Physical Science 101 -- Assignment 5


Ch. 8, Exercises: 1, 4, 8, 18, 22, 23, 30, 32, 35, 37 and this extra one:
E1 You are not harmed by contact with a charged metal ball, even though its voltage may be very high. Is the reason similar to the reason why you are not harmed by the sparks from a Fourth-of-July sparkler, even though the temperature of each of the sparks is greater than 1000 C? Defend your answer in terms of the energies involved.

8.1 Excess electrons rubbed from your hair leave it with a positive charge; excess electrons on the comb give it a negative charge.

8.4 The electrons are not the only charges in the penny! There are five thousand billion billion positively charged nuclei that attract the electrons and keep them from flying out of the penny.

8.8 For materials like metals that are both good electrical and heat conductors, the conduction is via electrons, which are loosely bound to nuclei, easy flowing, and easy to get moving.

8.18 The thick filament has less resistance and will draw (carry) more current than a thin wire connected across the same potential difference. (Important point: it is common to say that a certain resistor "draws" a certain current, but this may be misleading. A resistor doesn't "attract" or "draw" current, just as a pipe in a plumbing circuit doesn't "draw" water; it instead "allows" or "provides for" the passage of current when an electrical pressure is established across it.)

8.22 Your tutor is wrong. An ampere measures current, and a volt measures electric potential. They are entirely different concepts; voltage produces current in a conductor.

8.23 Automobile headlights are connected in parallel. The evidence for this is that one headlight lamp can be burned out, but the other still lights (if they were wired in series the burned out lamp would prohibit current from flowing through the functioning lamp).

8.30 How quickly a lamp glows after an electrical switch is closed does not depend on the drift velocity of the conduction electrons, but depends on the speed at which the electric field is established in the circuit -- about the speed of light!

8.32 The leaves, like the rest of the electroscope, acquire charge from the charged object and repel each other because they both have the same sign of charge. The weight of the conducting metal foil leaves is so small that even tiny forces are evident.

8.35 The electric field produced by the charged car polarizes the small droplets of paint. As we saw in class in different demonstrations, this induced polarization always causes an attraction: the attractive force between opposite (and closer) charges is always greater than the repulsion of like (and more distant) charges.

8.37 The cooling system of an automobile is a better analogy to an electric circuit because like an electric system it is a closed system, and it contains a pump, analagous to the battery or voltage source in a circuit. The water hose does not re-circulate the water as the auto cooling system does.

E1 You are not harmed by contact with a charged metal ball, even though its voltage may be very high. Is the reason similar to the reason why you are not harmed by the sparks from a Fourth-of-July sparkler, even though the temperature of each of the sparks is greater than 1000 C? Defend your answer in terms of the energies involved.

Yes, in both cases we have a high ratio of energy per ``substance.'' In the case of temperature, the ratio is energy/molecule. In the case of voltage it is energy/charge. Even with a small numerator, the ratio can be large if the denominator is small enough. Such is the case with the small energies involved to produce high-temperature sparklers and high-voltage metal balls. Harm (i.e. pain) is caused when a large energy is transferred to our bodies and these examples do not involve large energies.