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physics
electricity and magnetism

Conceptual Physics 11th edition Paul G. Hewitt - Solutions

Electronic types neglect the force of gravity on electrons. To see why, compute the force of Earth's gravity on an electron and compare it with the force exerted on the electron by an electric field of magnitude 10,000 V/m (a relatively small field). The mass and charge of an electron are given on
Atomic physicists ignore the effect of gravity within an atom. To see why, calculate and compare the gravitational and electrical forces between an electron and a proton separated by 10-10 m. The charges and masses are given on the inside back cover.
A droplet of ink in an industrial ink-jet printer carries a charge of 1.6 × 10-10 C and is deflected onto paper by a force of 3.2 × 10-4 N. Show that the strength of the electric field to produce this force is 2 million N/C.
An energy of 0.1 J is stored in the metal sphere on top of a Van de Graaff generator. A spark carrying 1 micro-coulomb (10-6 C) discharges the sphere. Show that the sphere's potential relative to ground is 100,000 V?
Find the voltage change when (a) An electric field does 12 J of work on a 0.0001-C charge; (b) The same electric field does 24 J of work on a 0.0002-C charge.
All bulbs are identical in the circuits shown. An ammeter is connected next to the battery, as shown. Rank the current readings in the ammeter, from greatest to least.
All bulbs are identical in the following circuits. A voltmeter is connected across a single bulb to measure the voltage drop across it. Rank the voltage readings from greatest to least.
Does more current flow out of a battery than into it? Does more current flow into a light bulb than out of it? Explain.
An electron moving in a wire collides repeatedly with atoms and travels an average distance between collisions called the mean free path. If the mean free path is less in some metals, what can you say about the resistance of these metals? For a given conductor, what can be done to lengthen the mean
A simple lie detector consists of an electric circuit, often from one finger to another. A sensitive meter shows the current that flows when a small voltage is applied. How does this technique indicate that a person is lying? (And when does this technique not indicate when someone is lying?)
It is commonly said that a certain resistor draws a certain current. Does this mean that the resistor "attracts" the current? Defend your answer.
What causes electric shock-current or voltage?
If a current of one- or two-tenths of an ampere were to flow into one of your hands and out the other, you would probably be electrocuted. But if the same current were to flow into your hand and out the elbow above the same hand, you would survive even though the current might be large enough to
Would you expect to find dc or ac in the filament of a light bulb in your home? In the headlight of an automobile?
Are automobile headlights wired in parallel or in series? What is your evidence?
A car's headlights dissipate 40 W on low beam and 50 W on high beam. Is there more or less resistance in the high beam filament?
The damaging effects of electric shock result from the amount of current that flows in the body. Why, then, do we see signs that read "Danger-High Voltage" rather than "Danger-High Current"?
Why are the wingspans of birds a consideration in determining the spacing between parallel wires in a power line?
One example of a water system is a garden hose that waters a garden. Another is the cooling system of an automobile. Which of these exhibits behavior more analogous to an electric circuit? Explain.
If electrons flow very slowly through a circuit, why does it not take a noticeably long time for a lamp to glow when you turn on a distant switch?
Why is the speed of an electric signal so much greater than the speed of sound?
Consider a pair of flashlight bulbs connected to a battery. Will they glow brighter if they are connected in series or in parallel? Will the battery run down faster if they are connected in series or in parallel?
If several bulbs are connected in series to a battery, they may feel warm to the touch but not visibly glow. What is your explanation?
In the circuit shown, how do the brightness's of the identical light bulbs compare? Which bulb draws the most current? What will happen if Bulb A is unscrewed? If Bulb C is unscrewed?
As more and more bulbs are connected in series to a flashlight battery, what happens to the brightness of each bulb? Assuming that heating inside the battery is negligible, what happens to the brightness of each bulb when more and more bulbs are connected in parallel?
What changes occur in the line current when more devices are introduced in a series circuit? In a parallel circuit? Why are your answers different?
When a pair of identical resistors are connected in series, which of the following is the same for both resistors-(a) voltage across each, (b) power dissipated in each, (c) current through each? Do any of your answers change if the resistors are different from each other?
When two identical resistors are connected in parallel, which of the following is the same for both resistors-(a) voltage across each, (b) power dissipated in each, (c) current through each? Do any of your answers change if the resistors are different from each other?
Batteries do have internal resistance, which is not always negligible. It shows when the current a battery supplies increases, whereupon the voltage it supplies decreases. Taking internal resistance of the battery into consideration, will the brightness of many bulbs diminish when connected in
Is the resistance of a 100-W bulb greater or less than the resistance of a 60-W bulb? Assuming the filaments in each bulb are of the same length and made of the same material, which bulb has the thicker filament?
If a 60-W bulb and a 100-W bulb are connected in series in a circuit, across which bulb will there be a greater voltage drop? How about if they are connected in parallel?
In which of the circuits shown does a current exist to light the bulb?
In periods of peak demand, power companies lower their voltage. This saves them power (and saves you money!). To see the effect, consider a 1200-W coffeemaker that draws 10 A when connected to 120 V. Suppose the voltage is lowered by 10% to 108 V. By how much does the current decrease? By how much
The wattage marked on a light bulb is not an inherent property of the bulb, but depends on the voltage to which it is connected, usually 110 or 120 V. How many amperes flow through a 60-W bulb connected in a 120-V circuit?
Using the formula power = current × voltage, find the current drawn by a 1200-W toaster connected to 120 V. Then, using the method from the previous problem, show that the resistance of the toaster is 12 Ω.
Show that operating a 100-W lamp continuously for 1 week when the power utility rate is 15cents/kWh costs $2.52.
A 4-W night-light is plugged into a 120-V circuit and operates continuously for 1 year. Find the following: (a) The current it draws, (b) The resistance of its filament, (c) The energy consumed in a year, (d) Then show that for a utility rate of 15 cents/kWh the cost for a year's operation is $5.25.
An electric iron connected to a 110-V source draws 9 A of current. Show that the amount of heat it generates in a minute is nearly 60,000 J.
Show in the previous problem that 540 C of charge flow through the iron in 1 minute. Previous problem An electric iron connected to a 110-V source draws 9 A of current. Show that the amount of heat it generates in a minute is nearly 60,000 J.
What is different about the magnetic poles of common refrigerator magnets and those of common bar magnets?
A friend tells you that a refrigerator door, beneath its layer of white-painted plastic, is made of aluminum. How could you check to see if this is true (without any scraping)?
Why will a magnet attract an ordinary nail or paper clip but not a wooden pencil?
Will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paper clip.
One way to make a compass is to stick a magnetized needle into a piece of cork and float it in a glass bowl full of water. The needle will align itself with the horizontal component of Earth's magnetic field. Since the north pole of this compass is attracted northward, will the needle float toward
A "dip needle" is a small magnet mounted on a horizontal axis so that it can swivel up or down (like a compass turned on its side). Where on Earth will a dip needle point most nearly vertically? Where on Earth will it point most nearly horizontally?
What is the net magnetic force on a compass needle? By what mechanism does a compass needle align with a magnetic field?
Since the iron filings that align with the magnetic field of the bar magnet are not solely little individual magnets, by what mechanism do they align themselves with the field of the magnet?
In what sense are all magnets electromagnets?
The north pole of a compass is attracted to the north magnetic pole of Earth, yet like poles repel. Can you resolve this apparent dilemma?
Your friend says that, when a compass is taken across the equator, it turns around and points in the opposite direction. Your other friend says that this is not true, that people in the Southern Hemisphere use the south magnetic pole of the compass to point toward the nearest pole. You're on; what
A strong magnet attracts a paper clip to itself with a certain force. Does the paper clip exert a force on the strong magnet? If not, why not? If so, does it exert as much force on the magnet as the magnet exerts on it? Defend your answers.
When iron-hulled naval ships are built, the location of the shipyard and the orientation of the ship in the shipyard are recorded on a brass plaque permanently attached to the ship. Why?
Can an electron at rest in a magnetic field be set into motion by the magnetic field? What if it were at rest in an electric field?
A cyclotron is a device for accelerating charged particles to high speed as they follow an expanding spiral-like path. The charged particles are subjected to both an electric field and a magnetic field. One of these fields increases the speed of the charged particles, and the other field causes
A beam of high-energy protons emerges from a cyclotron. Do you suppose that there is a magnetic field associated with these particles? Why or why not?
A magnetic field can deflect a beam of electrons, but it cannot do work on the electrons to change their speed. Why?
Inside a laboratory room there is said to be either an electric field or a magnetic field, but not both. What experiments might be performed to establish what kind of field is in the room?
To make a compass, point an ordinary iron nail along the direction of Earth's magnetic field (which, in the Northern Hemisphere, is angled downward as well as northward) and repeatedly strike it for a few seconds with a hammer or a rock. Then suspend it at its center of gravity by a string. Why
Residents of northern Canada are bombarded by more intense cosmic radiation than residents of Mexico. Why is this so?
What changes in cosmic-ray intensity at Earth's surface would you expect during periods in which Earth's magnetic field passed through a zero phase while undergoing pole reversals?
In a mass spectrometer (Figure 34.14), ions are directed into a magnetic field, where they curve and strike a detector. If a variety of singly ionized atoms travel at the same speed through the magnetic field, would you expect them all to be deflected by the same amount, or would different ions be
One way to shield a habitat in outer space from cosmic rays is with an absorbing blanket of some kind, which would function much like the atmosphere that protects Earth. Speculate on a second way for shielding the habitat that would also be similar to Earth's natural shielding.
If you had two bars of iron-one magnetized and one non magnetized-and no other materials at hand, how could you determine which bar was the magnet?
Historically, replacing dirt roads with paved roads reduced friction on vehicles. Replacing paved roads with steel rails reduced friction further. What recent step eliminates rail friction of vehicles? What friction remains after rail friction is eliminated?
If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the same iron near the south pole of the magnet?
Your study buddy claims that an electron always experiences a force in an electric field, but not always in a magnetic field. Do you agree? Why or why not?
Joseph Henry's wife donated part of her silk wedding gown to cover the wires of Joseph's electromagnets. What was the purpose of the silk covering?
A certain simple earthquake detector consists of a little box firmly anchored to Earth. Suspended inside the box is a massive magnet that is surrounded by stationary coils of wire fastened to the box. Explain how this device works, applying two important principles of physics-one studied in Chapter
How do the direction of the magnetic force and its effects differ between the motor effect and the generator effect, as shown in Figure 25.7?
Does the voltage output increase when a generator is made to spin faster? Defend your answer.
If you place a metal ring in a region in which a magnetic field is rapidly alternating, the ring may become hot to your touch. Why?
An electric saw operating at normal speed draws a relatively small current. But if a piece of wood being sawed jams and the motor shaft is prevented from turning, the current dramatically increases and the motor overheats. Why?
A magician places an aluminum ring on a table with a hidden electromagnet underneath. When the magician says "abracadabra" (and pushes a switch that starts current flowing through the coil under the table), the ring jumps into the air. Explain his "trick."
In the chapter-opening photograph, Jean Curtis asks her class why the copper ring levitates about the iron core of the electromagnet. What is the explanation, and does it involve ac or dc?
How could a light bulb near an electromagnet, but not touching it, be lit? Is ac or dc required? Defend your answer.
A length of wire is bent into a closed loop and a magnet is plunged into it, inducing a voltage and, consequently, a current in the wire. A second length of wire, twice as long, is bent into two loops of wire, and a magnet is similarly plunged into it. Twice the voltage is induced, but the current
Two separate but similar coils of wire are mounted close to each other, as shown. The first coil is connected to a battery and has a direct current flowing through it. The second coil is connected to a galvanometer.a. How does the galvanometer respond when the switch in the first circuit is
Why will more voltage be induced with the apparatus shown above if an iron core is inserted in the coils?
How does the current in the secondary of a transformer compare with the current in the primary when the secondary voltage is twice the primary voltage?
In what sense can a transformer be considered an electrical lever? What does it multiply? What does it not multiply?
Why can a hum usually be heard when a transformer is operating?
Why is it important that the core of a transformer pass through both coils?
In the circuit shown, how many volts are impressed across the lightbulb and how many amps flow through it?
Your friend says that according to Ohm's law, high voltage produces high current. Then your friend asks, "So how can power be transmitted at high voltage and low current in a power line?" What is your illuminating response?
If a bar magnet is thrown into a coil of high-resistance wire, it will slow down. Why?
Why are the armature and field windings of an electric motor usually wound on an iron core?
This exercise is similar to the previous one. Why will a bar magnet fall slower and reach terminal velocity in a vertical copper or aluminum tube but not in a cardboard tube?
The metal wing of an airplane acts like a "wire" flying through Earth's magnetic field. A voltage is induced between the wing tips, and a current flows along the wing, but only for a short time. Why does the current stop even though the airplane continues flying through Earth's magnetic field?
What is wrong with this scheme? To generate electricity without fuel, arrange a motor to operate a generator that will produce electricity that is stepped up with transformers so that the generator can operate the motor and simultaneously furnish electricity for other uses.
With no magnets in the vicinity, why will current flow in a large coil of wire waved around in the air?
Why is a generator armature harder to rotate when it is connected to a circuit and supplying electric current?
Would electromagnetic waves exist if changing magnetic fields could produce electric fields, but changing electric fields could not, in turn, produce magnetic fields? Explain.
Will a cyclist coast farther if the lamp connected to the generator on his bicycle is turned off? Explain.
When an automobile moves over a wide, closed loop of wire embedded in a road surface, is the magnetic field of Earth within the loop altered? Is a pulse of current produced? Can you cite a practical application for this at a traffic intersection?
An ideal transformer has 50 turns in its primary and 250 turns in its secondary. 12 V ac is connected to the primary. a. Find the volts ac available at the secondary. b. Show that a 10-Ω device connected to the secondary draws a current of 6 A. c. How much power is supplied to the primary?
A model electric train requires 6 V to operate. If the primary coil of its transformer has 240 windings, how many windings should the secondary have if the primary is connected to a 120-V household circuit?
Neon signs require about 12,000 V for their operation. What should be the ratio of the number of loops in the secondary to the number of loops in the primary for a neon-sign transformer that operates from 120-V lines?
100 kW (105 W) of power is delivered to the other side of a city by a pair of power lines between which the voltage is 12,000 V.a. How much current is carried in the lines?b. Each of the two lines has a resistance of 10 Ω. What is the voltage difference between the two ends of each line? (Think
The Sun is 1.50 × 1011 m from Earth. How long does it take for the Sun's light to reach Earth? How long does it take light to cross the diameter of Earth's orbit? Compare this time with the time measured by Roemer in the 17th century (Problem 1).
Show that it would take 2.5 s for a pulse of laser light to reach the Moon and to bounce back to Earth.

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