Conceptual Physics

1.7 A vacuum cleaner runs on a current of 12 amperes. How long will it take for 5.0 microcoulombs of charge to flow through it?

s

1.8 Physicists often demonstrate the power of electricity by simulating lightning with a device called a Tesla coil. Charge is

deposited onto a metal sphere until it amasses so much electric potential that it discharges through the air in the form of a

spark. With larger Tesla coils, this spark resembles a miniature lightning bolt. Find the average current into a Telsa coil that

amasses 1.38×10í^6 C of charge in 5.72 s.

A

1.9 The conducting sphere on top of a Van de Graaff generator is charged by a silk conveyer belt that picks up electrons at the

bottom of the generator's column, from grounded brushes consisting of certain substances (for example, fur), and releases

them at the top of the column to another set of brushes consisting of say, hard rubber. Suppose the belt of a certain Van de

Graaff generator is 10.0 cm wide, and it travels upward at 45.0 m/s, carrying a "current" of 175 mA. (a) What is the surface

charge density of the electrons on the belt? (b) Is the conventional "current" in the Van de Graaff generator flowing up or

down?

(a) C/m^2

(b) Up Down

Section 2 - Drift speed

2.1 In one of Bill Cosby's comedy routines, he talks about being so tired that "I turned off the light switch, and I was in my bed

before the light went out!" Suppose the drift speed of electrons in the wiring of the Cosby household is 3.0×10í^4 m/s, and that

there are 3.2 m of wiring between the light switch in his bedroom and the lamp. (a) In a direct current circuit, where the current

flows one way, how long would it take a conduction electron to go from the switch to the lamp? (b) Even though Cosby

couldn't really have gotten into bed before the light went out, could he at least have gotten into bed before one electron had

traveled from the switch to the lamp?

(a) s

(b)

2.2 In a typical household current, the electrons in the wire may have a drift speed of 1.3×10í^4 m/s. Actual household current is

not a direct current, but instead is an alternating (oscillating) current. (If you have ever received an electric shock from an

outlet, you have felt this rapid alternation as a painful vibration. A safe way to see the alternation is to wave your fingers

rapidly in front of a fluorescent light, and observe the stroboscopic stutter-silhouette that they form.) You can model the motion

of each conduction electron as simple harmonic motion, with a frequency of 60 Hz. The drift speed is the maximum speed,

what the speed would build up to if the electric field were applied continually, as in the case of direct current. What is the

approximate amplitude of an electron's oscillation?

m

Section 3 - Ohm’s law and resistance

3.1 A resistor has a potential difference of 5.65 volts applied across it, and a current of 345 mA flows through it. What is its

resistance?

ȍ

3.2 A device found at some amusement parks asks you to "test your personal force" by grasping two conducting bars, one in

each hand, and tolerating a muscle-clenching flow of direct current through your body. Assuming that your body is an ohmic

material, the resistance of your body between your two hands (if you are not sweating!) is about 1.00×10^4 ȍ. What voltage

does the machine have to apply to the conducting bars to drive a current of 30.0 mA through your body, causing strenuous

muscle contraction?

volts

3.3 The resistance across your (wet) tongue, from one side to the other near the tip, is 375 ȍ (assume that your tongue is an

ohmic material). If you connect two thick copper wires of relatively negligible resistance to the terminals of a 1.5 volt flashlight

battery, and place them on opposite sides of your tongue, you will be able to "taste" the electricity as it flows across,

producing a sour metallic flavor. What current is flowing through the saliva that coats your tongue?

A

3.4 A certain lightbulb filament when hot has a resistance of 205 ȍ. The potential difference across the filament at a certain

instant is standard household voltage, 120 V. (a) What current is flowing through the bulb at that time? (b) If the filament of the

bulb is a tungsten wire having a radius of 475 μm, what is the magnitude of the current density in the filament?

(a) A

(b) A/m^2

3.5 You find a used battery and discover that it is no longer strong enough to power your CD player. Out of curiosity, you decide

(^472) Copyright 2007 Kinetic Books Co. Chapter 25 Problems