145 votes
Accepted

Why is there no magnetic field around a wire connected to electricity at home?

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 ...
StessenJ's user avatar
  • 1,506
129 votes

If you are vacuuming your carpet and you wrap the cord around your body do you become a magnet?

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 ...
rob's user avatar
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127 votes
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If you are vacuuming your carpet and you wrap the cord around your body do you become a magnet?

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 ...
Lambda's user avatar
  • 4,721
89 votes
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If magnetic field lines don't exist, what are these iron filings doing around a magnet?

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 ...
knzhou's user avatar
  • 102k
83 votes
Accepted

How are magnets held together, and why do they not explode?

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 ...
Allure's user avatar
  • 20.5k
63 votes

Why does the Earth even have a magnetic field?

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 ...
leftaroundabout's user avatar
63 votes
Accepted

Why are the number of magnetic field lines finite in a particular area?

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 ...
Solomon Slow's user avatar
  • 14.6k
60 votes
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Why do we tap on cardboard to see magnetic field lines?

It’s like shaking a measuring cup half full of sugar to make it level out—in both cases there’s an energetically favored configuration you’re trying to reach, but without agitation, friction prevents ...
Ben51's user avatar
  • 9,705
59 votes

Is there a more "physically mature" way to think about the right hand rule with electromagnetism?

To expand on hft's answer: in my opinion, the most "physically mature" way to think about this is to internalize the (rather Zen) understanding that pseudovectors (like torque or magnetic ...
tparker's user avatar
  • 47.5k
50 votes
Accepted

Why are magnetic field lines imaginary?

Most of us will have experimented with placing iron filings around a magnet to get this sort of thing: This particular example is taken from Why iron filings sprinkled near a bar magnet aggregate ...
John Rennie's user avatar
48 votes
Accepted

Is the presence of a magnetic field frame-dependent?

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, ...
probably_someone's user avatar
44 votes

Why is Ampère's law violated if there are no fringe fields?

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 ...
Farcher's user avatar
  • 95.9k
42 votes

What is the difference between the magnetic $H$-field and $B$-field?

In layman's terms, E and B are the total electric and magnetic fields. D and H are the free electric and magnetic fields. P and M are the bound electric and magnetic fields. M would be the ...
SmallRed's user avatar
  • 553
41 votes

Is there just one EM field for the whole universe?

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 ...
knzhou's user avatar
  • 102k
40 votes
Accepted

Why are real photons so much less efficient in carrying momentum than virtual photons?

Within the usual handwavy accounts of virtual particles, the answer is "simple": Virtual particles aren't required to obey on-shell mass-energy relations (in this case $E=pc$), so there can ...
ACuriousMind's user avatar
  • 125k
36 votes
Accepted

Can a magnetic field exist without an electric field present?

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. ...
Phil Frost's user avatar
  • 4,161
36 votes

What is happening when magnetic field lines snap or break?

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) ...
honeste_vivere's user avatar
35 votes
Accepted

Is there an underlying physical reason why the Coriolis force is similar to the magnetic component of the Lorentz force?

As nobody has done this yet, let's try to give an answer to your question in the right framework, i.e. through the formalism of differential geometry (and of action principles, as far as the physics ...
Giorgio Comitini's user avatar
35 votes

Why are the number of magnetic field lines finite in a particular area?

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 ...
sintetico's user avatar
  • 3,513
35 votes
Accepted

What is the magnetic flux through a trefoil knot?

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 ...
Chiral Anomaly's user avatar
35 votes

If a compass is placed near the middle of the magnet, where will the compass needle point?

Let's find out! Here, I have a compass. The red end points to the Earth's North Pole, so that is the north side of the magnet by definition. Ignore the dial, since I've it rotated it to make the ...
Mark H's user avatar
  • 24k
33 votes
Accepted

Elementary Particle of Magnetic Field

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 ...
John Rennie's user avatar
32 votes
Accepted

Why are magnetic fields a consequence of special relativity?

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. ...
Zo the Relativist's user avatar
32 votes

Is the presence of a magnetic field frame-dependent?

This is a fantastic question, and you'd probably become famous for trying to solve it if it weren't for the fact that Einstein asked the same question over 100 years ago. The first sentence in his ...
Bob Knighton's user avatar
  • 8,460
32 votes

Why is there no magnetic field around a wire connected to electricity at home?

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 ...
hdhondt's user avatar
  • 10.9k
32 votes

In what sense, physically speaking, are electric and magnetic fields perpendicular?

The electric and magnetic fields are generally not perpendicular. Presumably you're thinking of electromagnetic waves propagating in vacuum, in which case the electric and magnetic fields are ...
J. Murray's user avatar
  • 69.1k
29 votes

How specifically does an MRI machine build an image from received radio waves?

The key to understanding image generation in MRI lies in realizing that the signal sent to the antenna (or coil) by the patient's tissues includes two different types of information: Information ...
Antoni Parellada's user avatar
29 votes

Why is there no magnetic field around a wire connected to electricity at home?

There are a number of reasons as to why you have been unable to detect the magnetic field due to a mains cable carrying current. First of all it might help to be conversant with some of the ...
Farcher's user avatar
  • 95.9k
29 votes
Accepted

Do magnetic field lines originate from the same point on a magnet?

There is no particular "point of origin" in case of magnetic field lines. This is because magnetic field lines form continuous closed loops. Even inside permanent magnets (like a bar magnet),...
Dodo's user avatar
  • 621

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