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University Physics Volume 3

Conceptual Questions

University Physics Volume 3Conceptual Questions

Conceptual Questions

6.1 Blackbody Radiation


Which surface has a higher temperature – the surface of a yellow star or that of a red star?


Describe what you would see when looking at a body whose temperature is increased from 1000 K to 1,000,000 K.


Explain the color changes in a hot body as its temperature is increased.


Speculate as to why UV light causes sunburn, whereas visible light does not.


Two cavity radiators are constructed with walls made of different metals. At the same temperature, how would their radiation spectra differ?


Discuss why some bodies appear black, other bodies appear red, and still other bodies appear white.


If everything radiates electromagnetic energy, why can we not see objects at room temperature in a dark room?


How much does the power radiated by a blackbody increase when its temperature (in K) is tripled?

6.2 Photoelectric Effect


For the same monochromatic light source, would the photoelectric effect occur for all metals?


In the interpretation of the photoelectric effect, how is it known that an electron does not absorb more than one photon?


Explain how you can determine the work function from a plot of the stopping potential versus the frequency of the incident radiation in a photoelectric effect experiment. Can you determine the value of Planck’s constant from this plot?


Suppose that in the photoelectric-effect experiment we make a plot of the detected current versus the applied potential difference. What information do we obtain from such a plot? Can we determine from it the value of Planck’s constant? Can we determine the work function of the metal?


Speculate how increasing the temperature of a photoelectrode affects the outcomes of the photoelectric effect experiment.


Which aspects of the photoelectric effect cannot be explained by classical physics?


Is the photoelectric effect a consequence of the wave character of radiation or is it a consequence of the particle character of radiation? Explain briefly.


The metals sodium, iron, and molybdenum have work functions 2.5 eV, 3.9 eV, and 4.2 eV, respectively. Which of these metals will emit photoelectrons when illuminated with 400 nm light?

6.3 The Compton Effect


Discuss any similarities and differences between the photoelectric and the Compton effects.


Which has a greater momentum: an UV photon or an IR photon?


Does changing the intensity of a monochromatic light beam affect the momentum of the individual photons in the beam? Does such a change affect the net momentum of the beam?


Can the Compton effect occur with visible light? If so, will it be detectable?


Is it possible in the Compton experiment to observe scattered X-rays that have a shorter wavelength than the incident X-ray radiation?


Show that the Compton wavelength has the dimension of length.


At what scattering angle is the wavelength shift in the Compton effect equal to the Compton wavelength?

6.4 Bohr’s Model of the Hydrogen Atom


Explain why the patterns of bright emission spectral lines have an identical spectral position to the pattern of dark absorption spectral lines for a given gaseous element.


Do the various spectral lines of the hydrogen atom overlap?


The Balmer series for hydrogen was discovered before either the Lyman or the Paschen series. Why?


When the absorption spectrum of hydrogen at room temperature is analyzed, absorption lines for the Lyman series are found, but none are found for the Balmer series. What does this tell us about the energy state of most hydrogen atoms at room temperature?


Hydrogen accounts for about 75% by mass of the matter at the surfaces of most stars. However, the absorption lines of hydrogen are strongest (of highest intensity) in the spectra of stars with a surface temperature of about 9000 K. They are weaker in the sun spectrum and are essentially nonexistent in very hot (temperatures above 25,000 K) or rather cool (temperatures below 3500 K) stars. Speculate as to why surface temperature affects the hydrogen absorption lines that we observe.


Discuss the similarities and differences between Thomson’s model of the hydrogen atom and Bohr’s model of the hydrogen atom.


Discuss the way in which Thomson’s model is nonphysical. Support your argument with experimental evidence.


If, in a hydrogen atom, an electron moves to an orbit with a larger radius, does the energy of the hydrogen atom increase or decrease?


How is the energy conserved when an atom makes a transition from a higher to a lower energy state?


Suppose an electron in a hydrogen atom makes a transition from the (n+1)th orbit to the nth orbit. Is the wavelength of the emitted photon longer for larger values of n, or for smaller values of n?


Discuss why the allowed energies of the hydrogen atom are negative.


Can a hydrogen atom absorb a photon whose energy is greater than 13.6 eV?


Why can you see through glass but not through wood?


Do gravitational forces have a significant effect on atomic energy levels?


Show that Planck’s constant has the dimensions of angular momentum.

6.5 De Broglie’s Matter Waves


Which type of radiation is most suitable for the observation of diffraction patterns on crystalline solids; radio waves, visible light, or X-rays? Explain.


Speculate as to how the diffraction patterns of a typical crystal would be affected if γ-raysγ-rays were used instead of X-rays.


If an electron and a proton are traveling at the same speed, which one has the shorter de Broglie wavelength?


If a particle is accelerating, how does this affect its de Broglie wavelength?


Why is the wave-like nature of matter not observed every day for macroscopic objects?


What is the wavelength of a neutron at rest? Explain.


Why does the setup of Davisson–Germer experiment need to be enclosed in a vacuum chamber? Discuss what result you expect when the chamber is not evacuated.

6.6 Wave-Particle Duality


Give an example of an experiment in which light behaves as waves. Give an example of an experiment in which light behaves as a stream of photons.


Discuss: How does the interference of water waves differ from the interference of electrons? How are they analogous?


Give at least one argument in support of the matter-wave hypothesis.


Give at least one argument in support of the particle-nature of radiation.


Explain the importance of the Young double-slit experiment.


Does the Heisenberg uncertainty principle allow a particle to be at rest in a designated region in space?


Can the de Broglie wavelength of a particle be known exactly?


Do the photons of red light produce better resolution in a microscope than blue light photons? Explain.


Discuss the main difference between an SEM and a TEM.

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