December catch-up and a lesson

Wow, I sure haven’t posted much this month.  It’ll be the worst month in the short history of this blog if I don’t work hard to catch up.

What do you know…I’ve been busy!  We had another Physics Olympics meet in December, but I did not host it at my school.  I just had to get a team of kids ready to compete!  Also, my online graduate course ended, and I had to take a final exam.  I’ve started a team of 7 kids on a competition called Real World Design Challenge, which is sponsored by the Department of Energy, and I spent a bunch of time on e-mailing and setting up accounts and installing software, etc.  Plus there was all that shopping and wrapping to do, and I even managed to go out twice!

Unfortunately, I have also wasted an inordinate amount of time on Facebook, updating my status and checking up on former students who spent much of December using their Facebook status to count down the days until they were on break (generally about a week before I was going to be on break, of course).  Several of them came to visit at school during the last three days before my break began, and I got to explain to some of my former AP physics students what I learned this semester about Newton’s 3rd Law of motion.  Namely, that it isn’t true.

(This is where you gasp.  Or gape.  Now shut your mouth and keep reading.)

I’m not kidding.  That whole thing about “for every action, there is an equal and opposite reaction…”  you know what I mean, right?  What it is supposed to mean is that when one object exerts a force on another object by interacting with it, the object exerting a force also experiences a force, and it’s the same amount of force but in the opposite direction.  So if I run into the wall, the wall runs into me.  If I push down on the floor, the floor pushes up on me.  These forces are electrical in nature: the electrons in me are repelled by the electrons in the wall, to such an extent that my atoms never even get close to the wall’s atoms (on the atomic scale) and I just bounce right off the wall.  If I am injured and the wall isn’t, that’s simply because I am squishier than the wall.  That’s an example of Newton’s 3rd Law working as expected.

However, let’s look at when this doesn’t work: in certain cases of the magnetic force between moving charged particles.  First, you need to understand a few things.  Magnetic fields are created by moving charged particles (like electrons moving inside atoms) and only interact with moving charged particles.  Any moving charged particle makes a magnetic field.  The earth’s magnetic field is created by moving charged particles in the earth’s core.  The magnetic field of a refrigerator magnet is caused by electrons in large clumps of atoms that are all moving in similar ways, so that their tiny magnetic fields all add up to make one strong enough to defy gravity and hold bits of paper to your refrigerator.

So if a moving charged particle gets into a magnetic field, it experiences a magnetic force that happens to be perpendicular to both the direction the particle is moving in and the direction the magnetic field is pointing.  Direction the field is pointing?  Yep, if you have a bar magnet filled with electrons moving in atoms such that they are adding up all their fields, the field has a direction.  It points away from the magnet at the “North” pole and it points toward the magnet at the “South” pole and alongside the magnet it points in the opposite direction from the way it points at the ends.  It is this directionality of the field that allows a compass needle to point toward the North pole of the earth, enabling you to do your orienteering or navigating or whatever.

So, the moving charged particle moves into the field, and it experiences a force to the side (compared to the way it was moving) because of that perpendicular business.  The field was created by another moving charged particle.  Does that particle experience an equal and opposite force?


In this example, the positive particle is moving to the left and the negative charged particle is moving down.  At a certain instant in time, the two particles are exactly above one another as shown.  The negative charged particle is in the field produced by the moving positive particle, and experiences a force to the right.  The positive charged particle is directly behind the negative charged particle, and in this location there is no magnetic field from the moving negative particle.  No magnetic field, no magnetic force on the positive particle, and goodbye Newton’s 3rd Law!


So I hope you enjoyed this little physics lesson, and maybe I will treat you to another one sometime!

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