What is B field: Definition and 173 Discussions

A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets. In addition, a magnetic field that varies with location will exert a force on a range of non-magnetic materials by affecting the motion of their outer atomic electrons. Magnetic fields surround magnetized materials, and are created by electric currents such as those used in electromagnets, and by electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, they are described as a map assigning a vector to each point of space or, more precisely—because of the way the magnetic field transforms under mirror reflection—as a field of pseudovectors.
In electromagnetics, the term "magnetic field" is used for two distinct but closely related vector fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter (A/m). B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. H and B differ in how they account for magnetization. In a vacuum, the two fields are related through the vacuum permeability,




B


/


μ

0


=

H



{\displaystyle \mathbf {B} /\mu _{0}=\mathbf {H} }
; but in a magnetized material, the terms differ by the material's magnetization at each point.
Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated and are both components of the electromagnetic force, one of the four fundamental forces of nature.
Magnetic fields are used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is conceptualized and investigated as magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.

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  1. M

    Type 1 Superconductors & Static B field density

    [credit to wikipedia] So it would appear that the type 1 meissner effect allows for the localized increase of magnetic flux density (as the displaced field must go somewhere, the B field is somewhat larger at the surfaces of the superconductor then elsewhere, and in fact is a higher value...
  2. S

    Will a metal rod keep moving if given a slight nudge in a magnetic field?

    imagine a metal rod floating in a region of magnetic field. You give the rod a slight nudge. Will the rod continue to go forever? Considering conservation of energy i think it wouldn't. But i can't come to terms with any reasonable reasoning.
  3. Z

    Does a straight non-time-dependent E field create a circular B field?

    I was under the impression that it does, you know from Ampere's law. But today, my professor told me that Electric and Magnetic fields are very similar (although I can't imagine they are exactly the same, since there are no magnetic monopoles), THAT a non-time dependent E field does not create...
  4. O

    Effect of Earth's B field on an e/m experiment

    I am doing an experiment to measure the charge to mass ratio of an electron. And I have found the method suggested by Thompson to account for the effect of the Earth's magnetic field[http://ajp.aapt.org/resource/1/ajpias/v58/i10/p1019_s1?isAuthorized=no" ]. But it only gives the equation, i.e...
  5. C

    Making Bar Magnets: Heating & B Field Placement

    If I wanted to make bar magnets would I heat up the metal and then place it in a B field of a solenoid and then cool it down in the field to keep the magnetization? It seems if i heated it up first the atoms would be more easily aligned in the B field, And then cool it down to preserve this state.
  6. O

    Energy In B field of infinite line current

    My professor had an example in class the other day where we had an infinite line of current uniformly distributed over a circular wire with radius R. He wanted to look at the energy of the B field produced by this current. He did the normal thing and integrated 1/2u*B^2 over all space. It...
  7. S

    B field of a wire carrying AC current

    Hi. New to the forums so bear with me. I am trying to calculate the magnetic field at a distance r from a wire carrying a 495Hz signal at 8Vpp and current 0.8mA. Initially I tried ampere's law, but fell into the trap of using the un-modified form (without maxwell's correction). I know that...
  8. zimo

    B field at the center of a large charges sheet

    Homework Statement In a plastic film factory, a wide belt of thin plastic material is traveling between two successive rollers with the speed v. In the manufacturing process, the film has accumulated a uniform surface electric charge density σ. What is B near the surface of the belt in...
  9. C

    Can aluminum alter the B field in a dielectric?

    Why do materials like aluminum cut down the value of the B field? Like suppose we had an infinite current carrying wire wrapped with a cylinder of aluminum. To find the B field we would use amperes law and the magnetic susceptibility of aluminum. For example, when a superconductor is...
  10. A

    Square coil rotated in a B field

    Homework Statement A 40 turn square coil has an area 11.0 cm2 and is rotated with an angular speed of 110.0 rads in a uniform 1.00 T magnetic field. The axis of the coil is perpendicular to the field at all times. (i.) What is the frequency of the sinusoidal voltage induced across its...
  11. C

    Conductor in a Static B Field: Impact on Electron Orbitals and Induction"

    What happens if I place a solid conductor in a static B field. I mean B fields can't do work so it won't induce a current unless its a superconductor. Will it shift the electron orbitals because of the Lorentz force? Any input will be much appreciated.
  12. B

    Motion in B Field: Deduce Conditions & Find Angle for x>b

    Homework Statement In a particular experiment, a particle of mass m and charge +q moves with speed v along the x-axis towards increasing x. Between x = 0 and x = b there is a region of uniform magnetic flied B in the y-direction. Deduce the conditions under which the particle will reach the...
  13. B

    Particle Focusing in a Uniform Magnetic Field

    Homework Statement Particles with charge e and mass m are emitted with velocity v from a point source. Their directions of emission make a small angle with the direction of a uniform constant flux density B. Show that the particles are focussed to a point at a distance 2pi mv/Be from their...
  14. B

    Motion in B Field: Solve for Focussing Distance

    Homework Statement Particles with charge e and mass m are emitted with velocity v from a point source. Their directions of emission make a small angle with the direction of a uniform constant flux density B. Show that the particles are focussed to a point at a distance 2pi mv/Be from their...
  15. J

    Induced Current in a Loop - Changing B Field

    Apologies if this is not in the proper location/format/etc; first post. In a very short (3 min) segment on MythBusters, they unsuccessfully tried to induce a current in a loop in the same plane as a current carrying wire (AC). I originally assumed this didn't work because the B field was not...
  16. C

    Electricity and Magnetism: What Causes B Fields from Moving Charges?

    In classical physics when we have a current we have a B field. So let's say we have a charged wire and then we apply a voltage and then current starts to flow. What happened when the charge was stationary and then moving to cause this B field. When the charge is at rest it has an E field, but...
  17. J

    Do E and B Fields Distort When Focusing Photons with a Lens?

    I have read different light frequncies have different lensing characteristics. Example, Gamma frequencies not capable of being focused with a lens. This made me think, what happens to E and B fields, for visible spectrum, when you focus photons with a lens? Anything? Do the fields distort...
  18. L

    How to find B field in paralle plate capasitor

    Homework Statement how to find B field in parallel plate capacitor the plates have a length L and width w and separated a distance h Homework Equations B dl=\mu I The Attempt at a Solution I am trying to find the B files i thought u would use B dl=\mu I but the book has the...
  19. C

    Is the B Field Constant Inside a Toroid?

    Is the B field constant inside a toroid? Like we would just use amperes law to find. B(2*pi*r)=(mu_0)NI But after talking to this kid he said it changes inside. But i thought it was constant.
  20. X

    Find B Field of Helmholtz Coils

    Homework Statement We have 2 circular loops, each of radius R with current I in same direction. This is called a Helmholtz coil. The loop axes are along the z axis and the centers are at z=+/-s/2. Find the magnetic field along the z axis between the 2 loops. Homework Equations...
  21. G

    Does magnetic flux change if the area is extended past the B field?

    Homework Statement A solenoid 2.50 cm in diameter and 30.0 cm long has 300 turns and carries 12.0 A. Calculate the flux through the surface of a disk of radius 5.00 cm that is positioned perpendicular to and centered on the axis of the solenoid. Homework Equations B = (permeability...
  22. C

    B field lines of an infinte solenoid?

    When I have an infinite solenoid , The b field lines point straight up the solenoid and the field is zero outside. But i thought that the field lines have to wrap back around on themselves. Does this not apply in this example . Or am i crazy?
  23. J

    Electron in B Field - Quantum

    Homework Statement Magnetic field in xz plane. \vec{B}=\hat{i}B_x+\hat{k}B_z Write down the hamiltonian operator for the interaction of the electron's intrinsic magnetic moment with this field and express it in matrix form. Find its eigenvalues and sketch these as a function of Bz...
  24. B

    How Is the Resistance of a Falling Wire in a Magnetic Field Calculated?

    Homework Statement A wire hoop is placed in front of a strong magnet, with the magnetic field B=1.1 T, pointing as shown. The strength and direction of the magnetic field are constant through the loop. The sides of the hoop have a track in which a copper wire of length 9.1 cm and mass 25 g, is...
  25. V

    Rotating B Field: Find Angle for Net Field of 99 nT

    Homework Statement Two concentric circular loops of wire carrying current in the same direction lie in the same plane. Loop 1 has radius 1.8cm and carries 3.6 mA. Loop 2 has radius 2.3cm and carries 6 mA. Loop 2 is to be rotated about a diameter while the net magnetic field set up by the two...
  26. Hepth

    E&M : Simulate/Calculate B field due to J

    My end goal is to analytically or numerically calculate the magnetic field inside and around a square conductor that has connection points at the top and bottom with a voltage applied. Are there any software packages (mathematica, or even just C) that can do this given ANY current density in...
  27. H

    Different treatment of E and B field in Brewster angle derivation

    Why does the Brewster angle phenomena happen only when the electric field vector lies in the plane of incidence (the magnetic field vector is perpendicular to that plane) but not when the magnetic vector field lies in the plane of incidence (now the electric field is perpendicular to that plane)?
  28. I

    Is the wire necessary to create a B field?

    Hi, In the Purcell book on E&M that we used during college (berkeley physics course volume 2), the b field of a current carrying wire was explained as the result of a relativistic transformation between two frames of reference... The explanation in purcell seems to require the...
  29. J

    Electron shot with initial velocity B field

    Homework Statement Electron shot in x direction with V_o=3.5*10^6 and is accellerated thru parallel plate capacitor with potential difference of 70V. after it travels on the x-axis for 5 mm what is its total field the electron produces? including magnetic and electric. Show direction and...
  30. F

    How do you find the B field between finite wires?

    I know the results for infinite, but how about finite? And where is the cutoff with the aproximation of infinite? I've heard the value 10% tossed around is that right?
  31. L

    Current needed through solenoid to produce given B field

    1. Magnetic resonance imaging needs a magnetic field strength of 1.5 T. The solenoid is 1.8 m long and 75 cm in diameter. It is tightly wound with a single layer of 2.20-mm-diameter superconducting wire. 2. B = mu0BI / l 3. Is there a way to do this without finding N? Either way...
  32. T

    Exploring the Lorentz Force and Fringing B Field of a Bar Magnet

    Preface: If a bar magnet is cut in half, we are left with say N-S--cut--N-S allinged along the z axis. Then the S and N ends are attracted to each other. This attraction is due to the "fringing" or non-uniform B field of the poles. That is the component of the B field that is normal to the...
  33. Z

    E field induced by changing B field

    Homework Statement The magnetic field inside a 5.0-cm-diameter solenoid is 2.0 T and decreasing at 4.90 T/s What is the electric field strength inside the solenoid at point on the axis?Homework Equations E=(r/2)*(dB/dt)The Attempt at a Solution E=(.025/2)*(4.9)= 0.06125 V/m Not sure what's...
  34. P

    Induced EMF in Changing B Field: A Paradox?

    Does a straight wire in changing B field(the field is perpendicular to the wire) has an induced emf?
  35. M

    How Does Changing Resistance Affect Induced Current Direction in Nested Loops?

    Homework Statement (a) If the resistance of the resistor in the figure below is slowly increased, what is the direction of the current induced in the small circular loop inside the larger loop? (b) What would it be if the small loop were placed outside the larger one, to the left? Homework...
  36. K

    Energy of a magnetic dipole in an external B field

    As in Feynman's lecture on physics vol 2, he showed that the "true energy" of a magnetic dipole(current loop) in an external B field is m*B, not -m*B. Basically the argument is if you want to pull a current loop from infinity to the final position, you must do extra work to keep the current in...
  37. C

    Superconductivity Critical B field, thermodynamical proof

    Homework Statement Use a thermodynamic argument to show that a strong magnetic field will cause a transition from a super-conducting to a normal state. Homework Equations The Attempt at a Solution Most books seem to use, the fact that the gibbs function of the super-conducting...
  38. S

    Induced Current - is there one if inside constant B field?

    Homework Statement A box of wire is moving within a constant B field. Is there an induced current? Homework Equations Slap rule The Attempt at a Solution Since the wire is experiencing constant B field, how would we figure out whether or not there is a current? I know there is a...
  39. S

    Discussion on Feynman lecture - E / B Field

    Greetings, In Feynman's lectures - vol. 2 either chap 19, 21 or 23 - I think 23 (I'm at work and don't have the lectures here), Feynman shows in a capacitor that a changing E-Field induces a B-field, then he shows that the B-field induces a new E-Field (he calculates the E-field) and then...
  40. C

    Anti-matter Bar Magnet B Field: Repulsion & More

    If I have bar magnet made of anti matter is the B field the same as regular matter and would their north poles repel and so on ?
  41. A

    Axially symmetric B field vector potential?

    Suppose you have an axially symmetric magnetic field for which the azimuthal component B_\phi = 0. This is all you know. What are some possible vector potentials \vec A (such that \vec B = \nabla \times \vec A) that would produce this field? (So we're working in cylindrical coordinates.) The...
  42. D

    Change in orbital magnetic dipole moment of the electron due to unifrom B field

    Homework Statement Assume that an electron of mass m and charge magnitude e moves in a circular orbit of radius r about a nucleus. A uniform magnetic field B is then established perpendicular to the plane of the orbit. Assuming also that the radius of the orbit does not change and that the...
  43. A

    B field due to a wire with an alternating current

    Homework Statement I need to find the magnetic field a distance r from a long, thin wire carrying a current I(t) = I_0 \sin \omega t.Homework Equations Field a distance r from a wire carrying a steady current I in the z direction: \vec B(r) = \frac{\mu_0 I}{2 \pi r} \hat \phi The Attempt at...
  44. N

    Electron in circular motion in B field problem check

    Homework Statement An electron of mass Me, with charge -e, is in a circular orbit in the xy-plane. There is a uniform magnetic field B in the positive z direction. It is moving at constant velocity V. Working in CGS units... Find R in terms of Me, -e, v, and B Find the angular...
  45. H

    Ion beam in E and B field problem

    Homework Statement A beam of ions enters a region in which the electric field E and magnetic flux density B are normal to each other and both normal to the velocity of the ions. Show that if the velocity of the ions is related to the fields by v=ExB/B2 then the ions pass through the...
  46. D

    How Does a Moving Conducting Loop React to a Uniform Magnetic Field?

    A region of space d = 35 cm wide is filled with a uniform magnetic field B = 2.3 T oriented normal to the plane of the screen and pointed into the screen. There is no magnetic field to the left or to the right of this region. An external agent drags a rectangular conducting loop with dimensions...
  47. M

    Path of least resistence, static wire in B field or moveable wire?

    I was curious, if the same voltage was applied to two conductors, I guess wired in parallel, and both passed through a magnetic field at a right angle to the field, and one was refrained from moving, while the other was allowed to move - which path would the electrical current travel?
  48. E

    Deriving torque from a B field on a magnetic Loop

    Homework Statement A wire ring lies in the xy-plane with its center at the origin. The ring carries a counter clockwise current I. A uniform magnetic fireld B is the the +x direction. There are a few parts to the question (Note: Everything except for I is a vector): 1. show that dL =...
  49. M

    Why is the b field inside a thick toroid largest along inner edge

    Why is the b field inside a thick toroid largest along inner edge and smallest along outer edge?
  50. I

    Finding Current and B Field given J(p) in Z Direction

    Homework Statement the current density is given by J(p) = (I/pi) * p^2 * e^(-p^2) in z direction The question is to first show that the cureent flowing through the wire is 'I' and then to find then to find the B field. Homework Equations stokes theorem. integral of B.dl = I I =...
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