A piece of iron is heated with a torch to a temperature of 900 K.How much more energy

Three bar magnets are placed near each other along a line, end to end, on a table. The net magnetic force on the middle one is zero, and its north pole is to the left. Make two sketches showing the pos- sible arrangements of the poles of the other magnets.

Sketch the shape of the magnetic field around a bar magnet

What happens to a ferromagnetic material when it is placed in a magnetic field?

What causes magnetic declination? Is there a place where the mag- netic declination is 180 (a compass points south)? If so, approxi- mately where?

Describe the three basic interactions between electricity and mag- netism.

Explain what superconducting electromagnets are. What advantages do they have over conventional electromagnets? What disadvantages do they have?

Name five different basic devices that use at least one of the elec- tromagnetic interactions.

In many cases, the effect of an electromagnetic interaction is per- pendicular to its cause. Describe two different examples that illus- trate this.

To test whether a material is a superconductor, a scientist decides to make a ring out of the material and then to see whether a current will flow around in the ring with no steady energy input. (a) Explain how a magnet could be used to initiate the current. (b) At some later time, how could the scientist check to see whether the current is still flowing in the ring but without touching the ring?

A coil of wire has a large alternating current flowing in it. A piece of aluminum or copper placed near the coil becomes warm even if it does not touch the coil. Explain why.

. In the particular accelerator shown in Figure 8.16, what are the pos- sible directions of the magnetic field that keeps the particles travel- ing around the circular path?

What is the motor-generator duality? Explain how it is used.

Explain why two wires, each with a current flowing in it, exert forces on each other even when they are not touching each other.

Explain why a transformer doesnt work with DC.

What would an audio speaker like that shown in Figure 8.23 do at the instant a low-voltage battery is connected to it?

The type of microphone described in Section 8.4 can be thought of as a speaker operating in reverse. Explain.

What is analog-to-digital conversion and how is it used in sound reproduction?

Explain how electromagnetic waves are a natural outcome of the principles of electromagnetism.

An electromagnetic wave travels in a region of space occupied only by a free electron. Describe the resulting motion of the electron.

List the main types of electromagnetic waves in order of increas- ing frequency. Give at least one useful application for each type of wave.

Alternating current with a frequency of 1 million Hz flows in a wire. What in particular could be detected traveling outward from the wire?

What are the main uses of microwaves? Explain how each process works.

A liquid compound is not heated by microwaves the way water is. What can you conclude about the nature of the compounds molecules?

Aircraft equipped with powerful radar units are forbidden from turning them on when on the ground near people. Explain why this is so.

Which type of EM wave does your body emit most strongly?

What is different about our perceptions of the different frequen- cies within the visible light band of the EM spectrum?

A heat lamp is designed to keep food and other things warm. Would it also make a good tanning lamp? Why or why not?

How are x-rays produced?

Why are x-rays more strongly absorbed by bones than by muscles and other tissues?

What is blackbody radiation? How does the radiation emitted by a blackbody change as its temperature increases?

A lightbulb manufacturer makes bulbs with different color temper- atures, meaning that the spectrum of light they emit is similar to a blackbody with that temperature. What would be the difference in appearance of the light from bulbs with color temperatures of 2,000 K and 4,000 K?

Referring to the lightbulbs described in Question 31 and assuming the emitting areas of the filaments in the two bulbs are the same, which of the two is the brighter? About how much brighter is this bulb than its companion?

Explain how infrared light can be used to detect some types of ani- mals in the dark. Can you think of a situation in which this would not work?

Stars emit radiation whose spectrum is very similar to that of a blackbody. Imagine two stars identical in size, each of which is at the same distance from us. One of the stars appears reddish in color, while the other one looks distinctly bluish. Based on this informa- tion, what can you say about the relative temperatures of the two stars? Which is the hotter? How do you know?

What effect does the ozone layer have on the EM waves from the Sun? What is currently threatening the ozone layer?

Describe the greenhouse effect that is occurring in Earths atmosphere.

How does the ionosphere affect the range of radio com- munications?

1. The charger cord used to recharge a cell phone contains a trans- former that reduces 120 V AC to 5 V AC. For each 1,000 turns in the input coil, how many turns are there in the output coil?

The generator at a power plant produces AC at 24,000 V. A trans- former steps this up to 345,000 V for transmission over power lines. If there are 2,000 turns of wire in the input coil of the transformer, how many turns must there be in the output coil?

Compute the wavelength of the carrier wave of your favorite radio station.

What is the wavelength of the 60,000-Hz radio wave used by radio- controlled clocks and wristwatches?

Compute the frequency of an EM wave with a wavelength of 1 in. (0.0254 m).

The wavelength of an electromagnetic wave is measured to be 600 m. (a) What is the frequency of the wave? (b) What type of EM wave is it?

Determine the range of wavelengths in the UV radiation band.

A piece of iron is heated with a torch to a temperature of 900 K. How much more energy does it emit as blackbody radiation at 900 K than it does at room temperature, 300 K?

The filament of a light bulb goes from a temperature of about 300 K up to about 3,000 K when it is turned on. How many times more radiant energy does it emit when it is on than when it is off?

A rectangular metal plate measures 0.15 m long and 0.1 m wide. The plate is heated to a temperature of 1,200 K by passing a current through it. Assuming that it behaves like a blackbody, how much power does the plate radiate under these conditions?

A copper cylinder with a temperature of 450 K is found to emit 175 J of energy each second. If the cylinder radiates like a blackbody, what is its surface area?

What is the wavelength of the peak of the blackbody radiation curve for the human body (T 310 K)? What type of EM wave is this?

What wavelength EM wave would be emitted most strongly by mat- ter at the temperature of the core of a nuclear explosion, about 10,000,000 K? What type of wave is this?

What is the frequency of the EM wave emitted most strongly by a glowing element on a stove with temperature 1,500 K?

The blackbody radiation emitted from a furnace peaks at a wave- length of 1.2 106 m (0.0000012 m). What is the temperature inside the furnace?

What is the lowest temperature that will cause a blackbody to emit radiation that peaks in the infrared?

1. Earths magnetic field lines are not parallel to its surface except in certain places: they actually dip downward at some angle to the ground. (An ordinary compass does not show this.) (a) At what places on Earth would the magnetic dip be the greatest? (b) Where on Earth would the dip be zero?

A cyclometer is a device mounted on a bicycle that records and dis- plays trip information such as elapsed time, distance traveled, and average speed. Modern versions employ small magnets attached to a spoke on either the front or rear wheel and a sensor located on the front fork or rear frame. As the magnet is carried past the detector once each wheel revolution, it produces an electrical pulse that is recorded and logged by an integrated circuit in the device. The dis- tance traveled equals the total number of revolutions multiplied by the circumference of the wheel. (a) A typical mountain bike has a wheel diameter of 26 in. How far will a cyclist have traveled in both miles and kilometers if her cyclometer indicates that her front wheel has made 25,000 revo- lutions during her trip? (b) If the cyclometer clock shows that the elapsed time for the trip was 2 hours and 40 minutes, what was the rider's average speed during this period?

A solenoid connected to a 60-Hz AC source will produce an oscillat- ing magnetic field, as we have seen. If a permanent magnet is inserted into the solenoid, it will oscillate, but not with the same fre- quency that an unmagnetized piece of iron would. Why? (This is why the hum or buzz from electrical devices is sometimes 60-Hz sound and sometimes 120-Hz sound.)

The right-hand rule is a way to determine the direction of the mag- netic field produced by moving charges. Imagine wrapping your right hand around the path of the charges so that the positive charges (or the current) flow from the little finger side of your fist to the thumb side ( Figure 8.52). Then your fingers circle the path in the same direction as the magnetic field lines. Use this rule to ver- ify the directions of the magnetic fields shown in Figures 8.8 and 8.10. How would you use the rule to find the direction of the magnetic field lines around a moving negative charge?

Sketch a diagram of an atom with one electron orbiting the nucleus. Use the right-hand rule (see Challenge 4) to determine the direction of the magnetic field produced by the electron

If a coil of wire is connected to a very sensitive ammeter and then waved about in the air, a current will be induced in it even if there are no magnets around. Why?

Even though the output voltage of a transformer can be much larger than the input voltage, the power output is nearly the same as the power input. (There is some energy loss from ohmic heating.) Use this to determine the relationship between the input and output currents and the number of turns in the input and output coils.

The highest frequency sound that can be recorded by a tape recorder depends on the size of the gap in the recording head. Why? Would a wider or a narrower gap be capable of recording higher frequencies?

Describe what happens to an electric charge as an electromagnetic wave passes through the region around it. Explain why the charge will produce another electromagnetic wave. (This process occurs in the ionosphere.)

Would an electromagnetic pump be able to move a liquid supercon- ductor? Why or why not?

The nuclei of carbon atoms that are found in Nature come in two main varieties called isotopes (more on this in Chapter 11). The carbon-14 nuclei have a greater mass than the carbon-12 nuclei. The two types of atoms can be separated from each other by ionizing them, accelerating them all to some uniform velocity, and then pass- ing them between the poles of a magnet. Why does this separate the two isotopes? Describe the motion of the two ion types after passing out of the magnetic field. What would happen if all the atoms did not have the same speed?