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How magnetic fields work at the most fundamental level?
What is magnetic force? How can objects affect each other without physically touching like in the case of magnetic attraction (or repulsion)? I'm dying to understand that at the *most* fundamental level.
Any idea?
I think the new startling M-theory (aka SuperStrings theory) could solve that question.
M-theory suggests that *everything*, matter and force, is made up of a single ingredient. An unimaginably small vibrating strands of energy called strings. In that context, the force is a string, so it can move out an reach other strings (matter) and affect them.
The problem is that I don't know if these strings are a real-physical entities or just a mathematical model that helps us understand reality?
If M-theory is right, and if these strings are real-physical entities, then the question would be easily solved.
7 Answers
- 1 decade agoFavorite Answer
The Difference between B and H
There are two quantities that physicists may refer to as the magnetic field, notated \mathbf{H} and \mathbf{B}. Although the term "magnetic field" was historically reserved for \mathbf{H}, with \mathbf{B} being termed the "magnetic induction," \mathbf{B} is now understood to be the more fundamental entity, and most modern writers refer to \mathbf{B} as the magnetic field, except when context fails to make it clear whether the quantity being discussed is \mathbf{H} or \mathbf{B}. See[2]
The difference between the \mathbf{B} and the \mathbf{H} vectors can be traced back to Maxwell's 1855 paper entitled 'On Faraday's Lines of Force'. It is later clarified in his concept of a sea of molecular vortices that appears in his 1861 paper On Physical Lines of Force - 1861. Within that context, \mathbf{H} represented pure vorticity (spin), whereas \mathbf{B} was a weighted vorticity that was weighted for the density of the vortex sea. Maxwell considered magnetic permeability µ to be a measure of the density of the vortex sea. Hence the relationship,
(1) Magnetic Induction Current causes a magnetic current density
\mathbf{B} = \mu \mathbf{H}
was essentially a rotational analogy to the linear electric current relationship,
(2) Electric Convection Current
\mathbf{J} = \rho \mathbf{v}
where ρ is electric charge density. \mathbf{B} was seen as a kind of magnetic current of vortices aligned in their axial planes, with \mathbf{H} being the circumferential velocity of the vortices.
The electric current equation can be viewed as a convective current of electric charge that involves linear motion. By analogy, the magnetic equation is an inductive current involving spin. There is no linear motion in the inductive current along the direction of the \mathbf{B} vector. The magnetic inductive current represents lines of force. In particular, it represents lines of inverse square law force.
The extension of the above considerations confirms that where \mathbf{B} is to \mathbf{H}, and where \mathbf{J} is to ρ, then it necessarily follows from Gauss's law and from the equation of continuity of charge that \mathbf{D} is to \mathbf{E}. Ie. \mathbf{B} parallels with \mathbf{D}, whereas \mathbf{H} parallels with \mathbf{E}.
In SI units, \mathbf{B} \ and \mathbf{H} \ are measured in teslas (T) and amperes per metre (A/m), respectively; or, in cgs units, in gauss (G) and oersteds (Oe), respectively. Two parallel wires carrying an electric current in the same direction will generate a magnetic field that will cause a force of attraction between them. This fact is used to define the value of an ampere of electric current.
[edit] Magnetic field of current flow of charged particles
Current (I) flowing through a wire produces a magnetic field () around the wire. The field is oriented according to the right hand grip rule.
Current (I) flowing through a wire produces a magnetic field (\mathbf{B}) around the wire. The field is oriented according to the right hand grip rule.
Charged particle drifts in a homogenous magnetic field. (A) No disturbing force (B) With an electric field, E (C) With an independent force, F (eg. gravity) (D) In an inhomgeneous magnetic field, grad H
Charged particle drifts in a homogenous magnetic field. (A) No disturbing force (B) With an electric field, E (C) With an independent force, F (eg. gravity) (D) In an inhomgeneous magnetic field, grad H
Substituting into the definition of magnetic field
\mathbf{B} = \mathbf{v}\times \frac{1}{c^2}\mathbf{E}
the proper electric field of point-like charge (see Coulomb's law)
\mathbf{E} = { 1 \over 4 \pi \epsilon_0} {q \over r^2} \hat{r}= {10^{-7}}{c^2} {q \over \ {r}^2} \hat{r}
results in the equation of magnetic field of moving charge, which is usually called the Biot-Savart law:
\mathbf{B} = \mathbf{v}\times \frac{\mu_0}{4 \pi}\frac{q}{r^2}\hat{r}
where
q is electric charge, whose motion creates the magnetic field, measured in coulombs
\mathbf{v} is velocity of the electric charge q that is generating \mathbf{B}, measured in metres per second
\mathbf{B} is the magnetic field (measured in teslas)
[edit] Lorentz force on wire segment
Integrating the Lorentz force on an individual charged particle over a flow (current) of charged particles results in the Lorentz force on a stationary wire carrying electric current:
F = I l \times B \,
where
F = forces, measured in newtons
I = current in wire, measured in amperes
B = magnetic field, measured in teslas
\times = vector cross-product
l = length of wire, measured in metres
In the equation above, the current vector I is a vector with magnitude equal to the scalar current, I, and direction pointing along the wire in which the current is flowing.
Alternatively, instead of current, the wire segment l can be considered a vector.
The Lorentz force on a macroscopic current carrier is often referred to as the Laplace force.
[edit] Properties
[edit] Magnetic field lines
Magnetic field lines shown by iron filings
Magnetic field lines shown by iron filings
The direction of the magnetic field vector follows from the definition above. It coincides with the direction of orientation of a magnetic dipole, such as a small magnet, a small loop of current in the magnetic field, or a cluster of small particles of ferromagnetic material (see figure).
[edit] Pole labelling confusions
See also Magnetic North Pole and Magnetic South Pole.
The end of a compass needle that points north was historically called the "north" magnetic pole of the needle. Since dipoles are vectors and align "head to tail" with each other, the magnetic pole located near the geographic North Pole is actually the "south" pole.
The "north" and "south" poles of a magnet or a magnetic dipole are labelled similarly to north and south poles of a compass needle. Near the north pole of a bar or a cylinder magnet, the magnetic field vector is directed out of the magnet; near the south pole, into the magnet. This magnetic field continues inside the magnet (so there are no actual "poles" anywhere inside or outside of a magnet where the field stops or starts). Breaking a magnet in half does not separate the poles but produces two magnets with two poles each.
Earth's magnetic field is probably produced by electric currents in its liquid core.
It can be more easily explained if one works backwards from the equation:
B=\frac {F} {I L} \,
where
B is the magnitude of flux density, measured in SI as teslas
F is the force experienced by a wire, measured in Newtons
I is the current, measured in amperes
L is the length of the wire, measured in metres
Demonstration of Fleming's left hand rule
Demonstration of Fleming's left hand rule
For a magnetic flux density to equal 1 tesla, a force of 1 newton must act on a wire of length 1 metre carrying 1 ampere of current.
1 newton of force is not easily accomplished. For example: the most powerful superconducting electromagnets in the world have flux densities of 'only' 20 T. This is true obviously for both electromagnets and natural magnets, but a magnetic field can only act on moving charge — hence the current, I, in the equation.
The equation can be adjusted to incorporate moving single charges, ie protons, electrons, and so on via
F = BQv \,
where
Q is the charge in coulombs, and
v is the velocity of that charge in metres per second.
Fleming's left hand rule for motion, current and polarity can be used to determine the direction of any one of those from the other two, as seen in the example. It can also be remembered in the following way. The digits from the thumb to second finger indicate 'Force', 'B-field', and 'I(Current)' respectively, or F-B-I in short. For professional use, the right hand grip rule is used instead which originated from the definition of cross product in the right hand system of coordinates.
Other units of magnetic flux density are
1 gauss = 10-4 teslas = 100 microteslas (µT)
1 gamma = 10-9 teslas = 1 nanotesla (nT)
[edit] Rotating magnetic fields
Main article: Alternator
The rotating magnetic field is a key principle in the operation of alternating-current motors. A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was conceptualized by Nikola Tesla, and later utilised in his, and others, early AC (alternating-current) electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and so, in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case. The ability of the three-phase system to create a rotating field, utilized in electric motors, is one of the main reasons why three-phase systems dominate the world's electrical power supply systems.
Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following the rotating magnetic field of a multicoiled stator. The short-circuited turns of the rotor develop eddy currents in the rotating field of the stator, and these currents in turn move the rotor by the Lorentz force.
In 1882, Nikola Tesla identified the concept of the rotating magnetic field. In 1885, Galileo Ferraris independently researched the concept. In 1888, Tesla gained U.S. Patent 381,968 for his work. Also in 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.
[edit] Hall effect
Main article: Hall effect
Because the Lorentz force is charge-sign-dependent (see above), it results in charge separation when a conductor with current is placed in a transverse magnetic field, with a buildup of opposite charges on two opposite sides of conductor in the direction normal to the magnetic field, and the potential difference between these sides can be measured.
The Hall effect is often used to measure the magnitude of a magnetic field as well as to find the sign of the dominant charge carriers in semiconductors (negative electrons or positive holes).
[edit] Extension to the Theory of Relativity
Einstein explained in 1905 that a magnetic field is the relativistic part of an electric field.[3] It arises as a mathematical by-product of Lorentz coordinate transformation of electric field from one reference frame to another (usually from co-moving with the moving charge reference frame to the reference frame of non-moving observer).
(However, the Lorentz transformation cannot be applied to electric fields unless it already presupposes the existence of magnetic fields and their inter relationship with electric fields under the terms of Maxwell's equations. As such, the magnetic field can hardly be considered as a by-product of the Lorentz transformation.)
When an electric charge is moving from the perspective of an observer, the electric field of this charge due to space contraction is no longer seen by the observer as spherically symmetric due to non-radial time dilation, and it must be computed using the Lorentz transformations. One of the products of these transformations is the part of the electric field which only acts on moving charges — and we call it the "magnetic field". It is a relativistic manifestation of the more fundamental electric field. A magnetic field can be caused either by another moving charge (i.e., by an electric current) or by a changing electric field. The magnetic field is a vector quantity, and has SI units of tesla, 1 T = 1 kg·s-2·A-1. An equivalent, but older, unit for 1 Tesla is Weber/m2.
The quantum-mechanical motion of electrons in atoms produces the magnetic fields of permanent ferromagnets. Spinning charged particles also have magnetic moment. Some electrically neutral particles (like the neutron) with non-zero spin also have magnetic moment due to the charge distribution in their inner structure. Particles with zero spin never have magnetic moment which is the consequence that a magnetic field is the result of motion of electric field.
A magnetic field is a vector field: it associates with every point in space a (pseudo) vector that may vary through time. The direction of the field is the equilibrium direction of a magnetic dipole (like a compass needle) placed in the field.
The Lorentz transformation of a spherically-symmetric proper electric field E of a moving electric charge (for example, the electric field of an electron moving in a conducting wire) from the charge's reference frame to the reference frame of a non-moving observer results in the following term which we can define or label as "magnetic field".
Maxwell did much to unify static electricity and magnetism, producing a set of four equations relating the two fields. However, under Maxwell's formulation, there were still two distinct fields describing different phenomena. It was Albert Einstein who showed, using special relativity, that electric and magnetic fields are two aspects of the same thing (a rank-2 tensor), and that one stationary observer may perceive a magnetic force where a moving observer perceives only an electrostatic force. Thus, using special relativity, magnetic forces are a manifestation of electrostatic forces of charges in motion and may be predicted from knowledge of the electrostatic forces and the velocity of movement (relative to some observer) of the charges.
A thought experiment one can do to show this is with two identical infinite and parallel lines of charge having no motion relative to each other but moving together relative to an observer. Another observer is moving alongside the two lines of charge (at the same velocity) and observes only electrostatic repulsive force and acceleration. The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines of charge more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an added attractive force, in a classical electrodynamics context, that reduces the electrostatic repulsive force and also increases in magnitude with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context.
A changing magnetic field is mathematically the same as a moving magnetic field (see relativity of motion). Thus, according to Einstein's field transformation equations (that is, the Lorentz transformation of the field from a proper reference frame to a non-moving reference frame), part of it is manifested as an electric field component. This is known as Faraday's law of induction and is the principle behind electric generators and electric motors.
[edit] See also
General
* Electric field — effect produced by an electric charge that exerts a force on charged objects in its vicinity.
* Electromagnetic field — a field composed of two related vector fields, the electric field and the magnetic field.
* Electromagnetism — the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field.
* Magnetism — phenomenon by which materials exert an attractive or repulsive force on other materials.
* Magnetohydrodynamics — the academic discipline which studies the dynamics of electrically conducting fluids.
* Magnetic flux
* Magnetic monopole
* SI electromagnetism units
- Robert TLv 41 decade ago
Magnetic force is an force that acts on materials containing iron.
Think of iron as having many tiny magnetic domains. When these domains are aligned in random fashion, the iron does not behave as a magnet. But when they are aligned all in the same direction, or nearly so, you have a magnet. What determines a domain? Atoms in a "cell" that have the same electron spin.
There are a few ways to turn a rod of iron into a magnet. Heat it up, and put it a magnetic field while it cools. Or stroke it with a strong magnet while it cools. Or use a coil of wire to make an "electromagnet" while it cools.
The counterpart to magnetic force is electrical force. Two parallel wires or plates, that are charged oppositely, will display an attractive force. Magnetic and Electric forces are strong forces, whereas gravity is considered a weak force.
- 1 decade ago
This is a tough one. Perhaps magnetic force fields are related to electromagnetic force fields like the ones set up by electrons, this field is what prevents your hand from passing through a solid object in spite of the fact that the solid object is mostly empty space. Like charges repel.
- ?Lv 45 years ago
Electrical engineering About the closest you can get to perpetual motion is, with a superconductor suspended over a magnet. As the earth revolves (and as long as it revolves) the superconductor will rotate 360 degrees every 24 hours.
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- 1 decade ago
no simple answer. basics of magets. when all eletrons spin in the same direction, you have a magnet. that is the basic part. after that you will have to study physics and engineering for a few years to even start to answer that question.
- Anonymous5 years ago
There are lots of people who would make fun of the prospect of changing their fates. This is due to the fact that it believes that nobody gets more that what is put in his destiny.
- Renaissance ManLv 51 decade ago
This question has not been satisfactorily answered yet by the greatest thinkers in history. You are more than welcome to try and solve it.