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,







{\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. T

    I Does B=0 at points out of the plane containing and normal to a point current I?

    Consider a point current I flowing at origin in the positive z direction. Biot Savart Law states that B field must move in an anticlockwise circle everywhere with the infinite line that the direction vector of the current, in this case the z axis, at the center. And its strength must be equal at...
  2. R

    B Seeing both B field lines and E field lines at the same time

    After watching this clip Electric Field Lines Lab I wonder if it is possible to see both electric field lines and magnetic field lines at the same time by swapping the two nails in the video with two bar magnets, as the conductors as we understand bar magnets are metals and metals are good...
  3. rmrribeiro

    Wrong B-Field Result: What Should I Have Done?

    I integrated B within the limits of a (from 0 to 0.007) teh result was 3.64E-10 T and it was wrong. the correcto one would be 5.8 E-4 T and it is a major diference (aprox 1 million times ) Waht shoud I have done? Regards
  4. FFXT

    I Faraday induction in constant B field, with non-conduction wires

    A standard textbook problem features a constant B field and a conducting loop that increases in area at constant rate. It is easy to work out the induced EMF and the associated electric field magnitude and direction (CW or CCW). The magnitude of the E field is E = B v where v is a velocity...
  5. A

    A charged conducting ring rotating B field -- referece frames

    Admittedly I found similar threads here already but due to my rather lacking math skills I wanted to go through this myself. As for the math side, I see various different equations with which this is treated can someone please provide the formulas for calculating B field from a rotating charged...
  6. A

    B field around a wire, single wire electromagnet

    A quick description. A single straight wire and a second straight wire, both wires are electrically as well as physically separated, the physical separation distance assume is very small in order for the B field experienced by the second wire to be sufficiently strong. In all cases one of the...
  7. G

    I Problem about the usage of Gauss' law involving the curl of a B field

    I am trying to derive that $$\nabla \times B=\mu_0 J$$ First the derivation starts with the electric field $$dS=rsin\varphi d\theta r d\varphi $$ $$ \iint\limits_S E \cdot dS = \frac{q}{4 \pi \varepsilon_0} \iint\limits_S \frac{r}{|r|^3} \cdot dS $$...
  8. E

    Function for the movement of a charged particle in a B field

    The movement in the z-direction is easy to solve for, as it's only affected by the gravitational force. However, if there's a magnetic field pointing down along the z-axis, the particle is going to be accelerated along the y-axis (F=q*v *B). The force is always going to be perpendicular to the...
  9. F

    B field between the plates of a charging capacitor (Ampere's law)

    A standard example consider a capacitor whose parallel plates have a circular shape, of radius R, so that the system has a cylindrical symmetry. The magnetic field at a given distance r from the common axis of the plates is calculated via Ampere's law: \oint_\gamma {\mathbf B} \cdot d{\mathbf...
  10. jisbon

    Rotating cylinder rolling without slipping in a B field

    Firstly, I need to determine what the electric field is causing. Using left hand rule, the force due to the field is acting down the slope. Hence my FBD looks like: Where the two arrows pointing towards the right represent the force due to the field and weight of the cylinder. Since ...
  11. ChrisXenon

    I E & B field phase relationship in EMR

    Common diagrams for the magnetic and electric field components of EMR show the fields at right angles in space with peaks aligned along the axis of propogation, for example Wikipedia here: https://en.wikipedia.org/wiki/Electromagnetic_radiation. However, Faraday's law says the E field depends...
  12. zehkari

    Find the position of a proton in an E and B field

    Hello all, I have a question with the helix path of proton in a magnetic field that I am a bit stuck on. Question: Equations: F = qv X B F = mv^2/r d=vt My Attempt: Think the graph drawn is good enough for questions (a). However, I am stuck on (b) and (c). Firstly I am not entirely sure...
  13. Spinnor

    B field of 1/2 infinite solenoid, equivalent current confguration

    I have wondered if there is a symmetric current configuration that gives the magnetic field of a half-infinite solenoid. With some thought I think I came up with such a configuration of current loops that produces the same magnetic field as a half-infinite solenoid Suppose we have a large but...
  14. J

    B field and H field confusion, physics GRE

    I'm getting ready for the exam that's coming up and I just took the 2017 practice exam to see what the last gaps are that I need to fill. Question 96 of this exam asks: "The magnetic field inside a long coil of wire (solenoid) has a certain magnitude and direction when the coil is air filled...
  15. G

    B Coil B field strength with different diameters

    Hi, I have a quick question. Say I have two metal rods or tubes and I want to make an electromagnet, one rod is with a diameter x and the other rod has a diameter of x2 (two times larger) Now I take the same gauge copper wire and wrap an equal amount of turns around each of the two rods. Then I...
  16. thee qs

    Radius of a particle's path in a B field and magentic momentum force

    ok so 1- the magnetic momentum is = to u in the k axis ( xyz - ijk ) and the magnetic field B = -A/z4 + Be^Cz) , also in the k axis orientation so the magnetic force F , that is applied on the magnetic momentum is given by 4 choices ; and as i can understnd it, 3 of them are...
  17. S

    Is there a B field in a charging spherical capacitor?

    Suppose a spherical capacitor is being charged. In this case the E field between the plates is growing with time which implies a displacement current which in turn implies a B field. How would one find this B field if it does exists? I'm guessing the B field is zero because of symmetry. I...
  18. R

    Faraday's law and linearly time dependent B field

    Homework Statement A positron is moving in a circular orbit of radius r = 2cm within a uniform magnetic field B0 = 50##\mu##T. The magnetic field varies over time according to the expression: B = 700t + Bo and, therefore, each orbit can be considered almost circular. (a) Calculate the...
  19. fluidistic

    I A question about antiferromagnetism in an external B field

    At first I was wondering whether antiferromagnetic materials exhibit magnetic hysteresis. From what I could read on wikipedia, I think they should, but for a very strange reason. Indeed, in the absence of any external ##\vec B## field, the magnetization of the 2 different sublattices have the...
  20. K

    Determining the B-field in center of current carrying loop

    Homework Statement Determine the B-field inside the middle of a circular loop of current. Homework Equations Attempt at using Ampere's law: ##\oint \vec{B} \cdot d \vec{l} = \mu_0 i## The Attempt at a Solution ##\oint B \cdot R d \theta = \mu_0 i \Rightarrow BR(2 \pi) = \mu_0 i \Rightarrow B...
  21. M

    What physical situation creates such a B field?

    Homework Statement We are given that the B-field is: $${\bf{B}} = C \delta(s-R) \hat{\phi}$$ where ##s## is the distance from some object and ##R## is the radius of some object. What kind of physical situation would create such a B-field? Homework Equations Stated above. The Attempt at a...
  22. O

    Ampere's law to calculate the B field from a balanced three-phase system

    I am a little confused when I try to use Ampere's law to calculate the B field from a balanced three-phase system. Consider the following, shown in the picture below: Lets say I want to calculate the value of B at a distance r (radius of my Amperian loop), the value of r is big enough so that...
  23. P

    B Can a time variable B field create

    A static E Field and vice Versa? Or a time variable field created also another time variable Field?
  24. PhysKid45

    What is the Direction of the B Field from Electron Motion?

    https://scontent-ord1-1.xx.fbcdn.net/v/t34.0-12/17092376_1311006885612822_239104452_n.png?oh=72f2336f20f39bd399888901812eae3e&oe=58BAB2BF Hello PF friends. I know what the answer is, but I do not understand why the answer is. By the right hand rule I put my thumb west, but pointer up, and...
  25. B

    Flux Density, B, of a Cylindrical Magent

    Hi, I'm trying to calculate the flux density of a magnet, I can get all but one of the values needed to calculate it. Does anyone know how/where to get the z(distance from a pole face on the symmetrical axis) value?
  26. A

    Finding E and B field of a weird charge distribution

    Homework Statement Initially there is a spherical charge distribution of with a radius ##R_0## and uniform charge density ##ρ_0##. Suppose the distribution expands spherically symmetrically such that its radius at time t is ##R_0 + V t##, where V is the velocity. Assuming the density remain...
  27. yabb dabba do

    Modeling E and B field of power lines

    I’m confused: Long distance electric power lines are modeled with distributed impedance and are therefore modeled as transmission lines. Long distance electric power lines are considered to be a two wire line (for a single phase of the electric power line anyway). What other types of...
  28. T

    I Applicability of E and B field Transformations

    I have a question regarding these transformation formulas: ##\begin{align} & E'_x = E_x & \qquad & B'_x = B_x \\ & E'_y = \gamma \left( E_y - v B_z \right) & & B'_y = \gamma \left( B_y + \frac{v}{c^2} E_z \right) \\ & E'_z = \gamma \left( E_z + v B_y \right) & & B'_z = \gamma \left( B_z -...
  29. A

    Two Coaxial Linear Magnetic Materials

    Homework Statement Consider a two-layered cylindrical wire with inner-layer permeability μ1 and outer-layer permeability μ2. A line current I runs through the center in the z direction. Calculate the bound currents and the magnetic field produced by the bound currents. Homework Equations [1]...
  30. Amrator

    Zero B Field Inside within Hollow Wire

    Homework Statement A long, thin wire carrying constant current I1 = 2 A into the page is surrounded by a concentric cylindrical hollow wire of inner radius a = 0.12 m, and outer radius b= 0.26 m, carrying total current I2 = 4 A directed out of the page, as shown. Assume the current in the...
  31. Salvador

    B Solenoid B field lines question

    Hi, I wonder what would the path of magnetic field be in my case.I have a solenoid , it's a metal rod that has a winding in the middle of it, I need the magnetic field to come out of the rod not at the ends of it as is usually shown in pictures but at the surface side along each end of the metal...
  32. Salvador

    Charged particles through various B field shapes quesion

    Hi, first of all I wan to ask a few simple questions , when we move a piece of wire perpendicular to a uniform magnetic field like between the faces of two magnets facing N-S we get a current either one or the other way in the wire , yet when we would shoot an electron beam the same path as the...
  33. RobertColman

    How can I derive a formula for change in B field strength?

    I'm a bit stuck on this question (which is homework so hints are more welcome than outright answers). The question is: A very long wire carrying a current I is moving with speed v towards a small circular wire loop of radius r. The long wire is in the plane of the loop and is too long to be...
  34. V

    Radius and period of charge in B field

    Homework Statement A doubly charged helium atom (mass = 6.68 x 10-27 kg) is accelerated through a potential difference of 4.00x 103 V. What will be the radius of curvature of the path of the atom if it is in a uniform 0.460 T magnetic field? Note: I hope this question is meant in advanced...
  35. Chris Fuccillo

    Does a coil with CW and CCW turns split its B field?

    Single Electro magnet with split field Single core with separated counter clock wise and clock wise turns My question: I’m not worried about field strength at the moment, I am just trying to make sure that the field will develop in this manner? Note: Please see picture. If you take a ferrous...
  36. J

    I What is the resolution to the problem with E cross B field and E>B?

    I have been unable to find a satisfactory explanation of this problem, elsewhere. Consider an uniform electric field, E, along the y axis. Consider also a uniform magnetic field, B, along the z axis. If we release a particle (charge=q, mass=m) at rest on the origin at time t=0, what will be...
  37. F

    Magnetostatics: Finding B field using Amperes Circuital Law

    I am preparing for an exam and I am going through a past paper which has solutions given for the questions but I need help understanding how the answer comes about. I suspect it may be just the algebra I don't get, but it may be the physics too. Wasn't sure if this was the correct forum either...
  38. S

    Cylinder parallel to a constant external B field

    Homework Statement A cylinder of permeability ##\mu## is placed in an external field ##B_0##. find the strength and direction of magnetic field inside the cylinder for: a) when axis of cylinder is parallel to external field. b) when axis of cylinder makes an angle ##\theta _0## with external...
  39. F

    Magnetostatics: Finding B field given current density

    Homework Statement Not sure if this is the correct place to post so move if needed. In a cylindrical conductor of radius R, the current density is givne by j_0 e^{- \alpha r} \hat{k}. Where ##\alpha## and ##j_0## are some constants and ##\hat{k}## is the unit vector along the z-axis. ...
  40. DoobleD

    Curl of the curl of E or B field

    A modern standard way of deriving the EM wave equation from Maxwell's equations seems to be by taking the curl of curl of E and B field respectively, and use some vector identity. See for instance on wikipedia. So, I have a basic understanding of the curl of a vector field. Defined as the...
  41. R

    Charged particles in a liquid and B field

    I would like to ask your opinions on what would happen if you had a liquid containing charged particles and then applied a sinusoidal magnetic field? 1. would the particles move in one direction and when the current changes direction then move in the other direction? 2. would there be a...
  42. Anode

    Torque on a Disk in a Magnetic Field (E&M)

    Hi, I've been struggling on this homework problem for quite a few hours now and would really appreciate a pointing in the right direction. It's a fairly entertaining problem that I'm getting stuck on in several places. 1. Homework Statement The problem goes: "An erratic engineer, who lives in...
  43. N

    Percentage of external B field absorbed by inductor?

    Hey all, if a magnetic field source is at A and an inductor is at B halfway from C, the measuring point will the field be attenuated and decrease in strength due to a percentage of it being absorbed by a solenoid if so how would one calculate the amount, I found how to calculate energy in an...
  44. Mnemonic

    How Is the Force on a Helium Ion Calculated in a Magnetic Field?

    Homework Statement What is the magnitude of the force experienced by a He2+ ion traveling with v = 1.3 × 10^5i m/s in a magnetic field B = 0.42 j T? Homework Equations F=qvbsinθ The Attempt at a Solution F=2*1.6e-19*1.3e5*0.42 I am unsure about the charge on the Helium ion. Is it 2*1.6e-19?
  45. J

    B Field of Large Ring Magnet Calculation

    Hi Everyone , Can anyone explain why I am getting negative result with a large Neodymium N35 ring Magnet ? Br: Remanence field, independent of the magnet's geometry z: Distance from a pole face on the symmetry axis D: Thickness (or height) of the ring Ra: Outside radius of the ring Ri...
  46. B

    Formula of Average Magnetic B Field Strength of Large Conductor

    Any one knows how to calculate the magnetic B field strength at a distance from a Large Conductor ? Do we go by average ? thanks
  47. D

    EM: B field at boundary with different permeabilities

    Hey this isn't so much a homework problem but one I have just had an exam over. I have absolutely no idea how to calculate it and in all past papers/tutorial questions and the notes, makes no mention of the sort of problem. I'm not bothered over the exact answer, just how you go about it...
  48. B

    Magnetic B Field Equation Due To Multi-Layer Solenoid

    Anyone knows the Magnetic B Field Equation Due To Multi-Layer Solenoid(with a core) ? where: Core Material "μ" is of radius R1(NOT shown in the below drawing) i - Current μ - Permeability (of Core Material)All length, diameter and pitch measurements are from centre to centre of the conductors...
  49. BiGyElLoWhAt

    Exploiting time varying B field to employ vacuum acceleratio

    So I've been wanting to build a particle accelerator for a while, and have kind of been brain storming ideas to make it work. I've been recently trying to figure out how to get the actual acceleration to happen. I have a few ideas, but the one that I like (assuming it's possible) is using a...