Please create an account to participate in the Slashdot moderation system


Forgot your password?
Check out the new SourceForge HTML5 internet speed test! No Flash necessary and runs on all devices. ×

Comment Re:Only 9 in 10 accept evolution? (Score 3, Interesting) 670

Given the poor quality of the questions in that poll, almost any results are possible.

On the subject of poor quality questions, one of the the questions to test the public's knowledge of science was

Electrons are smaller than atoms. (True/False)

46% of the general public said true and, at first, I was thinking that for more than half of the general public to not understand about atoms and electrons was a pretty poor showing.

But then I got to thinking about whether an electron is, in fact, smaller than an atom. Sure, the rest mass of an electron is much smaller than the rest mass of an atom. Maybe that's what the question was trying to ask. But the way the question is worded seems to imply a spatial size. When you're dealing with objects as light as electrons, the whole notion of size is non-intuitive (probability distributions described by wave functions).

Maybe they had their reasons for not simply asking whether an electron was more massive than an atom - or maybe whoever put the survey together some gaps in their own science education.

Comment Re:I still don't get the concept of a Monopole (Score 1) 104

I'm sure I'm ascribing an incorrect visualization to the phenomena, but my image of a magnetic pole is that of a motion in liquid - like a propeller in water - ...

Just to add my two cents, I visualize a magnetic field as three superimposed scalar fields of potential energy.

Classically, potential energy is the integral of force with respect to distance or, equivalently, force is the derivative of potential energy with respect to distance. To use slightly more sophisticated language, force is the gradient of the potential. In three dimensions, imagine a room with hot spots and cold spots. The temperature would correspond to the (scalar) potential energy (field) and arrows indicating changes from hot to cold would correspond to the (vector) force field.

Anyway, an electric field is a force field (rather than a potential (energy) field). The vectors of an electric field give the force on a test charge.

In contrast, a magnetic field is (the superposition of) three potential (energy) fields. Essentially, the orientation of the test magnetic dipole selects which of the three potential energy fields the dipole is interacting with. Further, to rotate the magnetic dipole requires exactly as much energy as the difference between the potential energy fields that are selected.

So, anyway once the orientation of the magnetic dipole has selected a particular combination of the three potential fields to interact with, the translational (as opposed to rotational) force on the dipole will actually be a combination of the gradients of those three potential fields.

To summarize, electric fields give the force on a test particle while a magnetic fields give the (potential) energy of a test particle.

Comment Re:nobel (Score 1) 104

Sure, the equations would be symmetrical, but since the field is not necessarily continuous,...

I was wondering similar things myself but then I got to thinking that the field would only be discontinuous in the classical approximation of a point "charge" and that you'd have to mess with quantum and wave functions to really understand what was going on.

... you could get energy for nothing (imagine that monopole following the field lines around a current carrying wire, getting faster and faster because of the infinite potential in the wire.)

The question of how a magnetic monopole would interact with an external magnetic field is quite interesting but also rather tricky. I assume that there are physicists who have worked it all out precisely but, just off the top of my head, it's not obvious to me what would happen.

Whereas an electric field can be thought of as the force on a test (electric) charge, the meaning of a magnetic field is quite a bit more complicated. For one thing, the force depends on the orientation of the test dipole but, more fundamentally, in a certain informal sense, it is actually the gradient of the magnetic field that determines the force on the dipole.

That is, the magnetic field vectors themselves don't actually indicate the direction of force on a test dipole. In particular, a spatially uniform field (for example, inside a solenoid) will not actually exert any force on a test dipole.

So, what happens to a magnetic monopole in an external magnetic field? The more I think about it the more I realize that I have no idea.

Comment Re:nobel (Score 1) 104

Bigger than that... A real magnetic monopole means real over-unity generators (aka "perpetual motion", aka "free energy").

I had actually been wondering about that myself. Do you have a reference? I did some google searches and looked over the Wikipedia page on magnetic monopoles but didn't see anything about magnetic monopoles violating the laws of thermodynamics.

There's a chance that a magnetic monopole might allow static magnetic levitation (Earnshaw's Theorem) but I haven't actually seen anything definitive on that either so it's pure speculation on my part.

Slashdot Top Deals

Real Programmers don't write in FORTRAN. FORTRAN is for pipe stress freaks and crystallography weenies. FORTRAN is for wimp engineers who wear white socks.