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Transcript
Chapter 11 1. While checking the temperature of an IC. chip the thermometer reads 500C. What is the temperature in 0F? 2. If the aluminum case of a power supply reaches 6600C it will begin to melt. What is this temperature in 0F? 3. A 25W soldering iron is on for 100 seconds. How many BTU’s of heat did the iron generate? 4. A power transistor is mounted to a heat sink. If the transistor mounting is 0.0005m thick, has an area of 0.0004m2, a thermal conductivity of 300J/(sm0C), is on for 500s, and the temperature of the sink is 250c and the temperature on the transistor is 520C, what is the heat flow into the sink? 5. The insulation in an attic is 1ft thick and has a thermal conductivity of 0.023BTU/(ft0Fh). What is the R-value? 6. An oven is wrapped in an insulating blanket with an R-value of 33 ft2 0 Fh/BTU. The total surface area of the oven is 6ft2, the oven is on for 3h with an inside temperature of 4500F sitting in a room of temperature 720F. What is the heat flow out of the oven in BTU? 7. A power transistor bank is mounted on a 1kg aluminum heat sink. The transistors dissapates 500W and all this heat flows into the sink. What is the change in temperature of the heat sink after 10s in Celcius? ( Hint Q= Pxt) 8. How much heat is needed to melt 1.0kg of ice at 00C? 9. The power capability of an IC chip lowers with rising ambient temperature. Suppose the power a chip can dissapate is the derated by 0.57W/0C above 250C. Therefore the formula for the power capability of the chip becomes Power = Rated Power – 0.57x (Temperature – 250c) for temperatures above 250C. If a 100W chip is running at 1000C with a derate 0f 0.57W/0c, what is the maximum power the chip can handle? 10. A 10 ohm resistor connected to a 12V battery is used as a heater in a 10kg pot of water. If the heater runs for 250 seconds, what is the change in temperature of the water in Celcius? 11. Water boils at 2120F. What is this temperature in Celsius? 12. How many watts of heat does a 5 ohm resistor generate when 0.5 A is flowing through it? 13. Calculate the heat flow in one hour through a 0.5in diameter aluminum rod 1.5ft in length with a thermal conductivity of 140 BTU/(ft0Fh) and a temperature difference across the ends of 1000F? 14.Heat sinks readily dissipate heat out of an IC. chip because they have large surface areas. If a heat sink has 5 fins measuring 5.0in x 1.0in, what is the total surface area? (Remember each fin has two sides). 15.A computer is cooled using a fan and having vent hole in the case. Heat also escapes out the sides of the metal case through conduction. If the case is 0.0625in thick and one side panel measures 18in x 24in, and the thermal conductivity is 120 BTU/(ft0Fh) and the temperature outside the computer is 720F and inside is 950F, and the computer runs for 8h, how much heat flowed through that one side? 16. Two houses are insulated with insulation having R-values of 30 and 20. How many times more does the house having the R-20 insulation loose heat versus the house with the R-30? 17. A pot of 1kg of water is being heated on the stove. If the water is to boil the temperature must rise from 220C to 1000C. How much heat in Joules must be added to the water? 18. If the stove in the previous problem is electric and the burner creates 2000W of heat, how long will it take for the water to boil? (Assume all the heat from the burner goes into the water). 19. How mush heat is needed to melt a 0.05kg ice cube at 0 0C? 20. How mush heat is needed to turn 0.05kg water into steam at 100 0C?