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How does the intensity of an electromagnetic wave depend on its electric field? How does it depend on its magnetic field?

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What is the physical significance of the Poynting vector?

It has the magnitude of the energy flux and points in the direction of wave propagation. It gives the direction of energy flow and the amount of energy per area transported per second.

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A 2.0-mW helium-neon laser transmits a continuous beam of red light of cross-sectional area 0.25 cm 2 . If the beam does not diverge appreciably, how would its rms electric field vary with distance from the laser? Explain.

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Problems

While outdoors on a sunny day, a student holds a large convex lens of radius 4.0 cm above a sheet of paper to produce a bright spot on the paper that is 1.0 cm in radius, rather than a sharp focus. By what factor is the electric field in the bright spot of light related to the electric field of sunlight leaving the side of the lens facing the paper?

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A plane electromagnetic wave travels northward. At one instant, its electric field has a magnitude of 6.0 V/m and points eastward. What are the magnitude and direction of the magnetic field at this instant?

The magnetic field is downward, and it has magnitude 2.00 × 10 −8 T .

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The electric field of an electromagnetic wave is given by
E = ( 6.0 × 10 −3 V/m ) sin [ 2 π ( x 18 m t 6.0 × 10 −8 s ) ] j ^ .
Write the equations for the associated magnetic field and Poynting vector.

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A radio station broadcasts at a frequency of 760 kHz. At a receiver some distance from the antenna, the maximum magnetic field of the electromagnetic wave detected is 2.15 × 10 −11 T .
(a) What is the maximum electric field? (b) What is the wavelength of the electromagnetic wave?

a. 6.45 × 10 −3 V/m; b. 394 m

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The filament in a clear incandescent light bulb radiates visible light at a power of 5.00 W. Model the glass part of the bulb as a sphere of radius r 0 = 3.00 cm and calculate the amount of electromagnetic energy from visible light inside the bulb.

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At what distance does a 100-W lightbulb produce the same intensity of light as a 75-W lightbulb produces 10 m away? (Assume both have the same efficiency for converting electrical energy in the circuit into emitted electromagnetic energy.)

11.5 m

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An incandescent light bulb emits only 2.6 W of its power as visible light. What is the rms electric field of the emitted light at a distance of 3.0 m from the bulb?

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A 150-W lightbulb emits 5% of its energy as electromagnetic radiation. What is the magnitude of the average Poynting vector 10 m from the bulb?

5.97 × 10 −3 W/m 2

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A small helium-neon laser has a power output of 2.5 mW. What is the electromagnetic energy in a 1.0-m length of the beam?

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At the top of Earth’s atmosphere, the time-averaged Poynting vector associated with sunlight has a magnitude of about 1.4 kW/m 2 .
(a) What are the maximum values of the electric and magnetic fields for a wave of this intensity? (b) What is the total power radiated by the sun? Assume that the Earth is 1.5 × 10 11 m from the Sun and that sunlight is composed of electromagnetic plane waves.

a. E 0 = 1027 V/m , B 0 = 3.42 × 10 −6 T ; b. 3.96 × 10 26 W

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The magnetic field of a plane electromagnetic wave moving along the z axis is given by B = B 0 ( cos k z + ω t ) j ^ , where B 0 = 5.00 × 10 −10 T and k = 3.14 × 10 −2 m −1 .
(a) Write an expression for the electric field associated with the wave. (b) What are the frequency and the wavelength of the wave? (c) What is its average Poynting vector?

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What is the intensity of an electromagnetic wave with a peak electric field strength of 125 V/m?

20.8 W/m 2

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Assume the helium-neon lasers commonly used in student physics laboratories have power outputs of 0.500 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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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?

a. 4.42 × 10 ‒6 W/m 2 ; b. 5.77 × 10 ‒2 V/m

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Suppose the maximum safe intensity of microwaves for human exposure is taken to be 1.00 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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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 μV/m (see below). (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 × 10 13 m 2 (a large fraction of North America), how much power does it radiate?

Figure shows waves incident on a dish antenna.

a. 7.47 × 10 −14 W/m 2 ; b. 3.66 × 10 13 W ; c. 1.12 W

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Lasers can be constructed that produce an extremely high intensity electromagnetic wave for a brief time—called pulsed lasers. They are used to initiate nuclear fusion, for example. Such a laser may produce an electromagnetic wave with a maximum electric field strength of 1.00 × 10 11 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 an 1.00 -mm 2 area?

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Questions & Answers

what is mass
Exildah Reply
mass is the amount of staff made up an object
Van
physic is anatural science that deals about scleral
Abdilahi
mass is accelaration divided force
Abdilahi
mass is force divided by acceleration
Van
what is the volume of a human being
Collins Reply
Why does the lines of force not touch each other 🇲🇲
Gbemisola Reply
what is physics
Exildah
physics is the study of matter and energy and the interaction between them
Van
what is unit
Rayyanu Reply
Please canu get more questions on electric field and electric flux please
Gbemisola
is electric field directly proportional to the squared of a distance
Benjamin Reply
No electric field is inversely proportional to the squared distance between the charges
Gbemisola
lets treat linear expansivity please
Ujah Reply
The bullet 2.00cm long is fired at 420/s and passes straight through a 10.0 cm thick board existing at 280 m/s.What is the average acceleration of the bullet through the board?
FAUSTINA
an unstretched spring is 12cm long .A load of 5N stretched it to 15cm .how long will it be under a load of 15N?
Morapeli
hi
Africa
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Benjamin
Benjamin how are u are u a freshman in the university
Africa
like 100 level
Africa
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Benjamin
l need a physics tutor
Benjamin
I think the best tutor is God and organic tutor in YouTube that guy is good
Africa
me too 100level
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Benjamin
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Benjamin
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Africa
studying what
Africa
Compare the electric flux through the surface of a cube of side length a that has a charge q at its center to the flux through a spherical surface of radius a with a charge q at its center.
Shari Reply
please I want to know how to solve increase in length
Ujah
Why a charged capacitor has potential difference but not emf
Gideon Reply
what is the dimension symbol of temperature?
Keren Reply
what is the dimension symbol of temperature?
Keren
what's the meaning of enthalpy in terms of latent heat, internal energy, phase change
Anthony Reply
Enthalpy is the degree ofdisorderlinessof a substance
Gbemisola
how to convert Kelvin to centigrade
Sangeetha Reply
what is the s, p, d, f in this table
Sangeetha
s, p, d, f in this table
Sangeetha
what kind of table this
Vengata
Periodic table
Gbemisola
what are waves
KENNETH Reply
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities
Abdikadir
Discuss how would orient a planar surface of area A in a uniform electric field of magnitude E0 to obtain (a) the maximum flux and (b) the minimum flux through the area.
KARAN Reply
I'm just doing the first 3 with this message. but thankyou for the time your obviously intending to support us with. viva la accumulation
Marcel
Find the net capacitance of the combination of series and parallel capacitors shown belo
jean Reply
what is ohm?
Sharafat Reply
Practice Key Terms 1

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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