Not related to electromagnetism, but some good clean Physics Phun

Professor of Physics at MIT Peter Fisher helps Conan O'Brien with the ring spin record...





I just had to share when I saw this.

Edit: Why do you think too much vaseline would be a bad thing for the ring spin? Discuss...

found via physicsknits blog

Topic open: DC Circuits

The following topic is now open for questions: Circuits. Anything about resistors, capacitors, currents, batteries, etc.

To pose a question, please post a comment to this post by clicking on the comments link below.

Question: Electric PE could be 0? This is very distrurbing...

I know we have down tons of questions where we have to find where V is zero (or similar like it..)

I never made the connection until now that V =0 means the electric potential energy at that point is 0 too! This is deeply disturbing.

Does this mean that if say a charge move between two other charges.. suddenly, middle of the way, it losts all of its potential energy? It is certain possible for V to be zero, which would imply electric PE be 0 as a consequence.

I am so used to the potential energy in a gravitational sense, where potential energy is 0 only when you hit the bottom.. This is like jumping off the a building then middle of the way, finding all of your potential energy all gone of a sudden. Where did it go?

Disturbing indeed.

Rest assured, physics is not broken, but there are two interesting points to be made here.

Firstly, while the analogy to gravitational energy is a good one, it is incomplete due to the fact that there is no negative mass, but there are indeed negative charges. This means that the potential surface can dip below zero such that a local minimum in potential energy may very well be "beyond zero". Really what the universe seeks is minima, not "zero", especially since we can arbitrarily choose where zero is (think about gravitational energy and choosing the top of a cliff to be at zero PE... one still loses PE if one falls to the bottom).

Secondly, what matters in terms of motion is force, not PE, and that is related to the slope of the potential surface, not the "value" of it.

Let's look at a potential map:



I started this off with some small rectangular regions at set voltage (these could be metal plates held at a potential for example) and then calculated the potential in between. We can see that there are some "hills" (white, or lighter yellow) and some "valleys" (darker shades) and a gradient in between representing a smooth potential surface (ignore the pixellation, I didn't calculate this on a very fine mesh).

In particular, take a look at the region between the highest positive potential and the lowest negative potential. We can see that there is an equipotential line running between them at V=0. But, if we think of this like an elevation map that V=0 crossing is on the middle of a hill! A positive charge (we would invert the map for a negative charge) moving across this potential surface would act like a marble rolling on an elevation map. If we put it on top of the white hill of +2V it would roll down into the -5V valley, still experiencing a force due to the electric field as it passed through 0V.

So, "zero potential" is a somewhat arbitrary thing, and because of negative charges it comes up naturally more often for electrostatic potential that you might be used to from gravitation. Also, the force, the thing which drives motion, is related to the slope (really a directional kind of slope which senses the steepest change).

The very useful thing about electrostatic "potential" is that it gives a convenient way to map out the landscape that a charge will "see" without reference to that charge, just as electric field maps out the force a charge would experience without reference to that charge, and the two are related by slope. Such a map can give an intuitive feel for what will happen when a charge is introduced even for complex arrangements of charges or plates.

I hope that helps. This is far from a complete explanation, so feel free to stimulate further discussion in the comments here!