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As the first half wavelength of the approaching wavefront reaches the atom - PowerPoint PPT Presentation


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As the first half wavelength of the approaching wavefront reaches the atom shown, the E -field. E. c. B. 1) pushes the electron cloud up, the nucleus down. 2) pushes the electron cloud left, the nucleus right. 3) pushes the electron cloud right, the nucleus left.

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Presentation Transcript
slide1

As the first half

wavelength of the

approaching wavefront

reaches the atom

shown, the E-field

E

c

B

1) pushes the electron cloud up, the nucleus down.

2) pushes the electron cloud left, the nucleus right.

3) pushes the electron cloud right, the nucleus left.

4) pushes the electron cloud down, the nucleus up.

slide2

As the electrons move down under the influence of the E-field, the oncoming B-field in the same half wavelength

pushes those electrons

E

1) toward the right of the screen.

2) toward the left of the screen.

3) up.

4) down.

5) forward, in the direction of c.

6) backward, into the screen.

c

B

slide3

As the protons move up under the influence of the E-field, the oncoming B-field in the same half wavelength

pushes those protons

E

1) toward the right of the screen.

2) toward the left of the screen.

3) up.

4) down.

5) forward, in the direction of c.

6) backward, into the screen.

c

B

slide4

Recall

1

2

1

2

1

2

1

2

Energy stored in the electric field within a capacitor: CV2

withC = oA/d and V=Ed

so PEEfield = (oA/d)(Ed)2 = o(Ad)E2

1

2

1

2

uE= oE2

Similarly

Energy stored in the magnetic field of a solenoid: LI2

whereL = NBA/Iand B=moI(N/)

1

2

so PEBfield = (NBA/I)I2 = NBA(B/moN)2

1

2mo

PEBfield = (A)B2

uB= B2

We say the “energy density”

within the magnetic field is

1

2mo

slide5

A

A

cDt

During a time Dt, an electromagnetic wave will

deliver energy to/through any surface A it

illuminates:

AcDt(uB+ uB )

energy transport intensity of em waves
Energy transport (intensity) of EM waves

The rate at which electromagnetic energy is being transferred by an EM wave through a certain area is:

Units: W / m2

Remember!

E and B fields fluctuate with time!

slide7

Compared to wave 1, the power

transported by wave 2 is about

1) half

2) the same

3) double

4) quadruple

slide8

You should recall that the

time average of E(t) is zero.

1

4

1

2

1

2

1

2

Eo2sin

2

Eo

Eosin

Eo

2

Eosin

What is the average E2(t)?

  • 0
  • Eo
  • Eo
  • 4) Eo
  • ( Eo)2
  • Eo2
average intensity and rms values

0

0

0

0

< Emax2 / oc

< Emax2 / oc

< Emax

< Emax

Emax2 / oc

Emax2 / oc

Emax

Emax

Average Intensity and RMS values

Energy

delivered

E field

Because intensity is constantly changing, we work with the average intensity

Average over one full wavelength. Since E varies sinusoidally,

(E2)avg = (1/2)(Emax2).

atomic spectra
Atomic Spectra

But spectrum of a heated rarefied gas shows lines:

Emission

spectrum

And when white light shines through a rarefied gas, dark lines appear in the continuous spectrum:

Absorption

spectrum

Blackbody spectrum is continuous:

Explaining these phenomena requires and understanding of the atom

slide12

1

2

Some answers to previous slides:

1) pushes the electron cloud up, the nucleus down

5) forward, in the direction of c.

5) forward, in the direction of c.

4) quadruple

6)Eo2