Chapter 6 Answers:
Review Questions:
1. When the hammer strikes, its kinetic energy is transformed into heat energy in the collision. The copper atoms vibrate faster after the blow so that the average kinetic energy of a copper atom is now greater than before. Since temperature measures the average kinetic energy of the molecules & atoms in a substance, the temperature is now higher (page 136).
4. Temperature measures the average kinetic energy per molecule (see above).
7. According to the ideal gas laws (which we discussed in class but not by name), this gas would have zero pressure at -273 degrees C, which is absolute zero. Actually, the gas would have already liquefied and then become solid before absolute zero. Absolute zero is practically unattainable (see #9 below).
9. Because the zero on the Kelvin scale is the lowest possible temperature reachable in theory or practice. That is why it is called absolute zero. At this temperature, all** relative motion of atoms ceases*** and therefore the average kinetic energy of any atom is zero.
12. Heat refers to the total energy content of an object resulting from the random motion of its molecules and atoms and is measured in Joules or calories. Temperature, on the other hand, refers to the average kinetic energy per molecule and is measured in Kelvin or Celcius by scientists.
16. 1 Calorie (a diet Calorie) = 1000 'heat' calories (page 139).
27. See figure 6.15 and read the explanation. Because of the shape and electrical nature of the water molecule, it forms the structure shown when it freezes. There is therefore more space between water molecules in an ice crystal than in an equal mass of melted water making ice have a lower density.
33. solid, gas, liquid, plasma
43. At higher pressures, the water molecules undergo more collisions with air molecules and other water molecules as they try to escape the surface of a liquid (pages 149-150).
48. Remember that increasing the temperature of a solid increases the average kinetic energy of the molecules & therefore their average speed. If a molecule in a solid vibrates fast enough, it can escape the clutches of its neighbor molecules momentarily. Such a molecule is captured and released repeatedly sort of like the 'square dance' analogy we acted out in class.
52. A liquid absorbs energy when reaching a higher energy state (e.g. when it evaporates). A liquid releases energy as it solidifies (goes to a lower energy state).
Exercises:
2. An iceberg has a tremendously higher amount of heat energy even though its temperature is much lower. This is a perfect illustration of the difference between heat energy and temperature. The average kinetic energy (that is, the temperature) of a water molecule at zero degrees C is lower than the average kinetic energy of a water molecule in a hot cup of coffee. But there are so many more water molecules in the iceberg that the total kinetic energy of all the molecules is much greater int the berg than the coffee. Here... I just did a quick calculation... it turns out that even for a small iceberg (about 500 tons) the heat energy contained is 1,400,000 times more than that contained by the coffee. The easy way to think of it is this: could you melt an iceberg with a hot cup of coffee?
16. We answered this together in class by heating the ring and ball over a flame. Remember that the size of the hole got larger!
19. The glass has to heat up first before heat is transferred to the mercury inside. Of course this means that the glass will momentarily expand while the mercury is still cool. The volume of the cavity inside the thermometer increases, causing the mercury level to drop momentarily. The effect doesn't last long, however, since the mercury is heated quickly by the glass and begins itself to expand.
Problems:
2. For a 10 degree C increase: Expanded Length = (1/100,000) * (1300 meters ) * (10 o) = .13 meters = 13 centimeters
4. First, the heat energy stored in the 10 g of steam, Q = (540 cal/g)*(10 g) = 5400 calories. So this is how much heat is available to melt the ice. Now it takes 80 of these calories to melt only one gram of ice so that: # grams melted = (5400 cal)/(80 cal/g) = 68 grams.
5. Q = CM(Tf - Ti) = (200cal/kg-oC)*(1kg)*(650 oC) = 130,000 calories.
.01 cal/kg*yr = Q/t
t = Q/(.01 cal/kg*yr) = 130000 cal / (.01 cal/kg*yr) = 13,000,000 years!
**This is true only of relative classical motion (the only kind studied during this semester course). Because of the uncertainty principle of quantum physics (see chapter 13), zero relative velocity for any particle would mean that its velocity would be completely certain...i.e. exactly zero. But this is an impossibility according to the uncertainty principle (first discovered in the 1920's). The principle states that the more precisely the velocity of a particle can be known the more uncertain its exact location becomes. The uncertainty principle actually predicts that atoms smear out and merge with each other into one super-atom at a few billionths of a degree above absolute zero. Albert Einstein was the first to notice this prediction of quantum theory and he thought it sounded kinda silly (just as you probably do). For him, this was just evidence that quantum physics wasn't a complete explanation of the way atoms behave. Something has to be wrong with a theory that predicts that such ridiculous results. Einstein and Satyendra Nath Bose got their names on this weird theoretical super-atom state of matter. It is called a Bose-Einstein condensate. But guess what! In 1995, Eric Cornell and Carl Wieman succeded in producing this state of matter in a super high-tech refrigerator.
***According to the second law of thermodynamics, it is practically impossible to attain absolute zero for completely different reasons (see chapter 7).