### Math Notes: Cylindrical r Coordinate Value to Trace a Spherical r Dependent on z

OK, so that was quite the title.  I haven't done one of these in a while, but classes are about to start again and i figured I may as well get started deriving things again.  Plus, I had to do it for the can crusher magnet simulation code[1] for the experiment[3].  Here's what's really going on.  I have a Sage function that will give me the magnetic field in the z direction produced by a coil of wire that sits at z = 0 and a has a radius of 'rcoil'.  I'd like to know the magnetic field produced by the loop of wire along a circular path that is perpendicular to the plane of the current carrying coil.  A circular path perpendicular to the plane of a coil kind of begs for spherical coordinates, but the routine I have takes a z coordinate and a radius coordinate in the cylindrical coordinate system.  In the picture above, the circular path is shown, and the coil of wire is at the diameter of the circle and perpendicular to the page.  Note:  For those reading on G+, the equations aren't going to come out correctly, what with the lack of Latex support and all... apologies, but you can get the full version at, and garner the vast majority of it from the figure below.

At each height z above the coil the magnetic field will be the same all the way around the coil for a given spherical radius.  We can exploit this radial symmetry and only calculate the value of the magnetic field once instead of at each point around the circle.  Then, we can simply make a plot of the magnetic field vs z with the understanding that the radius of the sphere is constant and specified.  Here's the transform to get the value of the magnetic field dependent on our position on the z axis, given the radisu of the sphere we're interested in and the z coordinate, (of course... don't you just love saying that?).  The only trick is to figure out what cylindrical value of radius to plug in for each value of z to keep all our points on the spherical circle.
For any point on the sphere, notice that the cylindrical r value can be written as

$r = r_{coil}*cos\theta$,

where $r_{coil}$ is the constant radius of the coil which is also the constant radius of the sphere that we're interested in.  Next, we note that at each point along the spherical path

$\theta = asin\left(\dfrac{z}{r_coil}\right)$

Plugging this into the first expression, we get

$r = r_coil cos\left(asin\left(\dfrac{z}{r_{coil}}\right)\right)$

There's one last little modification.  Arcsin's of other functions always creep me out just a little, so fortunately, there's a trig identity we can use to get rid of this one.  The identity[2] is

$cos\left(arcsin\left(x\right)\right) = \sqrt{1-x^2}$

By plugging this in, we get the final expression

$r = r_{coil} \sqrt{1-\left(\dfrac{z}{r_{coil}}\right)^2}$

Now, for each value of z, I just call the magnetic field routine, passing it the radius of the coil, the value of z, and the expression for r above that depends only on the radius of the coil and z!  The sage code looks like so:

 Bzradius(rcoil, z, curren) = dbcoilbz(rcoil, z, rcoil*(1-(z/rcoil)^2)^0.5, curren)

This gives me nice little graphs like the following.

References:
1.  github Can Crusher simulation project
https://github.com/hcarter333/cancrusher

2.  Trig identities
https://en.wikipedia.org/wiki/Trigonometry_idenities#Compositions_of_trig_and_inverse_trig_functions

3.  The hray experiment
http://copaseticflow.blogspot.com/2014/07/simulations-everywhere-simulations-lab.html

### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### Lost Phone

We were incredibly lucky to have both been in university settings when our kids were born.  When No. 1 arrived, we were both still grad students.  Not long after No. 2 arrived, (about 10 days to be exact), mom-person defended her dissertation and gained the appellation prependage Dr.

While there are lots of perks attendant to grad school, not the least of them phenomenal health insurance, that’s not the one that’s come to mind for me just now.  The one I’m most grateful for at the moment with respect to our kids was the opportunities for sheer independence.  Most days, we’d meet for lunch on the quad of whatever university we were hanging out at at the time, (physics research requires a bit of travel), to eat lunch.  During those lunches, the kids could crawl, toddle, or jog off into the distance.  There were no roads, and therefore no cars.  And, I realize now with a certain wistful bliss I had no knowledge of at the time, there were also very few people at hand that new what a baby…

### Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…