Exam (elaborations)
EGEE 101- Exam 1Questions and Answers
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Exam of 10 pages for the course EGEE at EGEE (EGEE 101- Exam 1)
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EGEE 101- Exam 1Radiation - answer Radiative heat transfer is how the "warmth" of the sun is transferred
to you when you're out or how you warm your hands in front of the fire.
Convective - answer Convection is the heat transfer due to bulk movement of molecules
within fluids. -The water molecules are free to roam about as they are not fixed in
relative positions (unless frozen) like the metal atoms. So as a water molecule heats up,
it has more energy and travels further. The molecules moving around have lots of
collisions (think bumper cars), but this "molecular billiards" occurs without energy loss,
and so, imagine breaking at pool but the balls never stop; they will keep on colliding
until there is a distribution of energy. This distribution will be a certain shape with a
certain portion of the molecules having certain energy levels. As the water warms up,
this distribution will simply shift to the right into a higher energy level (temperature).
The Refrigerator: - answer The casing is simply an insulated box, so the heat cannot get
in (remember that heat flows from hot to cold, so it would not be correct to say insulation
keeps the cold in). At the back, there is a heat exchanger, a coil where the heat from
inside the refrigerator can be dissipated—yes, this means that in the summer your
refrigerator is pumping out heat which you are removing with the air conditioner (silly,
isn't it!) But as heat flows from hot to cold, how did we manage to get it to flow the other
way? We used a heat pump, to which we give energy so we can move heat; this is
achieved because of the properties of the refrigerant and the use of the compressor (the
noise that comes intermittently from the refrigerator is the compressor, or, perhaps the
ice maker). The refrigerant will be a liquid with a low boiling point. Thus, we can easily
convert refrigerant from the liquid phase into the gas phase.
INCANDESCENT - answerLight bulbs need electricity, which is used to heat a tungsten
filament. The thin metal strand (the filament) gets so hot it becomes white-hot,
producing a nice white light. The bulb itself is simply an envelope in which we can put
inert gases so the filament does not burn.
- gives off too much heat
Efficiency Measure - answerEfficiency Measure
FLUORESCENT - answerMuch of the energy required to "light" a fluorescent light is
needed as soon as the light switch is turned on. The electrodes at either end of the
lamp are the starting point for the electron to "flow" through the tube (from one electrode
to the other). As they pass through the tube they excite the mercury vapor inside the
tube; the excited mercury gives off ultraviolet radiation.
, -The lower rate of energy use (wattage) will however still produce as much light as an
incandescent light because of the efficiency increase over the incandescent lighting
(look at lumens levels to compare)
LIGHT EMITTING DIODE (LED) - answer-LED's are small light sources that become
illuminated by the movement of electrons through a semiconductor material
(link is external)
-Recently, they have become a lighting option for white light (often directional lighting for
task lighting: reading lamps, counter tops, etc.)
-they are increasingly common in general lighting and have a nicer color than compact
fluorescent lighting
-used in traffic lights, streetlights, xmas tree lights, etc.
-They use less electricity, the light is closer to incandescent in color, and they last
longer.
HALOGEN LIGHTS - answerThese are great lights for producing lots of heat and very
bright light.
-Halogen lights do have their uses, however - exterior security lighting, sporting event
lighting, headlights in your car (for those driving the nicer cars anyway).
COMPACT FLUORESCENT LIGHTING - answer- 3 types
-The incandescent light bulb produces light by heating a tungsten wire to white hot
temperatures.
-The advantage is that your electricity bill will be less because for the same production
of light (how many lumens), less electricity is used.
-The advantage is that your electricity bill will be less because for the same production
of light (how many lumens), less electricity is used.
- more expensive to purchase but compact fluorescent bulbs make a great deal of
economic and environmental sense (electricity comes mostly from aging fossil fuel
utilities that produce air pollution).
-not much heat
R values - answerWe use R values to measure the insulating property of the material
(the resistance of an area of material to heat flow over time). The units are:
h= hours, ft2 is area, F is degrees F, and Btu is energy (British thermal units). So, as
that is a mouthful, we just report the R value without the units.
-the higher the R-value the better the insulating ability.
-increase R value by adding more insulation
-So, it makes sense to add insulation when the energy cost is high and the insulation
costs are cheap
-Highest R values are required for the attic
GEOTHERMAL HEATING & COOLING - answer-Pipes need only go down about 8 feet
before the earth is at a relatively constant temperature. Pumping a fluid through the
pipes allows for heat exchange and hence heating and cooling.
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