EGEE 101- Exam 1 (with Verified Solutions)

Radiaton correct answers 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 correct answers 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 : correct answers 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 correct answers Light 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 correct answers Efficiency Measure FLUORESCENT correct answers Much 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) correct answers -LED's are small light sources that become illuminated by the movement of electrons through a semiconductor material (link is external)