### Motors!!!

One of the kids' friends asked about magnets a few weeks ago.  This led to three weeks worth of play dates on electric circuits, electromagnets, and last but not least, Motors!!!  (It's nice to have a physicist in the family).

The motors were amazingly simple to put together, so I’m including the instructions.  Here’s a picture so we have something to talk about.

The parts are:
1 D cell battery
1 pound of 18 awg magnet wire, (you don’t need the whole pound, but Amazon sells It by the pound… seriously)
1 piece of cardboard out of the side of a box,
1 magnet
scotch tape

The How
Tape the D cell to the cardboard so it can’t move.  Next, cut two 4 inch pieces of magnet wire.  The next step is a bit of work, but use a kitchen knife, or a piece of sandpaper to scrape off the red insulation until you just see bare copper wire.  Place a dime size loop in one end of the wire, and then bend it over at a right angle to the rest of the wire to serve as a foot.  In the other end of the wire, make at least two pencil diameter loops to hold up the motor’s rotor, (the large coil of wire shown in the picture).  Tape the feet you made to the cardboard on either side of the battery, then tape the two wires to either side of the battery so that each of them is in contact with one of the poles, (the silver ends), of the battery.  Wrap the tape around a few times, and wrap it tight to make sure each wire is actually touching the pole.

Next, wrap magnet wire around the D cell to make the rotor coil.  Make about 10 turns.  This coil of wire is going to be the motor’s rotor, (a fancy name for the spinning bit).  Tie the ends of the wire through the loop so the loop can’t unravel, and leave about two inches of wire sticking out from opposite sides of the loop as shown in the picture.

Here’s the tricky bit.  Strip half the insulation off the pieces of wire sticking out from the coil.  Make the ends of the wire look like the diagram below.

If you strip off all the insulation, the motor won’t work.  The wire extending from the rotor needs to make electrical contact with the pencil diameter wire hoops attached to the battery only half the time.  It’ll spin through half a circle making electrical contact to the two posts you made earlier, and then during the next half circle it won’t.  Watch the wire on the left side of the rotor in the video . As it spins, you'll see that the side with insulation is visible, and then the side with bare metal.  (More on this in the ‘Why It Works’ section).

Now, insert the rotor into the pencil diameter hoops as shown in the picture.  Place the magnet on the battery pointed up at the rotor.  Then, give the rotor a little spin to get it moving, and it should spin happily around all on its own.

Next Time: Why it works

All the stuff for making motors:

### 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…

### The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simpl…

### Kids R Kapable

Just a little note to concerned ‘grownups’ everywhere.  If you look at a kid—and I mean really look—I don’t mean notice a person shorter than you, I mean make eye contact, notice their facial expression and observe their body language—If you look at a kid, don’t assume they need your help unless they’re obviously distressed, or ask for it.  You might think this is difficult call to make.  You might think, not having kids of your own, that you’re unable to make this determination.  You are.  You do in fact, already have the skills even if you’ve never been around kids  It’s a remarkably simple call to make, just use the exact same criteria you would for determining if an adult was in distress.  Because, guess what, kids and adults are in fact the same species of animal and communicate in the same way.  Honest.  If someone—adult or child—doesn’t need your help, feel free to say hello, give a wave, give a smile, but don’t—do not—try to force help on anyone that doesn’t want or need it.

Y…