New version page

UF PHY 2049 - Example

Documents in this Course
Subjects

Subjects

25 pages

Images

Images

6 pages

Magnetism

Magnetism

37 pages

Optics

Optics

30 pages

Circuits

Circuits

47 pages

PLAN

PLAN

3 pages

Subjects

Subjects

15 pages

Circuits

Circuits

30 pages

OUTLINE

OUTLINE

6 pages

Circuits

Circuits

22 pages

Light

Light

7 pages

Circuits

Circuits

15 pages

Images

Images

26 pages

PLAN

PLAN

6 pages

Lecture 6

Lecture 6

21 pages

Load more
Upgrade to remove ads

This preview shows page 1-2-3 out of 10 pages.

Save
View Full Document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience

Upgrade to remove ads
Unformatted text preview:

PHY2049: Chapter 331ExampleÎAn observer stands 1.8 m from an isotropic point source of light with power Ps= 250 W. Calculate the rms values of the electric and magnetic fields of the light at the position of the observer.Strategy: source power -> intensity at distance r -> E and B fieldsIntensity I ?Electric field Erms?2rms01EµcI =2244power totalat areaat power averagerπPrπrrIs==≡PHY2049: Chapter 332(continued)B field is very weak. ()()()V/m 488.1)250(308.125041041034780rms==××==−ππrPπµcEsT 106.11034878rmsrms−×=×==cEBPHY2049: Chapter 333Energy of EM WaveÎE/B=c, yet E and B are depicted with the same amplitude. Why?Compare energy densities uEand uBof the E and B fields.uE=uBeverywhere along the EM wave! Energy is shared equally by the E and B fields in the EM wave.2021EεuE=2021BµuB=BEuuu 22emw==(J/m3)PHY2049: Chapter 334(continued)Rewrite in terms of E and B:Energy transported per unit time through a plane drawn perpendicular to the propagation direction of the EM wave (a good definition of intensity): This is why the intensity has been defined to be <S> (Eq. 33-23) and the Poynting vector has the meaning claimed by Eq. 33-20.(J/m3)BEuuu 22emw==EBcµBµu020emw11==>=<>< EBµcu0emw1(J/m3)(m/s)=(W/m2)PHY2049: Chapter 335Momentum of EM Wave & Radiation PressureÎEM waves (e.g. light) carry energy and travel at velocity c. It is natural to expect that they also carry momentum, although they have no mass.Can be shown that they do, but the derivation is above the level of intro physics.ÎMomentum received by an objectCan show (derivation is tough)For total reflection back along path vs total absorption (analogy to completely elastic head-on collision)cU∆p∆ =for total absorptionabsref2 p∆p∆=Eq. 33-29 is misleadingPHY2049: Chapter 336(continued)ÎMomentum received -> Force received -> pressureFrom mechanicsIf EM wave is totally absorbed Intensity Therefore For total reflection back along path cU∆p∆ =for total absorptionabsref2 p∆p∆=impulse=momentum received/givent∆p∆F =At∆U∆I =≡areapower averagecIAt∆cU∆t∆p∆F ===AFpr≡cIpr=cIpr2=(p for pressure, not momentum)PHY2049: Chapter 337ExampleArtist’s view of the Cosmos 1 satellite with the solar sail unfolded. But the launching rocket failed.ÎThe intensity of the solar radiation near the equator is 1370 W/m2at noon. What is the radiation pressure on a completely reflecting surface that directly faces the sun?pr= 2I/c = 2(1370)/3x108= 9.1 x 10-6Unit? 9.1 x 10-6 Pa (Only ~10-10of atmospheric pressure)It is feeble but can be used:Pam NmWs/mm/sW/m222===PHY2049: Chapter 338Radiation PressureÎMore examplesRadiation pressure, together with gas pressure, prevents stars from collapsing due to gravity.Comet tails—shapes and directions.PHY2049: Chapter 339PolarizationÎPolarized EM wavesE field oscillates along a fixed direction, as does the B fieldRadio waves, EM waves for TV broadcastingLaser lightÎUnpolarized EM wavesLight from ‘ordinary’ sources (the Sun, light bulbs, etc.)ÎPeople--most mammals for that matter--cannot detect light polarization, but many insects and cephalopods can, as can some fish and amphibians.PHY2049: Chapter 3310Polarizing filterÎIdeally, absorbs only E field oscillating along one direction, unaffecting E field oscillating along the perpendicular direction, “polarizing” direction.Initially polarized light with an E field amplitude E0. Let θbe the angle between the direction of the E field and the polarizing direction yof the filter. Going through the filter, only the component parallel to the polarizing direction survives.Since light intensity is proportional to E2by definition, after the filter(polarized along y) If the light is initially unpolarized, after the filter(polarized along y)θEE cos0=θII20cos=2cos020IθII


View Full Document
Download Example
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Example and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Example 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?