# Chapter 24

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# Chapter 24 : Electromagnetic Waves - all with Video Solutions

### Problem 1

Verify that the correct value for the speed of light $c$ is obtained when numerical values for the permeability and permittivity of free space ($\mu_o$ and $\epsilon_o$) are entered into the equation $c = \dfrac{1}{\sqrt{\mu_o \epsilon_o}}$.

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### Problem 2

Show that, when SI units for $\mu_0$ and $\epsilon_0$ are entered, the units given by the right-hand side of the equation $c = \dfrac{1}{\sqrt{\mu_0 \epsilon_0}}$ are m/s.

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### Problem 3

What is the maximum electric field strength in an electromagnetic wave that has a maximum magnetic field strength of $5.00 \times 10^{-4} \textrm{ T}$ (about 10 times the Earth’s)?

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### Problem 4

The maximum magnetic field strength of an electromagnetic field is $5\times 10^{-6}\textrm{ T}$ . Calculate the maximum electric field strength if the wave is traveling in a medium in which the speed of the wave is $0.75c$ .

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### Problem 5

Verify the units obtained for magnetic field strength $B$ in Example 24.1 (using the equation $B = \dfrac{E}{c}$) are in fact teslas (T).

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### Problem 6

(a) Two microwave frequencies are authorized for use in microwave ovens: 900 and 2560 MHz. Calculate the wavelength of each. (b) Which frequency would produce smaller hot spots in foods due to interference effects?

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### Problem 12

Approximately what is the smallest detail observable with a microscope that uses ultraviolet light of frequency $1.20\times 10^{15}\textrm{ Hz}$?

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### Problem 13

A radar used to detect the presence of aircraft receives a pulse that has reflected off an object $6 \times 10^{-5} \textrm{ s}$ after it was transmitted. What is the distance from the radar station to the reflecting object?

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### Problem 16

If you wish to detect details of the size of atoms (about $1\times 10^{-10}\textrm{ m}$) with electromagnetic radiation, it must have a wavelength of about this size. (a) What is its frequency? (b) What type of electromagnetic radiation might this be?

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### Problem 17

If the Sun suddenly turned off, we would not know it until its light stopped coming. How long would that be, given that the Sun is $1.50 \times 10^{11} \textrm{ m}$ away?

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### Problem 18

Distances in space are often quoted in units of light years, the distance light travels in one year. (a) How many meters is a light year? (b) How many meters is it to Andromeda, the nearest large galaxy, given that it is $2.00\times 10^{6}$ light years away? (c) The most distant galaxy yet discovered is $12.0\times 10^{9}$ light years away. How far is this in meters?

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### Problem 19

A certain 50.0-Hz AC power line radiates an electromagnetic wave having a maximum electric field strength of 13.0 kV/m. (a) What is the wavelength of this very low frequency electromagnetic wave? (b) What is its maximum magnetic field strength?

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### Problem 20

During normal beating, the heart creates a maximum 4.00-mV potential across 0.300 m of a person’s chest, creating a 1.00-Hz electromagnetic wave. (a) What is the maximum electric field strength created? (b) What is the corresponding maximum magnetic field strength in the electromagnetic wave? (c) What is the wavelength of the electromagnetic wave?

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### Problem 21

(a) The ideal size (most efficient) for a broadcast antenna with one end on the ground is one-fourth the wavelength ($\dfrac{\lambda}{4}$) of the electromagnetic radiation being sent out. If a new radio station has such an antenna that is 50.0 m high, what frequency does it broadcast most efficiently? Is this in the AM or FM band? (b) Discuss the analogy of the fundamental resonant mode of an air column closed at one end to the resonance of currents on an antenna that is one- fourth their wavelength.

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### Problem 22

(a) What is the wavelength of 100-MHz radio waves used in an MRI unit? (b) If the frequencies are swept over a $\pm 1.00$ range centered on 100 MHz, what is the range of wavelengths broadcast?

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### Problem 23

(a) What is the frequency of the 193-nm ultraviolet radiation used in laser eye surgery? (b) Assuming the accuracy with which this EM radiation can ablate the cornea is inversely proportional to wavelength, how much more accurate can this UV be than the shortest visible wavelength of light?

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### Problem 24

TV-reception antennas for VHF are constructed with cross wires supported at their centers, as shown in Figure 24.27. The ideal length for the cross wires is one-half the wavelength to be received, with the more expensive antennas having one for each channel. Suppose you measure the lengths of the wires for particular channels and find them to be 1.94 and 0.753 m long, respectively. What are the frequencies for these channels?

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### Problem 25

Conversations with astronauts on lunar walks had an echo that was used to estimate the distance to the Moon. The sound spoken by the person on Earth was transformed into a radio signal sent to the Moon, and transformed back into sound on a speaker inside the astronaut’s space suit. This sound was picked up by the microphone in the space suit (intended for the astronaut’s voice) and sent back to Earth as a radio echo of sorts. If the round-trip time was 2.60 s, what was the approximate distance to the Moon, neglecting any delays in the electronics?

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### Problem 26

Lunar astronauts placed a reflector on the Moon’s surface, off which a laser beam is periodically reflected. The distance to the Moon is calculated from the round-trip time. (a) To what accuracy in meters can the distance to the Moon be determined, if this time can be measured to 0.100 ns? (b) What percent accuracy is this, given the average distance to the Moon is $3.84\times 10^{8}\textrm{ m}$ ?

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### Problem 27

Radar is used to determine distances to various objects by measuring the round-trip time for an echo from the object. (a) How far away is the planet Venus if the echo time is 1000 s? (b) What is the echo time for a car 75.0 m from a Highway Police radar unit? (c) How accurately (in nanoseconds) must you be able to measure the echo time to an airplane 12.0 km away to determine its distance within 10.0 m?

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### Problem 28

(a) Calculate the ratio of the highest to lowest frequencies of electromagnetic waves the eye can see, given the wavelength range of visible light is from 380 to 760 nm. (b) Compare this with the ratio of highest to lowest frequencies the ear can hear.

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### Problem 29

(a) Calculate the rate in watts at which heat transfer through radiation occurs (almost entirely in the infrared) from $1.0 \textrm{ m}^2$ of the Earth’s surface at night. Assume the emissivity is 0.90, the temperature of the Earth is $15^\circ\textrm{C}$, and that of outer space is 2.7 K. (b) Compare the intensity of this radiation with that coming to the Earth from the Sun during the day, which averages about $800 \textrm{ W/m}^2$, only half of which is absorbed. (c) What is the maximum magnetic field strength in the outgoing radiation, assuming it is a continuous wave?

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### Problem 32

Assume the helium-neon lasers commonly used in student physics laboratories have power outputs of 0.250 mW. (a) If such a laser beam is projected onto a circular spot 1.00 mm in diameter, what is its intensity? (b) Find the peak magnetic field strength. (c) Find the peak electric field strength.

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### Problem 33

An AM radio transmitter broadcasts 50.0 kW of power uniformly in all directions. (a) Assuming all of the radio waves that strike the ground are completely absorbed, and that there is no absorption by the atmosphere or other objects, what is the intensity 30.0 km away? (Hint: Half the power will be spread over the area of a hemisphere.) (b) What is the maximum electric field strength at this distance?

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### Problem 34

Suppose the maximum safe intensity of microwaves for human exposure is taken to be $1.00\textrm{ W/m}^2$ . (a) If a radar unit leaks 10.0 W of microwaves (other than those sent by its antenna) uniformly in all directions, how far away must you be to be exposed to an intensity considered to be safe? Assume that the power spreads uniformly over the area of a sphere with no complications from absorption or reflection. (b) What is the maximum electric field strength at the safe intensity? (Note that early radar units leaked more than modern ones do. This caused identifiable health problems, such as cataracts, for people who worked near them.)

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### Problem 35

A 2.50-m-diameter university communications satellite dish receives TV signals that have a maximum electric field strength (for one channel) of $7.50 \textrm{ }\mu\textrm{V/m}$ . (See Figure 24.28.) (a) What is the intensity of this wave? (b) What is the power received by the antenna? (c) If the orbiting satellite broadcasts uniformly over an area of $1.50 \times 10^{13} \textrm{ m}^2$ (a large fraction of North America), how much power does it radiate?

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### Problem 36

Lasers can be constructed that produce an extremely high intensity electromagnetic wave for a brief time—called pulsed lasers. They are used to ignite nuclear fusion, for example. Such a laser may produce an electromagnetic wave with a maximum electric field strength of $1.00\times 10^{11}\textrm{ V/m}$ for a time of 1.00 ns. (a) What is the maximum magnetic field strength in the wave? (b) What is the intensity of the beam? (c) What energy does it deliver on a $1.00\textrm{ mm}^2$ area?

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### Problem 37

Show that for a continuous sinusoidal electromagnetic wave, the peak intensity is twice the average intensity ($I_o = 2I_{\textrm{ave}}$, using either the fact that $E_o = \sqrt{2}E_{\textrm{rms}}$, or $B_o = \sqrt{2}B_{\textrm{rms}}$, where "rms" means average (actually root mean square, a type of average).

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### Problem 38

Suppose a source of electromagnetic waves radiates uniformly in all directions in empty space where there are no absorption or interference effects. (a) Show that the intensity is inversely proportional to $r^2$, the distance from the source squared. (b) Show that the magnitudes of the electric and magnetic fields are inversely proportional to $r$.

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### Problem 39

An $LC$ circuit with a 5.00-pF capacitor oscillates in such a manner as to radiate at a wavelength of 3.30 m. (a) What is the resonant frequency? (b) What inductance is in series with the capacitor?

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### Problem 41

Police radar determines the speed of motor vehicles using the same Doppler-shift technique employed for ultrasound in medical diagnostics. Beats are produced by mixing the double Doppler-shifted echo with the original frequency. If $1.50 \times 10^9 \textrm{ Hz}$ microwaves are used and a beat frequency of 150 Hz is produced, what is the speed of the vehicle? (Assume the same Doppler-shift formulas are valid with the speed of sound replaced by the speed of light.)

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### Problem 42

Assume the mostly infrared radiation from a heat lamp acts like a continuous wave with wavelength $1.50\textrm{ }\mu\textrm{m}$ . (a) If the lamp’s 200-W output is focused on a person’s shoulder, over a circular area 25.0 cm in diameter, what is the intensity in $\textrm{W/m}^2$ ? (b) What is the peak electric field strength? (c) Find the peak magnetic field strength. (d) How long will it take to increase the temperature of the 4.00-kg shoulder by $2.00\textrm{ C}^\circ$ , assuming no other heat transfer and given that its specific heat is $3.47\times 10^{3}\textrm{ J/kg}\cdot\textrm{C}^\circ$ ?

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### Problem 43

On its highest power setting, a microwave oven increases the temperature of 0.400 kg of spaghetti by $45.0\textrm{C}^\circ$ in 120 s. (a) What was the rate of power absorption by the spaghetti, given that its specific heat is $3.76 \times 10^3 \textrm{ J/kg}\cdot\textrm{C}^\circ$? (b) Find the average intensity of the microwaves, given that they are absorbed over a circular area 20.0 cm in diameter. (c) What is the peak electric field strength of the microwave? (d) What is its peak magnetic field strength?

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### Problem 44

Electromagnetic radiation from a 5.00-mW laser is concentrated on a $1.00\textrm{ mm}^2$ area. (a) What is the intensity in $\textrm{W/m}^2$ ? (b) Suppose a 2.00-nC static charge is in the beam. What is the maximum electric force it experiences? (c) If the static charge moves at 400 m/s, what maximum magnetic force can it feel?

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### Problem 45

A 200-turn flat coil of wire 30.0 cm in diameter acts as an antenna for FM radio at a frequency of 100 MHz. The magnetic field of the incoming electromagnetic wave is perpendicular to the coil and has a maximum strength of $1.00 \times 10^{-12} \textrm{ T}$. (a) What power is incident on the coil? (b) What average emf is induced in the coil over one-fourth of a cycle? (c) If the radio receiver has an inductance of $2.50 \textrm{ }\mu\textrm{H}$, what capacitance must it have to resonate at 100 MHz?

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### Problem 46

If electric and magnetic field strengths vary sinusoidally in time, being zero at $t=0$ , then $E = E_0 \sin{2\pi f t}$ and $B = B_0 \sin{2\pi f t}$ . Let $f = 1.00 \textrm{ GHz}$ here. (a) When are the field strengths first zero? (b) When do they reach their most negative value? (c) How much time is needed for them to complete one cycle?

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### Problem 47

A researcher measures the wavelength of a 1.20-GHz electromagnetic wave to be 0.500 m. (a) Calculate the speed at which this wave propagates. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

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### Problem 48

The peak magnetic field strength in a residential microwave oven is $9.20\times 10^{-5}\textrm{ T}$ . (a) What is the intensity of the microwave? (b) What is unreasonable about this result? (c) What is wrong about the premise?

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### Problem 49

An $LC$ circuit containing a 2.00-H inductor oscillates at such a frequency that it radiates at a 1.00-m wavelength. (a) What is the capacitance of the circuit? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

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### Problem 50

An $LC$ circuit containing a 1.00-pF capacitor oscillates at such a frequency that it radiates at a 300-nm wavelength. (a) What is the inductance of the circuit? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

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