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Chapter 38: Q 45 (page 1115)

The cosmic microwave background radiation is light left over from the Big Bang that has been Doppler-shifted to microwave frequencies by the expansion of the universe. It now fills the universe with 450 photons/cm3 at an average frequency of 160 GHz. How much energy from the cosmic microwave background, in MeV, fills a small apartment that has 95 m2 of floor space and 2.5-m-high ceilings?

Short Answer

Expert verified

Energy is equal to 70.72 MeV

Step by step solution

01

Step 1. Given information is :Universe is filled as 450 photons/cm3Average frequency = 160 GHz

We need to find out the energy from the cosmic microwave background, in MeV, that fills a small apartment that has 95 m2 of floor space and 2.5-m-high ceilings.

02

Step 2. Using the density of photons.

n=NVnisthedensity.V=AhAistheareahistheheightN=nVfnet=Nfavg.fnetisthenetfrequencyandfavg.istheaveragefrequency.fnet=nAhfavg.Enet=hfnetEnet=hnAhfavg.Enet=6.63脳10-34450脳106Ah160脳109HzEnet=1.133脳10-11J=70.72MeV

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Most popular questions from this chapter

In the following Figure is an energy-level diagram for a simple atom. What wavelengths, in nm, appear in the atom鈥檚 (a) emission spectrum and (b) absorption spectrum?

Compton scattering is relevant not only to x-ray photons but, even more so, to higher energy gamma-ray photons. Suppose a350keV gamma-ray photon backscatter (i.e., is scattered back toward the source) from a free electron. Afterward, what is the electron鈥檚 velocity in m/s?

The electron interference pattern of Figure 38.12 was made by shooting electrons with 50keVof kinetic energy through two slits spaced role="math" localid="1650737433408" 1.0渭尘apart. The fringes were recorded on a detector 1.0mbehind the slits.

a. What was the speed of the electrons? (The speed is large enough to justify using relativity, but for simplicity do this as a nonrelativistic calculation.)

b. Figure 38.12 is greatly magnified. What was the actual spacing on the detector between adjacent bright fringes?

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FIGURE Q38.9 is a simulation of the electrons detected behind two closely spaced slits. Each bright dot represents one electron. How will this pattern change if

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d. The left slit is closed?

Your answers should consider the number of dots on the screen and the spacing, width, and positions of the fringes.
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