Questions/comments about the homework


Mon, Apr 30, 4:21 p.m. - For #10 in Supp CH 11, I don't see how color is conserved.

A BIGGER (and related) issue to think about....why aren't lepton # or baryon # conserved? The answer is basically that when the X boson was around, quarks weren't confined to hadrons yet, so the "rules" of the game were very different. This is kind of like how with W bosons, strangeness doesn't have to be conserved. With X bosons lepton # & baryon # don't have to be conserved.


Wed, Apr 25, 5:32 p.m. - For Supp CH 11 #6, I realize that 1116 has less mass than 1192; hence, 1116 has less rest energy. Does less rest energy mean longer lifetime? I reread the chapter.

Not necessarily. It depends on whether or not a decay can happen via strong, EM, or weak force. Typically, the more massive a particle is, the more likely it can decay via strong force, but that isn't always the case. You need to check the possible decays for the two particles.


Thu, Apr 12, 7:51 a.m. - I'm confused on where to go after I've divided h bar from the given angular momentum value for Supp CH 7 #4

You need to then use equation 7.7 to solve for $l$. Since $n > l+1$, once you know $l$ your minimum possible $n$ will be $l+1$. Then use the equation for energy levels of the H-atom (Equation 7.6) to find the energy for this level.


Thu, Apr 12, 7:08 a.m. - "White light LED bulbs can be made by using blue or violet LEDs that are coated with phosphor (fluorescent) coatings. Explain why it would not be possible to make a white light LED with a red LED as the source." I am confused about this question.

First off, this statement isn't about the physics of LEDs, it is more about the physics of fluorescent coatings. To get "white light" what we need is a mix of blue, green, and red. If we produce a blue LED, we've got blue light covered. Since blue light has the highest energy photons of the 3 colors, we can use blue as the excitation wavelength for a fluorescent coating, allowing red and green photons to be emitted (along with the original blue).


Wed, Apr 11, 2:42 p.m. - Do we use speed of light for A74 (particularly d)? How do we calculate energy of non-photon particles based on wavelength?

ERROR: A74
PROBLEM NOT FOUND. NO PART (d)
PLEASE TRY AGAIN LATER


Tue, Apr 10, 1:45 p.m. - Is A62 1.32e(-45) m/s?

Nope. The answer is 1.3e(-21) m/s. I'm guessing you accidentally used $10^{12}$ m instead of $10^{-12}$ m


Wed, Apr 11, 2:49 p.m. - What is the answer for Supp CH 5 #5?

I get: a) 1/5 b) 1/10 c) 0.5 d) 0


Sun, Mar 4, 9:08 p.m. - For problem A47 what value are we supposed to use for "D" in rayleigh's criteron, I feel like it should be diameter of the human eye but I am not sure

For problem A47, the D in question is the diameter of the aperture of your eye, which is your pupil diameter. Grab a ruler and go look in the mirror to get an idea of the value for this.


Sun, Mar 4, 12:51 p.m. - For A47, it asks "How does your calculated separation compare with Prof. Bowen's estimate of the actual separation between lines (5 cm)?" How are we supposed to compare the two? Do we just have to roughly comment on whether the values are close enough or off by a large factor? Or, do we have to calculate the uncertainty of each measurement, propagate the error, and see if the measurement falls within the average +/- the propagated error? Just to add on to my question about A47, can we just give a percent error to compare the theoretical posed by Prof. Bowen and experimental calculated by us?

Compare it. How close are you? Are you off by 5%, 50%? What do you think could contribute to this difference?


Mon, Feb 26, 6:32 p.m. - What is the answer to CH 14 #39? I don't have the Wolfson Volume 1 with me.

For future reference, the answers for CH14 are also in the back of Volume 2. CH14 #39: (a) 280 Hz (b) 70 Hz (c) 210 Hz


Sat, Feb 17, 9:04 a.m. - Hello, can you please put up the answers for CH 14: 17, 21 (for those who don't have Wolfson Volume 1)? These problems are from last Friday's assigned problem set.

14-17: (a) 300 m; (b) 1.58 m; (c) 3.0cm; (d) 8 microns; (e) 500 nanometers; (f) 3.0 Angstroms.

14-21: (a) 1.3 cm; (b) 9.1 cm; (c) 0.20 s; (d) 45 cm/s; (e) -x direction.