In the chapter-opening photograph, Jean Curtis asks her

Electromagnetic Induction What must change in order for electromagnetic induction to occur in a wire coil?

Electromagnetic Induction What important discovery did physicists Michael Faraday and Joseph Henry make?

Faraday’s Law State Faraday’s law.

Faraday’s Law What are the three ways in which voltage can be induced in a loop of wire?

How does the frequency of induced voltage relate to how frequently a magnet is plunged in and out of a coil of wire?

What are the basic differences and similarities between a generator and an electric motor?

Generators and Alternating Current Is the current that is produced by a common generator ac or dc?

What is the common frequency of ac in homes in the United States?

Who discovered electromagnetic induction, and who put it to practical use?

What is an armature?

Power Production What commonly supplies the energy input to a turbine?

Power Production Is it correct to say that a generator produces energy? Defend your answer.

Power Production What are the principal differences between an MHD generator and a conventional generator?

Does an MHD generator employ Faraday’s law of induction? Explain.

What name is given to the rate at which energy is transferred?

Transformers Is it correct to say that a transformer boosts electric energy? Defend your answer.

Which of these does a transformer change: voltage, current, energy, power?

Transformers How does the power input to an efficient transformer compare with the power output?

Transformers What exactly does a step-down transformer step down?

In a step-down transformer, how does the input current compare with the output current?

Transformers Why does a transformer require ac?

Transformers What is the principal advantage of ac over dc?

Self-Induction When the magnetic field changes in a coil of wire, voltage in each loop of the coil is induced. Will voltage be induced in a loop if the source of the magnetic field is the coil itself?

Power Transmission What is the purpose of transmitting power at high voltages over long distances?

Power Transmission Does the transmission of electric energy require electric conductors between the source and receiver? Cite an example to defend your answer.

Field Induction Who extended Faraday’s law to changing electric fields?

Field Induction What is induced by the rapid alternation of an electric field?

Field Induction What is induced by the rapid alternation of a magnetic field?

Field Induction Are wires needed in Maxwell’s view of Faraday’s law?

What do we call electromagnetic waves in the range of frequencies that match what our eyes can see?

A video game console requires 6 V to operate correctly. A transformer allows the device to be powered from a 120-V outlet. If the primary has 500 turns, show that the secondary should have 25 turns.

\(\text { Transformer relationship: } \frac{\text { Primary voltage }}{\text { Number of primary turns }}=\frac{\text { secondary voltage }}{\text { number of secondary turns }}\) The primary of a transformer connected to 120 V has 100 turns. The secondary has 10 turns. Show that the output voltage is 12 V. This is a step-down transformer.

The primary of a transformer connected to \(120 \mathrm{~V}\) has \(10\) turns. The secondary has \(100\) turns. Show that the output voltage is \(1200 \mathrm{~V}\). This is a step-up transformer. Equation Transcription: Text Transcription: 120 V 10 100 1200 V

A model electric train requires 6 V to operate. When it is connected to a 120-V household circuit, a transformer is needed. If the primary coil of the transformer has 360 turns, show that the secondary coil should have 18 turns.

A transformer for a laptop computer converts a 120-V input to a 24-V output. Show that the primary coil has five times as many turns as the secondary coil.

If the output current for the transformer in the preceding problem is 1.8 A, show that the input current is 0.36 A.

A transformer has an input of 6 V and an output of 36 V. If the input is changed to 12 V, show that the output would be 72 V.

An ideal transformer has 50 turns in its primary and 250 turns in its secondary. 12-V ac is connected to the primary. Show that (a) 60 V ac is available at the secondary; (b) 6 A of current is in a 10-? device connected to the secondary; and (c) the power supplied to the primary is 360 W.

Neon signs require about 12,000 V for their operation. Consider a neon-sign transformer that operates off 120-V lines. How many more turns should be on the secondary than on the primary?

A power of 100 kW (\(10^{5}\) W) is delivered to the other side of a city by a pair of power lines, between which the voltage is 12,000 V. (a) Use the formula P = IV to show that the current in the lines is 8.3 A. (b) If each of the two lines has a resistance of 10 \(\Omega\), show that there is a 83-V change of voltage along each line. (Think carefully. This voltage change is along each line, not between the lines.) (c) Show that the power expended as heat in both lines together is 1.38 kW (distinct from power delivered to customers). (d) How do your calculations support the importance of stepping voltages up with transformers for long-distance transmission? Text Transcription: 10^5 Omega

Bar magnets are moved into the wire coils in identical quick fashion. Voltage induced in each coil causes a current, as indicated on the galvanometer. Ignore the electrical resistance in the loops in the coil, and rank from greatest to least the readings on the galvanometer.

Each of the transformers shown is powered with \(100 \mathrm{~W}\), and all have \(100\) turns on the primary. The number of turns on each secondary varies as indicated. Equation Transcription: Text Transcription: 100 W 100

Why does the word change occur so frequently in this chapter?

A common pickup for an electric guitar consists of a coil of wire around a small permanent magnet, as described in Figure 25.5. Why will this type of pickup fail with nylon strings?

Why does an iron core increase the magnetic induction of a coil of wire?

Why are the armature and field windings of an electric motor usually wound on an iron core?

Why is a generator armature harder to rotate when it is connected to a circuit and supplying electric current?

Why does a motor also tend to act as a generator?

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?

At the security area, people walk through a large coil of wire and through a weak ac magnetic field. What is the result of a small piece of metal on a person that slightly alters the magnetic field in the coil?

A piece of plastic tape coated with iron oxide is magnetized more in some parts than in others. When the tape is moved past a small coil of wire, what happens in the coil? What is a practical application of this?

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?

When you turn the shaft of an electric motor by hand, what occurs in the interior coils of wire?

Your friend says that if you crank the shaft of a dc motor manually, the motor becomes a dc generator. Do you agree or disagree?

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?

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 one of the chapter-opening photographs, 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 lightbulb 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 is the same as that produced in the single loop. Why?

Two separate but similar coils of wire are mounted close to each other, as shown. The first coil is connected to a battery. The second coil is connected to a galvanometer. (a) How does the galvanometer respond when the switch in the first circuit is closed? (b) After being closed, how does the meter respond when the current is steady? (c) How does the meter respond when the switch is opened?

Why will more voltage be induced with the apparatus shown above if an iron core is inserted in the coils?

Why won’t a transformer work if you are using dc?

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?

What is the principal difference between a step-up transformer and a step-down transformer

Why can a hum usually be heard when a transformer is operating?

Why doesn’t a transformer work with direct current? Why is ac required?

Why is it important that the core of a transformer pass through both coils?

Are the primary and secondary coils in a transformer physically linked, or is there space between the two? Explain.

A 120-volt ac source powers the circuit shown. How many volts are impressed across the lightbulb, and how many amps flow through it?

In the circuit shown, how many volts are impressed across the meter, and how many amps flow through it?

How would you answer the preceding question if the input were 12 V ac?

Can an efficient transformer step up energy? Defend your answer.

If a bar magnet is thrown into a coil of high-resistance wire, it will slow down. Why?

With no magnets in the vicinity, why will current flow in a large coil of wire waved around in the air?

We know that the source of a sound wave is a vibrating object. What is the source of an electromagnetic wave?

What does an incident radio wave do to the electrons in a receiving antenna?

How do you suppose the frequency of an electromagnetic wave compares with the frequency of the electrons it sets into oscillation in a receiving antenna?

A friend says that changing electric and magnetic fields generate one another and that this gives rise to visible light when the frequency of change matches the frequencies of light. Do you agree? Explain.

Would electromagnetic waves exist if changing magnetic fields could produce electric fields but changing electric fields could not, in turn, produce magnetic fields? Explain.

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 by applying two important principles of physics- one from Chapter 2 and the other in this chapter.

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?

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?

Your physics instructor drops a magnet through a long vertical copper pipe and it moves slowly compared with the drop of a nonmagnetized object. Provide an explanation.

This exercise is similar to the preceding 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?

Although copper and aluminum are not magnetic, why is a sheet of either metal more difficult to pass between the pole pieces of a magnet than a sheet of cardboard?

A metal bar, pivoted at one end, oscillates freely in the absence of a magnetic field. But when it oscillates between the poles of a magnet, its oscillations are quickly damped. Why? (Such magnetic damping is used in a number of practical devices.)

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.