# Questions tagged [magnetostatics]

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### How to derive Biot-Savart's law for the magnetic field of a surface charge

The Biot-Savart law gives the magnetic field of a steady line current: $$B(r)=\frac{\mu_0}{4\pi}\int\frac{I\times\hat r}{r^2}dl.$$ Now according to Griffiths, for surface currents, the Biot-Savart ...
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### How do magnetic shields work?

I've come across the fact that magnetizable material can shield magnetic fields. For example, if I put a ferromagnetic plate between the magnet and a nail that is hanging on a thread pulled towards ...
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### Cancellation of surface integrals (involving Maxwell's equation)

In our physics class today, we wanted to derive ${curl}(\bar B) =\mu_0 \bar j$ from the Maxwell equation $$\oint_C \bar B(\bar r).d\bar l = I_{net}\mu_0$$ We did this using Stokes theorem, and in ...
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### What is the physical quantity which makes a Reed switch open or close?

I would like to generate the activation map of a Reed switch, similar to the following: Using a magnetic finite element modeling software such as FEMM. Assuming the Reed switch closes for X Ampere-...
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### Force Lines of Magnetic Field of Circle Loop

Magnetic field of a loop with a stationary current in a xy plane with center in the origin is usually computed on the z axis. Out of the z axis the calculation results into elliptic integrals. I would ...
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### How to get image current in magnetostatics? (magnetic slab)

I want to know the image current and its location which satisfies the boundary condition at the interface. This problem was originated from the problem 6-33 in Fields and wave electromagnetics, D. ...
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### Is Biot-Savart Law valid for time-varying currents unlike Ampere's law?

I have just finished learning the basics of magnetism, and it should be noted that I am not very familiar with Maxwell's equations. Note: In the question, when I say "Ampere's Law", I am referring ...
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### Behaviour of a magnetic compass needle near a magnet

We all know that a magnetic compass needle when suspended by a thread and placed near a bar magnet, will rotate. However like a bar magnet, will the magnetic compass needle get attracted towards the ...
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### Finding the vector potential of a spinning spherical shell with uniform surface charge?

I have problem solving the following magneto-static problem. I would greatly appreciate help and guidance. This is how the problem is stated: A spherical shell of radius $R$, carrying a uniform ...
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### Is current always constant with time in electrostatics?

In electrostatics, electric and magnetic fields are not time-variant. In that situation, I guess that current must be stationary. Otherwise, $\partial E/\partial t \ne 0$ because if it were stationary ...
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### Work by magnetic field [duplicate]

we all know that magnetic field doesn't work....if a current carrying element pulls a metal piece then work is done by the source or the battery not the field.....now let us assume that a bar magnet ...
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### Magnetic Scalar and Vector Potential

Electric potential is single value since curl E =0. According to me magnetic scalar potential should be single valued ( since curl B=0) and the vector potential should be multi valued. I can just not ...
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### Vector Potential and Zero Divergence

In the following problem and many others in magnetostatics (Griffiths book) I'm asked to check that $\nabla . A = 0$. While "It is always possible to make the vector potential divergenceless." also "...
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### How does the continuity of $\bf A$ ascertain the parallel components $\boldsymbol\nabla(A_{i,1}-A_{i,2})$ being zero but not the normal one?

I was reading in this pdf about the discontinuity of the derivative of the vector potential $\mathbf A$ across a boundary with $\hat{\mathbf n}$ acting as the normal to the plane having surface ...
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### Electric flux density for a hollow cylinder using Gauss's law [closed]

In the textbook of "Engineering Electromagnetics": I understood it except the part highlighted with yellow(i.e. how did he find the charge distribution of the outer cylinder? What are the principles ...
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### Magnetic field of uniformly charged rotating hollow sphere

I want to compute the magnetic field due to a homogeneously charged, rotating sphere with radius $R$, angular velocity $\vec{\omega} = \omega\hat{z}$ and total charge $Q$. I want to use Biot-Savart ...
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### Lifetime of the magnetization of a ferromagnet

In a ferromagnetic material, there's spontaneous magnetisation, meaning in the absence of a $\mathbf{B}$ field $\mathbf{M}\neq 0.$ This means that there may be different domains in the sample where ...
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### Why do magnetic forces scale down so poorly?

Im told by experts in MEMS and nanotech that magnetic forces scale down very poorly. This is one reason we have many MEMS devices based on electric forces/electrostatics but not on magnetics. Can ...
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### How to understand Steady Currents within the limits of Magnetostatics

I know that the precise mathematical statement of local charge conservation is $\nabla \cdot \vec{J}=-\frac{\partial \rho}{\partial t}$, where $\vec{J}$ is the volume current density and $\rho$ is the ...
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### Magnetic field between two rings [closed]

Two rings are placed parallel atop each other along an axis with a distance $\epsilon$ which is much smaller than the radius of both rings a. Through one of the rings there is a current I and through ...
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### Magnetic field inside and outside cylinder with varying current density [closed]

I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current $I$ flows down a long cylindrical wire of radius $a$. Find the ...
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### Most fundamental quantity in magnetostatics and magnetodynamics [closed]

In both elctrostatics and electrodynamics, the electric charge is defined as the most fundamental quantity. What is the most fundamental quantity in case of magnetostatics and magnetodynamics ?
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### Does Biot-Savart law apply in changing electric field or it needs a modification just like Ampere's law, to become as true as Maxwell's 4th equation?

Does Biot-Savart law apply in changing electric field or it needs a modification just like Ampere's law, to become as true as Maxwell's 4th equation? In the update to Ampere's circuital law, the ...
Solve simple electrostatic or magnetostatic problems using only Maxwell equations. For example: In every book there is an excercise to find a magnetic field outside a thin wire of radius $a$ with ...