## Questions/Comments about Induction, Waves, and Light

*Wed, Mar 7, 10:27 p.m.*-

**How do you find the direction of propagation for an electromagnetic wave?**

There are a couple of different ways to get this. 1) If you know the direction of the E and B fields, you can get the propagation direction from $\vec{v} = \vec{E} \times \vec{B}$. 2) You can look at the phase of the wave. The axis along which is wave is propagating is whatever variable you are multiplying $k$ by. And you can determine whether it is travelling in the negative or positive direction by looking at whether you are adding or subtracting $\omega t$. For example, if the expression for the phase is $kx - \omega t$ then the wave is propagating in the +x direction. If the expression for the phase is $kz + \omega t$ then the wave is propagating in the -z direction. The axis is whatever variable you are multiplying

*Tue, Mar 6, 8:15 p.m.*-

**For A93c, I got -.5 rads as my answer. Is this okay? The back of the book lists .5 rad. I got -.5 rads because I had the phase shift of the original two waves as (-3pi/4). The reason why I said positive and not negative is because if you take the first solid wave to be at a phase shift of 0, then the dotted wave has to be (3pi/4) "ahead" of that wave if they are moving to the right, i.e. the solid is 3pi/4 shifted from the dotted wave but the dotted is -3pi/4 shifted from the solid wave. However, I'm not sure if we can even determine the direction in which the wave is moving. Can you please provide some clarification for this problem? Thank you!**

Yes, in this case it is okay. Because we're looking at a snapshot of the waves, we can't tell which direction these waves are traveling, so we don't know which wave "leads" the other.

*Sun, Mar 4, 1:47 p.m.*-

**Can you explain "Got It? 29.3" on page 552? I would have thought that it could propagate in the +x or -x direction (answer c) but the answer says it is b (-x direction). Does it have something to do with the direction of propagation being the cross product of vectors ExB?**

Yes. $\vec{v} = \vec{E} \times \vec{B}$. So, if you take the cross product of E(+y) and B(-z) you get -x.

*Wed, Mar 7, 5:07 p.m.*-

**How does the shaking flashlight work from the most recent lecture? I was confused about this.**

I'm just gonna point you here: https://en.wikipedia.org/wiki/Mechanically_powered_flashlight#Shake_type_design

*Tue, Mar 6, 1:47 p.m.*-

**Was CH 14 #36 unassigned? Are we expected to know this for the test? Will beats be covered on Exam #2?**

CH 14 #36 was unassigned, and we removed the objective for it. You are not expected to know anything about beat frequency for the exam. On the other hand, this beat frequency stuff is super cool and is why certain chords sound the way that they do. If you want to nerd out on this stuff, I'd love to chat about it post-exam!

*Wed, Feb 28, 7:39 p.m.*-

**Is the imaginary part of a wave function not relevant? It seems like we only ever care about the real part (horizontal component).**

It is relevant! Though the real part of the wave is usually the physical oscillation itself, if we don't also keep track of the imaginary part or the wave, we can't correctly deal with interference.

*Sat, Mar 3, 9:40 p.m.*-

**How can I solve problem 2.12 without knowing the temperature of the cavity?**

You don't need the temperature for the cavity. Step through the example that we did in class to find the number of photons in a particular mode, and you should see how this scales with mode.

*Sat, Mar 3, 5:07 p.m.*-

**When using 1/2(Kb)(T), should T be in Kelvin?**

Yes. Since $k_B$ is typically expressed in units of either J/K or eV/K, T must be in Kelvin (K) to get energy units for $1/2 k_B T$.

*Mon, Feb 26, 3:06 p.m.*-

**For A92, should we just have a graph of the added solution? Just the table? What is the correct answer?**

As stated in the problem, you should fill out a table of values and then plot the solution on the graph. If you'd like to check your graph, come by pooled office hours.

*Mon, Feb 26, 10:57 a.m.*-

**How do we know that the screen is a long distance away in CH 32 #34? I do not see where that is specified in the problem or how that can be inferred.**

In CH32 #34, the slit spacing is 1.5 $\mu$m. You are correct that the problem should specify $L$, the distance between the slits and the screen. In this case, the slit spacing is pretty small, so I would just specify "assuming $L >> d$" in your solution.

*Sat, Feb 24, 7:15 p.m.*-

**Are we allowed to use the dsin(theta)=m(lambda)?**

This is a special case formula, so you can use it if you specify why it is applicable in a particular situation. On the other hand, if you go with $\Delta \phi = \frac{2 \pi \Delta r}{\lambda}$, it is applicable in all situations, and we expect for you to know how to use and apply it.

*Sat, Feb 17, 7:54 a.m.*-

**When trying to play the X problems, Mac users (with newer Macs) should be advised to try using Firefox instead of Safari. Newer Macs do not have QuickTime enabled (in Safari): https://support.apple.com/en-us/HT205081.**

Thanks for looking into this. Students that are struggling to get the X problems to work on their personal devices can also use the computers in the library.

*Sat, Feb 17, 7:53 a.m.*-

**The answer to A31(f) is not given.**

(f) $\pm ~ y, \pm ~ x$

Thanks for letting up know, we'll update this for future versions of the supplement.

*Fri, Feb 16, 9:08 a.m.*-

**I cannot play the X1 problem. I get a link to the following error when I try to play the video: https://support.apple.com/en-us/HT205081**

If you can't see it on the calendar page, try going directly to Wolfram link. If that still doesn't work, you can either do this problem on a campus computer (the library has several options) or see the helpdesk to get your quicktime plugin issue resolved.