# Tag Info

137

There are two wires, the second wire carries the equal (!) return current. The magnetic fields from the two wires cancel out, except at very short distance. For measuring the current from the field you must clamp only one of the wires. The field at a distance can be further reduced by twisting the wires ("twisted pair") or by adopting a coaxial structure ("...

127

No. The power cord on a vacuum cleaner has both supply and return conductors, which produce opposing magnetic fields. The region of nonzero magnetic field is limited to a few cable diameters away from the cable. Also the magnetic field due to an alternating current changes direction at 50-60 Hz, depending on your local power supply frequency. If you ...

125

Earth has a magnetic field. You can verify this yourself; it is why a compass works. Just take any magnet and hang it carefully from a string. As long as there's nothing else magnetic around and it's well-balanced and free to rotate, it will line up with Earth's magnetic field. We have measured the Earth's magnetic field all over the surface and up into ...

125

Okay, accuse me of having too much time on my hands, but here's what I did: If you can't tell from the pic. I wrapped the vacuum cord around a steel bar. I turned on the vacuum and tried to pick up the screw. Absolutely nothing not even a hint of attraction so maybe BowlofRed has a point. In case the comment gets deleted later, here is BowlofRed's comment: ...

81

Here's a map of the barometric pressure in the United States. The map contains isobars, which are lines of constant pressure. These are constructed by starting from an arbitrary point, and following the direction where the pressure doesn't change. Isobars don't "exist", in the sense that there isn't literally a big white line in the sky hovering over New ...

78

Individual iron filings will align their long dimension with the magnetic field. But then they will also feel the induced magnetization in other iron particles nearby, and they will tend to move toward each other till their points touch. This is what creates the strings of iron particles. When the needles cannot move from their site, one does not get lines.

78

The chemical bonds of the material keep it together. If the magnets you're thinking of are made of metal, then the chemical bond is the metallic bond, which is quite strong. You can get a sense of how strong it is if you try to rip a metal bar into two. Unless you are exceptionally strong, you probably won't manage – but you are probably able to pull a bar ...

66

The Earth's climate isn't quite as stable as you think. The Earth's climate has toggled back and forth between a greenhouse Earth and an icehouse Earth for the last 600 million years or so. During the icehouse Earth phases, the climate can enter an ice age, an extended period of time during which the climate in oscillates between glaciations and ...

61

why there are gaps between the the iron filling lines? Iron filings are ferromagnetic. They don't just show the field, they change it. ...hence the iron filings align themselves to stronger field lines. The filings self-organize into distinct lines because their presence concentrates the field. Magnetic field lines prefer to go through a ferromagnetic ...

59

Most electromagnetic radiation is of very high frequency - the magnetic field changes many times per second. This means that the compass just doesn't have time to "follow" the magnetic field changes. The only thing that does affect a compass is a DC magnetic field - usually this is a large piece of iron etc. that gets magnetized (e.g. by the earth's ...

59

The crucial part is that earth's outer core is fluid, and that it's conductive. That the material happens to be iron which we know as ferromagnetic is actually rather unimportant, because the geomagnetic field is not created as a superposition of atomic spins like in a permanent magnet. Rather, it's generated via Ampère's law from macroscopic currents,...

51

$\def\VA{{\bf A}} \def\VB{{\bf B}} \def\VJ{{\bf J}} \def\VE{{\bf E}} \def\vr{{\bf r}}$The Biot-Savart law is a consequence of Maxwell's equations. We assume Maxwell's equations and choose the Coulomb gauge, $\nabla\cdot\VA = 0$. Then $$\nabla\times\VB = \nabla\times(\nabla\times\VA) = \nabla(\nabla\cdot\VA) - \nabla^2\VA = -\nabla^2\VA.$$ But \nabla\...

42

You are indeed correct about the frame-dependence of magnetic fields. The reason the point charge doesn't affect the compass is because the compass and the charge are both moving at the same speed, both being on the Earth, and therefore, the compass sees the charge as stationary. This means no magnetic field is produced. As a side note, you hit upon an ...

41

Although the relationship between special relativity and magnetic fields is often stated as making magnetic fields irrelevant, this is not quite the correct way to say it. What actually disappears is the need for magnetic attractions and repulsions. That's because with the proper choice of motion frames a magnetic force can always be explained as a type of ...

40

You can define a "wind field" for the Earth by putting a weather vane at every point. You've probably seen drawings of these wind fields in weather reports; you can even define 'wind field lines' in analogy with electric and magnetic field lines. Then a completely analogous question is, "is there just one wind field on the Earth, or does every storm have its ...

40

Consider the two paths $ABCDA$ and $EFGHE$. Path $AB$ contributes a positive value to the $\vec B\cdot d\vec l$ integral but the other parts of the loop contribute nothing, so overall there is a finite value for the $\vec B\cdot d\vec l$ integral but no enclosed current which violates Ampere's law. Again path $EF$ contributes a positive value to ...

37

Electric forces are attractive or repulsive forces between "charged objects", e.g. comb and dry hair after some friction. Charged objects are those that carry some nonzero electric charge $Q$. The lightest – and therefore easiest to move – charged particle is the electron so the surplus or deficit of electrons is the most typical reason why some objects are ...

34

The number of field lines is not a meaningful physical quantity, but only a useful tool to visualize the magnetic of electric fields. It is not a meaningful quantity because it is not measurable, for the reason that, as you said, "One can draw/imagine as many unique (curved/straight)lines as he/she wants in some specified finite area (assuming that each ...

33

The gauge boson associated with the magnetic field is the photon. Electric and magnetic fields are in effect different views of the same thing, i.e. the electromagnetic field, and the gauge boson for the electromagnetic field is of course the photon. Consider you are looking a static charge, which obviously has just a static electric field. But now suppose ...

32

As you already indicated, physical units need to be considered. When working in SI units, the ratio of electric field strength over magnetic field strength in EM radiation equals 299 792 458 m/s, the speed of light $c$. However, the numerical value for $c$ depends on the units used. When working in units in which the speed of light $c=1$, one would ...

32

My understanding is that field lines are just a visualization tool showing points of equipotential magnetic moment tangent to the line. Yes, field lines are just visualization tools we (humans) invented, they are not physical objects. I don't understand, intuitively, how an equipotential line could snap or break, or why that would result in a release of ...

31

Special relativity makes the existence of magnetic fields an inevitable consequence of the existence of electric fields. In the inertial system B moving relatively to the inertial system A, purely electric fields from A will look like a combination of electric and magnetic fields in B. According to relativity, both frames are equally fit to describe the ...

31

Let's forget about anything quantitative at all. Special relativity gives you length contraction -- so, when you're moving at a certain speed, distances along your direction of motion are compressed. Amongst many other things, this means that volumes will shrink, which also means that densities will increase. Now, electromagnetism tells us that the ...

31

I can see 2 possible reasons why your phone does not detect a magnetic field. First, a magnetic field is generated when current flows. A simple wire carries no current. So, unless the wire is part of an active circuit (e.g. a lamp that is turned on) there will be no magnetic field. Even if there is a current flowing, it will be a 50 Hz current, so the ...

31

Every knot is the boundary of an orientable surface. Such a surface is called a Seifert surface.$^\dagger$ For any given knot (with a given embedding in 3-d space), the flux is the same through two such surfaces. As usual, the flux can be calculated either by integrating $\mathbf{B}$ over the surface, or by integrating $\mathbf{A}$ around the knot. Figure 6 ...

30

So then you get moving electrons and all of a sudden you have a "magnetic" field. But at the same time if you take a magnetic dipole (a magnet as we know it) and move it around you will all of sudden get an electric field. It was a great step forward in the history of physics when these two observations were combined in one electromagnetic theory in ...

30

The "magnetic field" is a concept within classical electrodynamics. Maxwell's equations were developed in the mid 19th century at a time where basic atomic physics was still a nascent field of study. Viewed in the contemporary historical context, a permanent magnet is a perfectly fine example of a magnetic field without an electric field. Within the theory ...

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