Electricity Demystified, 2nd edition
Stan Gibilisco
Explanations for Quiz Answers in Chapter 2
1. If we have two objects with different electrical charges, we can have absolute confidence that a potential difference (voltage) exists between them. Choice D represents the best answer. A current might flow between two objects having different charge; a conductive path might exist between the objects; an essentially infinite resistance might exist between the objects. Therefore, choice A, B, or C each represent a possible scenario, but we don't know for certain that any one of them is true based only on the fact that the two objects have different charges.
2. If we make certain that we drive a constant current through a variable-resistance electrical component, then the potential difference (voltage) across the component varies in direct proportion to the resistance. The correct choice is A. Choices B and C have no relevance in this scenario, and choice D is absolutely wrong; conductance varies in inverse proportion to the resistance, not in direct proportion.
3. Choice D is correct here. Conductance represents the mathematical reciprocal (or so-called multiplicative inverse) of resistance. As resistance goes up, conductance goes down; as resistance goes down, conductance goes up. Choices B and C are, as in the previous question and answer, irrelevant, and we know from our answer to the previous question that choice A can't be right.
4. In any object or medium that conducts current, the electrons flow in the opposite direction from the conventional current. Electrons always move from a more negatively charged region to a more positively charged region. Conventional current, however, flows from plus to minus. The correct choice is A.
5. If we place a constant voltage across a variable-resistance component, then the current through that component will vary in inverse proportion to the resistance. Therefore, if we increase the resistance by a factor of 4, the current will decrease by a factor of 4; that is, the current will become 1/4 as great. The correct answer is B.
6. One coulomb (1 C) of charge represents 6.24 x 1018 elementary charge units (ECU). A charge quantity of 1.56 x 1018 ECU equals exactly 1/4 of 6.24 x 1018 ECU, or 1/4 of a coulomb (0.250 C). The correct choice is D.
7. We can easily see the solution to this problem when we note that 3.12 x 1018 ECU equals exactly 1/2 of a coulomb (0.500 C). Therefore, 0.500 C flow past the point in one second (1 s), representing 0.500 A of current. The correct choice is B.
8. Your body acquires an electrostatic charge if you walk around on a carpeted floor in dry weather while wearing hard-soled shoes. The correct choice is C. While the voltage under these conditions can rise to quite high levels, making you think for a moment that choice A could be right, the voltage doesn't present a deadly hazard because there aren't enough charge carriers to deliver a lethal current through your body when discharge occurs (a fact that makes choice B wrong as well).
9. A proton always carries a positive charge, so choice C is correct. An atom can have no charge, a negative charge, or a positive charge, depending on the number of electrons relative to the number of protons, so choice A won't work. An electron always has a negative charge, so B is wrong. A neutron has no charge at all, so D is wrong.
10. Electrons always carry the same amount of electrostatic charge, that is, 1 ECU. If we place two electrons close together in a vacuum, they'll repel each other. If we vary the distance between the electrons, and if nothing else interferes with the electrostatic force between them, then the force will vary inversely in proportion to the square of the distance. Increasing the separation by a factor of 10 will therefore cause the force to diminish by a factor of 102, or 100. In other words, the force will become 1/100 as great. The correct choice is B.