Chapter 14 Physics
CQ: How is heat transfer related to temperature?
It's proportional as indicated by equations 𝑞 = 𝑚𝑠∆𝑇 and 𝑞 = 𝐶∆𝑇.
What are the units for a Joule?
Nxm
What is the rate of heat transfer formula?
Q/t
What is the Stefan-Boltzmann constant?
theta = 5.67x10^-8 J/(sxm^2xK^4).
CQ: On cold, clear nights horses will sleep under the cover of large trees. How does this help them keep warm?
The trees act as an insulator allowing the horses body heat to only rise so far and to remain in the vicinity.
What are the three methods in which heat transfer takes place?
-Conduction - heat transfer via contact of two substances -Convection - heat transfer by the macroscopic movement of a fluid. -Radiation - heat transfer via electromagnetic radiation (light waves)
What is heat?
-the spontaneous transfer of energy due to thermal differences -the transfer of heat due to random, thermal motion Note that in these definitions, heat is a verb (it is the transfer of energy), therefore it is given in Joules or calories.
What is the relationship between Cal, kcal, and cal?
1 Cal = 1 kcal = 1000 cal
What three things does temperature change depend on?
1. mass of the substance 2. make-up of the substance 3. phase of the substance
What is C in q=CDeltaT.
C is heat capacity and is s x mass. It is the temp needed to raise the temp of the entire substance by 1 degree C. It does depend on mass, therefore it is an extensive property.
CQ: When heat transfers into a system, is the energy stored as heat? Explain briefly.
Heat is a verb. Therefore, it's not a substance to be stored. Once energy is transferred via heat, it can be stored in the kinetic energy of the molecules through their random molecular motion.
CQ: Explain the functions of the various parts, such as the vacuum, the silvering of the walls, the thin-walled long glass neck, the rubber support, the air layer, and the stopper.
If the liquid inside were hot, its molecules are moving faster than if it were cold. The molecules bump up against the walls of the container transferring some energy to the molecules in the walls (conductance). If the vacuum wasn't present, the air would bump up against the container walls and pick up that energy and carry it away from the container (conductance), and the hot liquid would cool down. The silvered walls has the purpose of insulation to try and reflect hear back into the liquid as well as protecting the glass from any degradation from whatever chemical it contains. (Things like NaOH can react with the glass over time.) The rubber support is to provide the inner glass container supported. Many dewars are just the glass container without the extra outer layer. But if it's really intended for drinking purposes, we like a flat surface for it to stay upright.
CQ: Satellites designed to observe the radiation from cold (3 K) dark space have sensors that are shaded from the Sun, Earth, and Moon and that are cooled to very low temperatures. Why must the sensors be at low temperature?
If their temperature is not lower than the radiation, then they will actually emit their own heat , which interferes with the waves their trying to receive.
CQ: Loose-fitting white clothing covering most of the body is ideal for desert dwellers, both in the hot Sun and during cold evenings. Explain how such clothing is advantageous during both day and night.
It reflects the sun's energy during the day to help the person stay cooler. At night, it acts as an insulator keeping the person's body heat from being drawn away as easily. (It does this during the day, too.)
CQ: Describe a situation in which heat transfer occurs. What are the resulting forms of energy?
One example is energy from a hot cup of coffee is transferred into the surrounding air. The resulting forms of energy can be considered the kinetic energy of the air molecules because that quantity increases.
CQ: What three factors affect the heat transfer that is necessary to change an object's temperature?
Temperature change depends on (1) mass of the substance, (2) the make-up of the substance, and (3) the phase of the substance.
What are the most common units for energy?
The Joule and calorie. 1 cal = 4.184 J
CQ: On average, would Earth be warmer or cooler without the atmosphere? Explain your answer
The atmosphere acts as an insulator retaining the heat that the Earth has. The atmosphere also acts to absorb some of the light coming from the Sun. Therefore, the Earth would be cooler without the atmosphere.
CQ: Some electric stoves have a flat ceramic surface with heating elements hidden beneath. A pot placed over a heating element will be heated, while it is safe to touch the surface only a few centimeters away. Why is ceramic, with a conductivity less than that of a metal but greater than that of a good insulator, an ideal choice for the stove top?
The ceramic will not transfer the heat to the surrounding ceramic very easily but will transfer it easily to the metal pot.
CQ: Why are cloudy nights generally warmer than clear ones?
The clouds act as an insulator allowing our body heat to only rise so far and to remain in the vicinity.
CQ: When watching a daytime circus in a large, dark-colored tent, you sense significant heat transfer from the tent. Explain why this occurs.
The dark color absorbs the Sun's rays and transfers that heat inside the tent where a person's body picks it up through convection.
CQ: Why are thermometers that are used in weather stations shielded from the sunshine? What does a thermometer measure if it is shielded from the sunshine and also if it is not?
To keep from heating up. They measure the air temperature if they're shaded, and they measure the temperature of the incoming waves if they're not shaded.
What are our two main equations for heat?
q=msDeltaT and q=CDeltaT. s is specific heat and it is the heat required to raise the temp 1 degree C in one gram. It is intensive property (not dependent on amount)
