid fusion liquid vapor vaporization Latent heat energy required to change phases Q mL Property of substance transition Example 11 3 Example 11 4 1 0 liters of water is heated from 12 C to 100 C then boiled away a How much energy is required to bring the water to boiling b How much extra energy is required to vaporize the water c If electricity costs 75 per MW hr what was the cost of heating and boiling the water a Q 8 8x104 cal 3 68x105 J Consider Bobby Joe from the previous example If the 80 of the 130 kcals from her soda went into heat which was taken from her body from radiation how much water was perspired to maintain her normal body temperature Assume a latent heat of vaporization of 540 cal g even though T 37 C 193 g b Q 5 4x105 cal 2 26x106 J c 5 5 A can of soda has 325 g of H20 Some fluid drips away Conduction Three Kinds of Heat Transfer Conduction Shake your neighbor pass it down Examples Heating a skillet losing heat Power depends on area A thickness x temperature difference T and conductivity of material through the walls Convection Move hot region to a different location Examples Hot water heating for buildings Circulating air Unstable atmospheres Radiation Light is emitted from hot object Examples Stars Incandescent bulbs P kA T x Conductivity is property of material Example 11 5 A copper pot of radius 12 cm and thickness 5 mm sits on a burner and boils water The temperature of the burner is 115 C while the temperature of the inside of the pot is 100 C What mass of water is boiled away every minute DATA kCu 397 W m C m 1 43 kg Conductivities and R values Conductivity k Property of Material SI units are W m C T T P kA A x R R x k R Value Property of material and thickness x Measures resistance to heat Useful for comparing insulation products Quoted values are in AWFUL units Conducitivities and R values What makes a good heat conductor ARGH Free electrons metals Easy transport of sound lattice vibrations Stiff is good Low Density is good Pure crystal structure Diamond is perfect Example 11 6a Example 11 6b An large pipe carries steam at 224 C across a large industrial plant The outside of the pipe is at room temperature 24 C The pipe is 120 m long and has a diameter of 70 cm The pipe is constructed of an insulating material of conductivity k 2 62 W m C In order to reduce the rate of heat loss through the pipe by a factor of 1 2 an engineer could An large pipe carries steam at 224 C across a large industrial plant The outside of the pipe is at room temperature 24 C The pipe is 120 m long and has a diameter of 70 cm The pipe is constructed of an insulating material of conductivity k 2 62 W m C In order to reduce the rate of heat loss through the pipe by a factor of 1 2 an engineer could a b c d e a Make the pipe using a new material with twice the conductivity 5 24 W m C b Re design the pipe to double the R value c All of the above d None of the above Reduce the length of the pipe by a factor of 1 2 Reduce the diameter of the pipe by a factor of 1 2 Increase the thickness of the pipe by a factor of 2 All of the above None of the above Example 11 6c An large pipe carries steam at 224 C across a large industrial plant The outside of the pipe is at room temperature 24 C The pipe is 120 m long and has a diameter of 70 cm The pipe is constructed of an insulating material of conductivity k 2 62 W m C In order to reduce the rate of heat loss through the pipe by a factor of 1 2 an engineer could a Reduce the density of steam in the pipe by a factor of 1 2 b Reduce the temperature of the steam to 124 C c Reduce the velocity of the steam through the pipe by a factor of 1 2 d All of the above e None of the above R values for layers Consider a layered system e g glass air glass P A T R T T1 T2 T3 PR PR PR 1 2 3 A A A P R1 R2 R3 A R R1 R2 R3 Example 11 7 Consider three panes of glass each of thickness 5 mm The panes trap two 2 5 cm layers of air in a large glass door How much power leaks through a 2 0 m2 glass door if the temperature outside is 40 C and the temperature inside is 20 C DATA kglass 0 84 W m C kair 0 0234 W m C Convection If warm air blows across the room it is convection If there is no wind it is conduction Can be instigated by turbulence or instabilities P 55 8 W Why are windows triple paned Transfer of heat by radiation All objects emit light if T 0 Colder objects emit longer wavelengths red or infra red Hotter objects emit shorter wavelengths blue or ultraviolet To stop convection Stefan s Law give power of emitted radiation 4 P e AT Emissivity 0 e 1 usually near 1 Example 11 8 5 6696x10 8 W m2 K4 is the Stefan Boltzmann constant Example 11 9 If the temperature of the Sun fell 5 and the radius shrank 10 what would be the percentage change of the Sun s power output 34 DATA The sun radiates 3 74x1026 W Distance from Sun to Earth 1 5x1011 m Radius of Earth 6 36x106 m a …
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