# Physics H133: Problem Set #3

Here are some hints, suggestions, and comments on the problem set.
## Two-Minute Problems

Remember to give a **good** explanation, no longer than
two sentences.

- Q3T.2: Review the discussion of the wave model predictions in Sec. Q3.3.
- Q3S.2: How is the intensity of the light related to the number
of photons per second? Be careful, the energy per photon depends
on the frequency!

## Chapter Q Problems

- Q3B.5: What can you say about the electron kinetic energy when
they are just ejected? How does W come in?
- Q3S.3: How do you convert a number for power=energy/second into
one for number per second?
- Q3S.4: Similar to the problem done in class.
How is the maximum voltage developed related to the
maximum kinetic energy of electrons liberated? What equation relates
maximum kinetic energy to the wavelength of the incident light? If
you have two data points, is that enough to find hc and W?
- Q3R.1: How many photons per second must the eye register from the star
in order to "see" it? How many photons per second does the star emit?
How does the distance of the star come into the game?
You'll need to estimate
an average energy for the photons from the sun; you can do this
by assuming an "average" wavelength.
- Q3A.1: 4-momentum is a 4-component vector (E/c**2,px,py,pz) which
energy and 3-momentum in the same way as the position 4-vector
(ct,x,y,z) combines time and space. In SR units, c=1, and the momentum
4-vector reads (E,px,py,pz). So in Eq. (Q3.20), the first line describes
energy conservation, the second line px conservation, and so on.
Figure out which of the four terms in Eq. (Q3.20) describes the
4-momentum of the electron (photon) before and after the collision.
For part (d), you must return to SI units by setting E -> E/c**2
in the 4-momentum.

Your comments and
suggestions are appreciated.

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**Physics H133: Hints for Problem Set 3.**

Last modified: 01:53 pm, March 30, 2012.

furnstahl.1@osu.edu