What is Magnetic field: Definition and 1000 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. Z

    Magnitude of magnetic field at different points near a capacitor.

    Here is a picture depicting the capacitor and the points of interest. I approached this problem by applying the Ampere-Maxwell law. For each point I used an circular Amperian loop that I denote by ##P##, enclosing a circular surface ##S##. Thus, for point ##b## we have...
  2. lonely penguin

    I How relative is the magnetic field of a current carrying conductor?

    According to the special theory of relativity the electric and magnetic fields observed from a charge distribution should depend on the relative motion of the observer. For example consider a linear arrangement of point charges which are fixed relative to each other. An observer who is...
  3. S

    emf in coil rotating inside magnetic field

    My answer is (B) but the answer key is (A). My working: $$\varepsilon=-\frac{d\phi}{dt}$$ $$=-AB\frac{cos\omega t}{dt}$$ $$=AB\omega \sin \omega t$$ Why the answer is zero? I thought the flux will be zero, not the emf. Thanks
  4. S

    I Quantum Hall Effect and Hall Voltage

    I understand that the Quantum Hall Effect explains how both the transverse and longitudinal resistance vary with magnetic field strength. I don’t get why the hall resistance is equal to the hall voltage over the current . I know current isn’t a vector quantity but isn’t the hall resistance an...
  5. Z

    How to obtain correct negative sign in calculation of motional emf?

    My issue is with the signs. We have $$\mathcal{E}=\oint \vec{v}\times\vec{B}\cdot d\vec{s}$$ $$=\oint v_0\hat{i}\times B_0(-\hat{k})\cdot dy\hat{j}$$ $$=\int_0^h v_0B_0\hat{j}\cdot dy \hat{j}$$ $$=v_0Bh$$ Now, apparently, the sign should be negative. Intuitively, the magnetic force on any...
  6. C

    Force on rectangular loop by a current in a long straight wire

    Hi, I am struggling to get the right answer for this question. My first thought was that I should consider to what direction does each segment of wire have a force towards. I found the direction to be in the following (see red arrows): My past attempt was: Floop = IlooplloopBwire Since Bwire...
  7. Juanda

    I Pushing a rod in a magnetic field

    I will present the case as an exercise because I believe it will be easier to understand that way. I didn't know if I should post it under the mechanics or electromagnetism sections because I am actually trying to link both. Since it feels like a fairly simple problem, I considered the Classical...
  8. Silver2007

    Magnetic field due to electric wire

    In my opinion, the magnetic field at point P should cancel each other, because the magnetic field caused by the two wires has the same direction, like this. but the solution does not assume that the magnetic fields cancel each other out. I don't know where I'm wrong, please help me, thanks.
  9. L

    Biot-Savart-Law Superposition

    For the following conductor loop, determine the magnetic field along the ##z##-axis, which passes through the center of the conductor loop and is perpendicular to it. The conductor loop consists of an infinitely long wire through which a constant current ##I## runs. Is it possible to determine...
  10. M

    What does magnetically sealed possibly mean?

    I remember one of my favorite parts of the original Star Wars movie was when Luke, Leia, Chewie and Han jump into the trash compactor. Han tries to blast his way out and the blaster bolt ricochets off the walls of the compactor until it eventually hits a piece of garbage. Luke screams “WILL YOU...
  11. mister i

    B How does a compass know that it is in a magnetic field?

    Sorry for this question: how does a compass (or its electrons) know that it is in a magnetic field? Is it the information of the photons that cross it? (photons are what transmit electromagnetic forces)
  12. hongseok

    I Does Diamagnetism Occur Even in a Constant Magnetic Field?

    I learned that diamagnetism is due to Lenz's law. But doesn't Lenz's law only apply when the magnetic field changes? Why does diamagnetism occur even when a constant magnetic field is applied without change?
  13. M

    Magnetic field due to a long, straight wire

    From Ampere's law, ##\displaystyle\oint\vec B\cdot d\vec l=\mu_0 I## where ##r## is the distance from the wire ##B\cdot 2\pi r=\mu_0 I## ##\displaystyle 91.4\times 2\pi\left(\frac{2}{100}\right)=4\pi\times 10^{-7} I## ##I=91.4\times 10^5\ A=9.14\ \rm{MA}## But the answer given in the...
  14. M

    B How much electricity is required to create a magnetic field as large & strong as the Earth's?

    How much power (Megawatts) would it require if we were to create a magnetic field as large and strong as the Earth's magnetic field?. I haven't learned how to calculate this, but just curious.
  15. A

    I Simple electromagnetic wave including delay

    I want to derive the electric and magnetic field at a point around an infinitely long wire which carries a sinusoidal alternating current. And I want the answer includes the phase delay according to the limited speed of propagation of the wave. Surely the answer must satisfy the Maxwell's...
  16. M

    A Effects of magnetic fields on material (Magnetic resonance)

    Hi, I was wondering if a Magnetic Resonance (+contrast) can in some way modify the structure of artificial chordae used in heart surgery. My first answer would be "No". There ain't any medical article about that. Also (as far as i know) I don't think magnetic field can break chemical bond, so it...
  17. M

    Tangential acceleration of proton due to a changing magnetic field

    ##\displaystyle R=\frac{mv}{qB}\implies v=\frac{RqB}{m}## where ##v## is the speed of the proton ##\displaystyle\frac{dv}{dt}=\frac{Rq}{m}\frac{dB}{dt}## On substituting the values, I get ##\displaystyle\frac{dv}{dt}=9.58\times 10^4\ m/s^2## This answer, however, is incorrect. Where have I...
  18. M

    In which direction is the magnetic field at point P?

    Consider a circular Amperian loop oriented counterclockwise that is concentric with the circles in the figure and passes through P. By symmetry, ##\vec{B}## is everywhere tangent to this circular loop and has the same magnitude B everywhere on the circle. By Ampere's law...
  19. H

    B Consequences of a 1 Tesla magnetic field coming into the proximity of Earth?

    What would be the consequence if a powerful (1 Tesla) magnetic field came into proximity of Earth? Options open concerning size of generating body and distance. Thank You.
  20. bmhiggs

    Multipole ring magnet generator

    I've dissected a pig (flashlight) and found a hand crank generator that has a multipole ring magnet. I used a bar magnet to establish that it was a multipole magnet, because I was wondering if I could figure out the direction of the current based on the direction of the magnetic field. My...
  21. Vectronix

    B Electromagnetic radiation interacting with a magnetic field

    Why isn't EM radiation attracted/repelled by a magnetic field?
  22. april7

    I Measuring change of magnetic field strengh over distance

    I have few cylindrical neodymium magnets of same diameter and different heights and different magnetic inductions in the geometric center of the magnetic pole surface. Knowing those values of magnetic induction, how can I measure what the induction is at a certain distance from that center? For...
  23. S

    Calculations for accelerating particles in a Cyclotron

    For this question part d, KE=mv^2/2=q^2B^2r^2/2m (I rearranged B=mv/qr for v and subbed into mv^2/2). q^2b^2r^2/2m=2F_cyc^2r^2m(pi)^2 But when I subbed the values in I got 16.45MeV but the answer says 165keV instead. I'm not sure what went wrong? What's a good explanation for part e also?
  24. A

    B Measuring electric field vs magnetic field

    Hello. Sorry. I am not a physicist and I have very superficial knowledge of the subject. I would like to ask a question about the practical field. I only have a device that measures the electric field. When I measure the signal coming from a cell phone tower before it is reflected from a metal...
  25. G

    I Railgun: Significant magnetic field where projectile is?

    Hi. Very simplified schematics of railguns all look like this: I have trouble properly understanding this. So apparently there is still a significant magnetic field where the projectile is, even though that's where the current stops flowing through the rails? Of course the magnetic field...
  26. T

    I Is the magnetic field from this current rotationally symmetric?

    Imagine a very large disc with current converging to it's center as $$ I=- \frac{k}{r} \hat{r}$$ Obviously ##\theta## is absent in the equation so the magnetic field by this current should look the same no matter which angle one is observing. Then does this mean that the magnetic field is...
  27. S

    I Question: Interpretation of Current and Magnetic Field Using Relativity

    The current in a wire is said to produce a magnetic field and a force on a charge. It is said that the correct way to interpret the effect is because of relativity. So does that mean that that the most basic non-relativistic equations that offer basic calculations of the forces and fields are...
  28. Hak

    Spring-mediated dipole in a magnetic field

    Here is my attempt to answer the questions: a. The system's motion is a combination of two types of motion: the translational motion of the center of mass and the rotational motion of the charges around the center of mass. The forces acting on the system are the Lorentz force, which is the...
  29. Hak

    Magnetic field in a solenoid with rectangular cross-sectional area

    I was thinking of using magnetic monopoles, but I don't know how to do. I tried to calculate the magnetic flux through the cross section of the solenoid if a current ##I## flows through it. Magnetic induction inside can be given as ##B = \mu_0 n I##, whereas magnetic induction at a distance...
  30. P

    Electric field in nonmagnetic material given a magnetic field

    Hello, I am stuck on a problem that I don't quite understand, which looks like this: "Given a nonmagnetic material with the magnetic field H = 50⋅exp(−100⋅x)⋅cos(2π⋅10⁹⋅t − 200⋅x)⋅ŷ determine the electric field strength E" I don't understand how I am supposed to find the solution for this...
  31. ktmsud

    I Hall effect -- is it always applicable?

    A current carrying conductor experiences magnetic force in a magnetic field. F=BILsinθ Where, B = Magnetic flux density I = Current L = Length of conductor and θ = Angle between magnetic field and current This force is due to free electrons moving in a...
  32. L

    Does a Bent Wire Affect Induction Calculations?

    Hi, unfortunately, I am not sure if I have calculated the task correctly here Task a I have now proceeded in such a way that I thought that the magnetic field only flows through the area drawn in red. Which ##\frac{1}{4}## corresponds to the area of a circle. By the fact that the magnetic...
  33. harryharns

    C/C++ Perform RK4 between 2 Clusters in Magnetic Field

    I have a simulated data of charged particles in a magnetic field. I have selected clusters, each cluster contains a set of points(x,z) and I want to perform RK4 between the first and second clusters and fill the positions in a histogram. I have selected the clusters with the initial...
  34. ergospherical

    I Average magnetic moment of atom in magnetic field ##B##

    from the partition function - am trying to show that ##\langle \mu \rangle = \beta^{-1} (\partial \log Z / \partial B)## where ##Z## is the canonical partition function for one atom, i.e. ##Z = \sum_{m=-j}^{j} \mathrm{exp}(\mu_0 \beta B m)##, and ##\mu = \mu_0 m##. The average...
  35. D

    B Magnetic field and generator power output

    Hi, I am confused about whether decreasing the magnetic field used for a generator could increase the generator's power output. I used four equations: 1. Torque = Force x radius 2. Torque = NIAB (N = number of turns, I = current, A = area of armature, B = magnetic field). 3. emf =...
  36. R

    B Exploring the Electric Field of a Moving Charge

    How does an electric field of a moving charge, for example a moving electron, inside a wire looks like? Does it looks like this with distorted circular radial lines?
  37. milkism

    A diamagnetic sphere in a uniform magnetic field

    Problem: Solution part a) where formula 6.14 is just M x n. We need to do part b without seperation of variables, I'm quite stuck. Will B just be the magnetic field inside a solenoid? How can I find the other fields.
  38. 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...
  39. M

    Electron in a time variable magnetic field

    TL;DR Summary: Find acceleration of electron in dB/dt >0 Hello. Here is a problem that i'm not so sure about: Inside a solenoid there is a time-dipendent magnetic field B, so we have dB/dt = b (constant). We want to know the acceleration of an electron: a) placed in the center of the solenoid...
  40. S

    Magnetic field force between two perpendicular wires

    There are a couple of problems with the same setup. On plugging in the values and solving for the integral, I am not getting the expected values of the force. Is there something wrong in the solution attached?
  41. F

    Determining Radius from Magnetic Field of a Single-Wire Loop

    So I thought I knew how to do this problem but I've run into some issues that make the algebra feel impossible and I am beginning to feel like I'm taking the wrong approach, I ended up rewriting it in a doc because I was concerned maybe my handwriting was the cause of my error so the work is...
  42. Brian Tsai

    I A math confusion in deriving the curl of magnetic field from Biot-Savart

    I am recently reading "Introduction to Electrodynamics, Forth Edition, David J. Griffiths " and have a problem with the derive of the curl of a magnetic field from Biot-Savart law. The images of pages (p.232~p233) are in the following: The second term in 5.55(page 233) is 0. I had known...
  43. DomDominate

    B What exactly is spin? Does the standard model work without spin?

    I did some research online and found that "When certain elementary particles move through a magnetic field, they are deflected in a manner that suggests they have the properties of little magnets." To explain this phenomenon, physicists invented the concept of spin. So far so good. What I...
  44. S

    B Magnetic Field & Particle Spin: Does It Matter?

    Does the magnetic field caused by moving particles depend on the particle spin value? Eg a stream of say protons spin 1/2 is creating a magnetic field. If the particles are (say) lithium nuclei spin 3/2 instead, does that create the same strength field ? (same conditions of course)
  45. Grizzly_1

    B Confusing diagram of a rotating coil in a magnetic field

    Hello all, I am currently studying for a physics a-level qualification in the UK, I use the AQA specification and I am having trouble understanding this image representing a scenario I found in my textbook. The first image in the three part diagram shows this rotating coil and to me, it makes...
  46. A

    B Magnetic field coil ferrous material for loudspeaker

    Hi everyone. there are materials called soft magnetic and they are halfway between a permanent magnet and a ferromagnetic material. I would like to try to make a field coil loudspeaker where the maximum amount of flux-density is very important but also the amount of current it takes to create...
  47. J

    Thin heartshaped wire - Calculate the magnetic field at the origin

    Hi, So I know I am to use Biot Savarts law dB= (my_0/4pi)* (I dl x (r-r')/|r-r'|^3 where r=0 because its in origo and r'=r'_c(r'_hat). This makes (r-r')= -r'_c(r'_hat) and |r-r'|^3= r_c^3. From previous questions, I have defined dl' as the infinitesimal displacement of r'(phi) when phi' is...
  48. O

    Motional EMF in the presence of a time-varying magnetic field

    My question arises from the following problem: We have a uniform magnetic field into the page, decreasing at a constant rate dB/dt< 0, causing the bar to move to the right. Find the velocity of the bar as a function of the time, and in terms of the known parameters: the resistor R and the...
  49. A

    Relationship between magnetic field lines and magnetic field

    As stated in the problem, I want to demonstrate mathematically that field line density is directly related to the magnitude of B. How would I be able to do this, other than simply using the flux equation and showing that for a higher flux in the same area, the magnetic field must be rise...