# Tag Info

## Hot answers tagged magnetic-fields

11

The centripetal acceleration that the protons feel as they circulate in the LHC is roughly: $$a = \gamma^2 \frac{v^2}{r}$$ This is the usual equation for centripetal acceleration but multiplied by a factor of $\gamma^2$ to allow for the time dilation the protons experience. The speed $v$ is approximately $c$. The radius of the LHC is about 4.3km but the ...

4

There are classes of neutron star called soft gamma ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs), which emit bursts of high energy radiation that last (usually) from less than a second to maybe 100s. It is thought that these objects have extremely strong fields of up to $10^{11}$ T, which is produced during a core collapse supernova when some ...

3

Magnetic attraction (and repulsion) force tails off quite quickly with distance, as anyone who has manipulated magnets will know from experience. For instance, for two point-like magnetic poles the attractive force $F \propto \frac{1}{x^2}$ with $x$ the distance between the poles. So when you double the distance then the force is reduced by a factor $4$. ...

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The force is given by: $$\vec F = I \oint_{\gamma_2} d\vec r \times \vec B(\vec r).$$ Where $\vec B(\vec r)$ is the field generated by coil 1 and $I$ is the current in coil 2 and the integral is along the curve $\gamma$ which is described by the wires of the coil 2. Note, that there might also be a torque. Knowing the maximal or homogeneous field $B$ in ...

2

I think the quantity you are looking for is susceptibility, which is a measure of how the magnetization $M$ of a sample depends on the applied magnetic field $H$. For a paramagnetic (or diamagnetic) material, the susceptibility $\chi$ is defined by $M = \chi H$. But for a ferromagnet, things are more complicated, because the magnetization can be nonzero in ...

2

These magnets are orders of magnitude too weak to damage or even influence a card. You need several Volt to change the state of a flash cell, and one of these magnets won't have the power to do that. The easiest way to "prove" this is to look at the internal storage of the devices, which is usually using flash technology as well. The internal storage is ...

2

If it were two wires on your desk with steady currents between them then before hand there was a strong magnetic field, for instance between the two wires the fields from each added together to make a stronger field. And magnetic fields have energy, really and truly. If you want to create a strong magnetic field you will have to do work. And if a strong ...

2

The external force on the whole current-carrying wire in region $V$ can be expressed as $$\mathbf F = \int_V \rho\mathbf E_{ext} + \mathbf j \times \mathbf B_{ext}\,d^3\mathbf x.$$ where $\mathbf E_{ext}, \mathbf B_{ext}$ is the external electric and magnetic field and $\rho,\mathbf j$ is the charge and current density. Stationary and moving wire will ...

1

I would use something like this - a stack of bars in parallel to the direction you want for the field, above and below the box: Since you have limited wire, you want to wrap the largest possible amount of metal bars: the ratio circumference / area gets better as the thing you wrap is larger. But there is no point to make it larger than the area of the ...

1

In the situation you mentioned, if your "wire" is simply a straight or non-closed conductor an emf gets generated across the ends of the conductor,known as motional emf Do note that this is not equivalent to a current getting generated. This emf gets generated in equilibrium conditions ,when the electric field force balances the magnetic field force on the ...

1

Static magnetic field is a harmonic function (its curl and divergence vanish, you can then derive that its Laplacian vanishes), so it does not have local minima or maxima. EDIT (10/3/2015): I am grateful to the authors of the comments for their criticism. What I wrote seems to be applicable to the components of static magnetic field (their Laplacian is ...

1

If you place two magnets end to end with a gap in between, there will be a local minimum in the magnetic field between the poles. And inside a coil there is a local maximum. So I don't know where you get your statement from. It seems to be wrong. Or it applies under certain conditions which you did not specify.

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For a stationary point to be a local maximum or local minimum, then the eigenvalues of the Hessian matrix must either be all negative or all positive respectively. However, a neat trick is that the trace of the Hessian is equal to the sum of the eigenvalues and is also the Laplacian. So, to answer the question we ask what the Laplacian of the magnetic ...

1

It's a good idea. The basic reason no one's done it is that diamagnets are 4-5 orders of magnitude smaller permeability. Added to which, if you have a superconducting set up you can get a superconducting magnet which is multiples stronger than a permanent magnet. The set up you show would probably need to be ~1000 times higher to work. Maybe in space ...

1

Any ammeter which uses a shunt resistor will ignore magnetic fields (if it is well-designed), and measure only the voltage across the shunt. Since the shunt is a known resistance, $$i = \frac{V}{R}$$ For instance, this is the standard way a DMM measures current.

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The Earth's outer core is essentially made of molten iron, nickel, cobalt and other ferromagnetic metals, which are in constant motion. With these elements inside the outer core and in motion, they help create Earth's magnetic field. This is the basic idea of the hydromagnetic dynamo model: Convection currents of magma in the Earth's outer core, driven ...

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