Download Chapter 11 1. While checking the temperature of an IC. chip the

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?