Physics Demystified, 2nd edition Stan Gibilisco Explanations for Quiz Answers in Chapter 1 1. The term gravitational acceleration refers to the straight-line acceleration (rate at which the speed increases) of a free-falling object in a gravitational field. The standard unit of acceleration is the meter per second squared (m · s-2), so the correct choice is D. 2. When we multiply power by time, we get energy. If we ever encounter a unit called a kilowatt-second, therefore, we'll know right away that it quantifies energy. Of the four choices given here, only one of them states a unit of energy, the joule (equivalent to a watt-second). That's choice C. 3. Let's refer to Table 1-3 on page 47. In the fourth row up from the bottom of the table, we see that one candela (1 cd) equals 1.464 milliwatts per steradian (1.464 mW · sr-1). Scientists define the candela as an expression of light intensity at the peak visible wavelength (the wavelength at which the human eye is the most sensitive). We can calculate that5.0 cd = 5.0 x 1.464 = 7.32 mW · sr-1 which rounds off to 7.3 mW · sr-1. The correct choice is A. 4. Weight expresses the force that a mass exerts when subjected to a certain amount acceleration. If we want to convert weight to mass (or vice-versa), we must know the magnitude of that acceleration. Usually, when we talk about the weight of an object, we imply that its mass experiences gravitational acceleration. The correct choice is D. 5. The standard unit of inductance is the henry (H). The standard unit of magnetic flux quantity is the weber (Wb), which represents the equivalent of exactly one ampere-henry (1 A · H), or 1 A of current flowing through an inductance of 1 H. A millihenry (mH) equals precisely 0.001 H, while a milliampere (mA) represents exactly 0.001 A. This question describes a current of 500 mA (0.500 A) flowing through a coil whose inductance equals 500 mH (0.500 H). The magnetic flux quantity is therefore 0.500 x 0.500 = 0.250 Wb. The correct choice is B. 6. By definition, absolute zero represents the complete absence of heat, the lowest temperature that can possibly exist. Also by definition, a temperature of absolute zero equals 0 K. The correct choice is C. 7. When we talk about electrical current, we refer to the rate, per unit time, at which charge carriers flow past a certain point along a conductive path. The most common expression for this phenomenon is the coulomb per second (C · s-1). The correct choice is D. 8. The correct choice is A. Candela quantify power (such as microwatts) per unit solid angle, but not power as a quantity in its own right (such as microwatts alone). Moles and coulombs both specify overall quantity; they're terms like "a dozen" or "a gross" in everyday use, so choice B doesn't apply. Meters and feet both specify distance, so choice C doesn't apply. Light years also specify distance, just as meters do, so choice D doesn't apply. 9. By definition, the steradian is the standard unit of solid angular measure. The correct choice is C. 10. The astronomical unit (AU) quantifies distance, as we can see by carefully examining Table 1-3 on page 46 (about halfway down). An astronomical unit, by the way, is the average distance between the earth and the sun. (You already knew that, right?) The correct choice is C.