Physics Demystified, 2nd edition Stan Gibilisco Explanations for Quiz Answers in Chapter 6 1. If we have a sealed chamber containing pure water vapor, we can't change the state of the vapor by heating it. The water vapor will remain in a gaseous state no matter how much heat energy we transfer to it. The answer is C. In order to get any of the vapor to become liquid or ice (choices A, B, or D), we'd have to cool it down. In other words, it would have to give up heat energy. 2. Remember that power quantifies the rate at which energy is transferred, expended, radiated, or dissipated. We can always express power as energy per unit time. Of the four choices given here, only A meets that criterion, so that's the correct answer. Calories (choice B) express energy directly. Calories per gram (choice C) express energy per unit mass. Kelvins per kilogram (choice D) express absolute temperature per unit mass. 3. If we let s represent a particular solid object's change in linear dimension (in meters), d represent the object's starting linear dimension (also in meters), and T represent the change in temperature (in degrees Celsius), then we can calculate the coefficient of linear expansion α (in units of per degree Celsius) asα = s / (dT) In the statement of this problem, we're told that the rod initially measures 2.0000 m long, so we can set d = 2.0000 m. The initial temperature is +30.0şC and the final temperature is +70.0şC, so we know that the temperature change is T = +40.0şC. The final linear dimension (rod length in this case) equals 2.0800 m. That's 0.0800 m longer than the initial linear dimension, so we can set s = +0.0800 m. When we input these values into the formula for the coefficient of linear expansion, we get α = +0.0800 / [2.0000 x (+40.0)] = 0.00100 = 1.00 x 10-3 /şC The correct choice is B. 4. This problem looks rather intimidating, doesn't it? Actually, when we pay close attention to our definitions and reduce the problem to a calculation process, it's easy. Let's start by reviewing the appropriate formula, then "plugging in" the given values, and finally "grinding out" the arithmetic. If we symbolize a material sample's heat of vaporization (in kilocalories per kilogram) as hv, the heat added to or given up by the sample (in kilocalories) as h, and the mass of the sample (in kilograms) as m, thenhv = h / m This formula works if and only if the sample remains at its vaporization temperature throughout the entire process. In this case, we're assured that it does; the vaporization temperature is 150.5şC, and the sample stays at that temperature from the beginning of the process to the end. We have 100.000 g of material. That's 0.100000 kg, so we can set m = 0.100000 kg. We transfer 300.00 kcal of heat to the substance, and as a result, it changes state from all liquid to all vapor. Therefore, we know that h = 300.00. Now we can calculate hv = 300.00 / 0.100000 = 3000 kcal/kg The answer is B. We round off to four significant figures because the vaporization temperature parameter is specified only to that level of accuracy. 5. By definition, heat of vaporization quantifies the amount of thermal energy required to convert a unit mass (such as 1 g or 1 kg) of liquid to the same mass of the same substance in the gaseous state. The correct choice is D. 6. When we have an extremely high temperature and we're limited as to accuracy (in terms of significant figures), kelvins and degrees Celsius figures are essentially the same. So, in this situation, choice C is valid. When we want to convert kelvins to degrees Rankine, we must multiply kelvins by exactly 1.8. In this case, the absolute temperature in kelvins is 2.00 x 107, so we have 2.00 x 1.80 x 107 = 3.60 x 107 degrees Rankine. Choice A also works! As with kelvins and degrees Celsius, degrees Rankine and degrees Fahrenheit values are essentially the same when we have an extremely high temperature and a limited expression of accuracy, so choice B is valid. We've determined that A, B, and C will all answer this question satisfactorily, so we can settle on D, "All of the above," as the correct choice. 7. When we have a sample of matter in the liquid state at its vaporization temperature and we impart heat energy to the the liquid, it changes to the gaseous state over a certain period of time. (The length of time depends on how rapidly the sample absorbs the heat energy.) During this process, the liquid portion of the matter remains at the vaporization temperature, which in this case equals 450 K. When we analyze the statement of this problem, we can see that 150 cal (or 0.150 kcal) is nowhere near enough energy to convert all of the liquid to a gas. Therefore, we know that we'll have plenty of liquid remaining after we transfer 150 cal to it; moreover, that liquid will still have a temperature of 450 K. The correct choice is C. 8. After we've imparted 150 cal of heat energy to the liquid sample described in Question 7, some of the liquid will have changed into the gaseous state. We started with 2.000 kg of liquid, so we'll end up with less than 2.000 kg of liquid. The correct choice is B. (The total mass of the sample, including both the liquid and the gas, will still equal 2.000 kg, of course.) 9. The coldest possible temperature is absolute zero (the complete absence of heat), which by definition equals 0 K or -273.15şC. The answer is C. 10. We can quantify specific heat in terms of calories per gram per degree Celsius (cal/g/şC). The correct choice is A.