PHYSICS
PHYSICS
5th Edition
ISBN: 2818440038631
Author: GIAMBATTISTA
Publisher: MCG
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Chapter 27, Problem 93P

(a)

To determine

The energy and momentum of one laser photon if the laser wavelength is 670 nm .

(a)

Expert Solution
Check Mark

Answer to Problem 93P

The energy of the photon is 1.9 eV and its momentum is 9.9×1028 kgm/s .

Explanation of Solution

Write the equation for the energy of the photon.

  E=hcλ                                                                                                                     (I)

Here, E is the energy of the photons, h is the Planck’s constant, c is the speed of light in vacuum, and λ is the wavelength of the photon.

Write the equation for the energy of the momentum of the photon in terms of energy.

  E=pc

Here, p is the momentum of the photon.

Rewrite the above equation for p .

  p=Ec                                                                                                                      (II)

Conclusion:

The value of hc is 1240 eVnm and the value of c is 3.00×108 m/s .

Substitute 1240 eVnm for hc and 670 nm for λ in equation (I) to find E .

  E=1240 eVnm670 nm=1.85 eV1.9 eV

Substitute 1.85 eV for E and 3.00×108 m/s for c in equation (II) to find p .

  p=1.85 eV1.6×1019 J1 eV3.00×108 m/s=9.9×1028 kgm/s

Therefore, the energy of the photon is 1.9 eV and its momentum is 9.9×1028 kgm/s .

(b)

To determine

The number of photons per second emitted by the laser.

(b)

Expert Solution
Check Mark

Answer to Problem 93P

The number of photons per second emitted by the laser is 3×1015 photons/s .

Explanation of Solution

The number of photons per second emitted by the laser is given by the ratio of power of the laser to the energy per photon.

Write the equation for the rate of photon emission.

  rate=PE                                                                                                                (III)

Conclusion:

Given that the output power of the laser pointer is 1 mW .

Substitute 1 mW for P and 1.85 eV for E in equation (III) to find the photon emission rate.

  rate=(1 mW1 W103 mW)(1.85 eV1.6×1019 J1 eV)=1×103 W2.96×1019 J=3×1015 photons/s

Therefore, number of photons per second emitted by the laser is 3×1015 photons/s .

(c)

To determine

The average force on the laser due to the momentum carried away by the emitted photons.

(c)

Expert Solution
Check Mark

Answer to Problem 93P

The average force on the laser due to the momentum carried away by the emitted photons is 3×1012 N .

Explanation of Solution

The impulse momentum theorem states that the impulse is equal to the change in momentum. The impulse is the product of the average force applied and the time interval during which the force acts.

Write the mathematical expression for the impulse-momentum theorem.

  Δp=FavΔt

Here, Δp is the change in momentum, Fav is the average force applied and Δt is the time interval.

Rewrite the above equation for Fav .

  Fav=ΔpΔt                                                                                                               (IV)

Use equation (II) to write the expression for the change in momentum.

  Δp=ΔEc                                                                                                                 (V)

Write the equation for Δt .

  Δt=ΔEP                                                                                                                (VI)

Put equations (V) and (VI) in equation (IV).

  Fav=ΔE/cΔE/P=Pc                                                                                                          (VII)

Conclusion:

Substitute 1 mW for P and 3.00×108 m/s for c in equation (VII) to find Fav .

  Fav=(1 mW1 W103 mW)3.00×108 m/s=1×103 W3.00×108 m/s=3×1012 N

Therefore, the average force on the laser due to the momentum carried away by the emitted photons is 3×1012 N .

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Chapter 27 Solutions

PHYSICS

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