## Introduction

Modeling a magnet realistically is a process finest performed numerically. Even the simplified mannequin of two separated disks with uniform floor magnetization ##pm~sigma_M## includes elliptic integrals simplifying assumptions. As a mannequin, the purpose dipole could also be unrealistic to some however the math is tractable and accessible. The usefulness of the purpose dipole mannequin in electrodynamics is analogous to that of the purpose mass in kinematics and dynamics. It brings forth the salient options and serves as a place to begin for extra concerned therapies. Certainly, analyses of the falling magnet discovered within the literature depend on modifications of the purpose dipole equations with an eye fixed towards bringing their predictions into higher settlement with experiments.

The objective of this text is to assemble in a single place what may be mentioned a couple of magnet dipole falling by way of a solenoid. A second article, now in preparation, has the identical objective however for some extent magnetic dipole falling by way of a strong conducting pipe.

### The motional emf

The inverse Lorentz transformation equations for the parts of the electrical and magnetic fields are $$start{align}& mathbf{E}_{parallel}=mathbf{E’}_{parallel}~;~~mathbf{B}_{parallel}=mathbf{B’}_{parallel}nonumber & mathbf{E}_{perp} =gammaleft(mathbf{E’}_{perp} -mathbf{v}timesmathbf{B’}proper)~;~~mathbf{B}_{perp}=gammaleft(mathbf{B’}_{perp}-frac{1}{c^2}mathbf{v}timesmathbf{E’}proper).nonumber finish{align}$$

Think about a magnetic level dipole ##mathbf{m}=m~hat z## falling with velocity ##mathbf{v}=v~hat z##. Taking “down” as optimistic and for strange magnet velocities (##gamma approx 1##), we write in cylindrical coordinates,$$mathbf{E}_{perp}=-mathbf{v}occasions mathbf{B’}=-mathbf{v}occasions mathbf{B}=-v~hat ztimesleft(B_{rho}~hat {rho}+B_z~hat {z}proper)=-vB_{rho}~hat {theta}.$$The road integral ##int_a^b mathbf{E}_{perp}cdot dmathbf{l}## is the motional emf between factors ##a## and ##b##.

The relativistic remedy may be a bit an excessive amount of, however it helps get the indicators proper. Anyway, from right here on we’ll discontinue using primes to indicate coordinates within the shifting body. As an alternative, we’ll use primes for supply coordinates and no primes for coordinates of factors of curiosity. Integrals might be taken over primed coordinates.

### A single ring

At focal point ##mathbf{r}(rho ,z)## when the dipole is at ##mathbf{r’}(rho’,z’)## the dipolar discipline is $$mathbf{B}=frac{mu_0}{4pi} left[ frac{ left(3 mathbf{m}cdot (mathbf{r}-mathbf{r}’))(mathbf{r}-mathbf{r}’)-|mathbf{r}-mathbf{r}’|^2 mathbf{m}right)}{|mathbf{r}-mathbf{r}’|^5} -frac{mathbf{m}}{ |mathbf{r}-mathbf{r}’|^3} right].$$We place the dipole on the ##z##-axis and orient it in order that ##mathbf{m}=m~hat z##. At ##rho=a##, the radial element of the magnetic discipline is $$B_{rho}=frac{3mu_0ma}{4pi} frac{ z-z’} {left[a^2+(z-z’)^2right]^{5/2}}.$$

The motional emf in a coaxial single ring of radius ##a## at ##z’## is $$start{align}textual content{emf}_{textual content{ring}}=int mathbf{E}_{perp}cdot dmathbf{l}=-vB_{rho}(2pi a) = -frac{3mu_0ma^2}{2} frac{ (z-z’)~v} {left[a^2+(z-z’)^2right]^{5/2}}.finish{align}$$Word that the ring may very well be an intangible loop, not essentially conducting, and the emf round it might nonetheless exist. If, nonetheless, the ring is a conducting loop, there might be an induced present in it. A plot of the spatial dependence of the emf is proven in Determine 1 beneath. We notice that when the dipole is farther than about 4 radii from the ring’s heart, the emf is beneath 1% of its peak worth.

Because the dipole falls in free house, a wave of motional emf with the waveform proven in Determine 1 is touring with velocity ##v(t)## down an intangible tube consisting of loops of radius ##a.## Within the subsequent part, we’ll discover what to anticipate when a conducting finite solenoid of radius ##a## occupies a part of that house.

### From ring to solenoid

We think about the solenoid as a stack of ##N## turns. The start of the ##okay##th flip is the tip of the ##(k-1)##th and the tip of the ##okay##th flip is the start of the ##(okay+1)##th. Thus, the turns are a set of rings linked in collection. We’re looking for an expression for the emf throughout the free ends of a solenoid of ##N## turns, radius ##a##, and size ##L##. It is going to be an integral of ##textual content{emf}(z)## over the size of the solenoid.

The variety of rings in a stack of top ##dz’## is ##dn=dfrac{N}{L}dz’##. Integrating over the size, $$start{align}textual content{emf}(z)=frac{N}{L}int_{frac{-L}{2}}^{frac{L}{2}}textual content{emf}_{textual content{ring}}dz’=frac{mu _0 N m a^2}{2 L} left{ frac{v}{left[a^2+left(z-frac{L}{2}right)^2right]^{3/2}}-frac{v}{left[a^2+left(z+frac{L}{2}right)^2right]^{3/2}}proper}.finish{align}$$

We assume that the dipole has mass and is in free fall. If the solenoid ends are open, there might be an emf throughout them however no present. Even when a voltmeter is linked to measure the emf, the present by way of it’s anticipated to be negligible . In both case, eddy present braking is negligible and we’ll ignore it along with air resistance. Below these circumstances, the usual kinematic equations underneath fixed acceleration apply. Taking the origin on the midpoint of the solenoid, we write an expression for the place of the dipole. We assume that it begins from relaxation at top ##h## above the highest of the solenoid (##z_0=-h-frac{L}{2}##.) Substituting $$z=(-h-frac{L}{2}+frac{1}{2}g~t^2)~textual content{ and }~v=g~t$$ in equation (2), we get hold of the emf throughout the solenoid as a operate of time. $$start{align}textual content{emf}(t)=frac{mu _0 N m a^2}{2 L} left{ frac{g~t}{left[a^2+left(-h-L+frac{1}{2}g~t^2right)^2right]^{3/2}}-frac{g~t}{left[a^2+left(-h+frac{1}{2}g~t^2right)^2right]^{3/2}}proper}finish{align}$$

### The emf profile

Determine 2 beneath is a plot of emf(t) vs ##t## with out the general multiplicative fixed. For the plot, we used ##g=980## for the acceleration, ##a=1## for the radius, ##h=6a## for the discharge top above the highest and ##L=10 a## for the size. Thus, if the items for the radius ##a## are centimeters, the numbers on the abscissa might be seconds. The strong vertical grid traces mark the occasions when the dipole is on the high, midpoint, and backside of the solenoid. The pairs of crimson dashed traces mark the occasions when the dipole is inside one solenoid diameter (##pm~2a##) from the highest (left) and the underside (proper).

Alternatively, we are able to get hold of an expression for the emf as a operate of place ##z## by substituting ##v=sqrt{2g[z-(-h-frac{L}{2})]}## in equation (2) to get $$start{align}textual content{emf}(z)=frac{mu _0 N m a^2}{2 L} left{frac{sqrt{2 g left(z+h+frac{L}{2}proper)}}{left[a^2+left(z-frac{L}{2}right)^2right]^{3/2}}-frac{sqrt{2 g left(z+h+frac{L}{2}proper)}}{left[a^2+left(z+frac{L}{2}right)^2right]^{3/2}}proper}.finish{align}$$A plot of the emf(##z##) vs. ##z## will not be proven as a result of it’s equivalent to the one in determine 2 apart from the labeling of the abscissa. Each plots characteristic extrema, one unfavourable and one optimistic, with a area between when/the place the emf is close to zero. That is qualitatively according to Faraday’s legislation. The magnetic flux adjustments most quickly when the dipole is close to the ends and hardly in any respect when it’s close to the center of the solenoid.

### Time period corrections

Examination of determine 2 reveals that the extrema happen when the dipole is close to the ends of the solenoid. At ##z=+L/2## the primary time period in equation (4) has a most whereas the second time period is small; the reverse is true at ##z=-L/2##. A precise willpower of the extrema would contain fixing ##frac{d}{dz}textual content{emf}(z)=0##, a frightening process. Contemplating, nonetheless, that the true extrema are very near ##z=pm frac{L}{2}##, it’s expedient to do collection expansions of equation (4) about ##z=pm frac{L}{2}## to second-order and discover corrections to the zeroth-order phrases. Right here, we summarize the outcomes and relegate among the particulars to the Appendix.

The fractional corrections to the place and worth of the minimal are $$frac{delta z}{(-L/2)}=-5.5times 10^{-3}~;~~frac{delta(textual content{emf})}{textual content{emf}(-L/2)}=1.1times 10^{-3}.$$The corresponding numbers for the utmost are $$frac{delta z}{(+L/2)}=2.1times 10^{-3}~;~~frac{delta( textual content{emf})}{textual content{emf}(L/2)}=1.6times 10^{-4}.$$The numbers present that the corrections have a small impact on the zeroth order phrases. Thus, we’ll ignore them and make the algebra simpler.

### A relation between amplitudes

We outline as amplitudes the magnitudes of the extrema of ##textual content{emf}(t)## (Determine 2). Within the approximation that the extrema happen at ##z=pm ~frac{1}{2}##, equation (4) offers, the amplitudes $$start{align} & A_1=left|textual content{emf}(-L/2)proper|=sqrt{2 g h}left[frac{1}{a^3}-frac{1}{left(a^2+L^2right)^{3/2}}right]nonumber & A_2=textual content{emf}(+L/2)|=sqrt{2 g (h+L)}left[frac{1}{a^3}-frac{1}{left(a^2+L^2right)^{3/2}}right].nonumber finish{align}$$

We write the distinction between the 2 equations as $$start{align} & A_2-A_1=textual content{const.} left(sqrt{frac{2 (h+L)}{g}}-sqrt{frac{2 h}{g}}proper) nonumber & textual content{the place }textual content{const.}=frac{mu _0 N m a^2}{2 g L}left[frac{1}{a^3}-frac{1}{left(a^2+L^2right)^{3/2}}right].nonumber finish{align}$$We acknowledge the expression in parentheses multiplying the fixed because the transit time ##T## of the dipole from one finish of the solenoid to the opposite. This simplifies the equation much more to $$start{align}A_2-A_1=(textual content{const.})occasions T.finish{align}$$

It’s simple to think about an experimental protocol to check equation (5) with tools that samples and data the emf throughout the solenoid as a operate of time. Nonetheless, one ought to make sure that beginning heights ##h## are better than the efficient dipole vary of 4 solenoid radii. This may decrease experimental uncertainties that grow to be important at small values of ##h##. A PF thread posted by @billy_t right here describes such an experiment. Nonetheless, the evaluation centered on the amplitudes themselves and never on their relation to the time of transit.

## Abstract

Modeling a magnet as some extent dipole falling by way of a solenoid is enough to offer a smart description of the form and normal options of the emf(##t##) curve that’s in settlement with what one would anticipate qualitatively. The flux varies most quickly when the dipole is getting into and exiting the solenoid and least quickly when it’s close to the solenoid’s center. The amplitude of the fitting peak is larger than the left. This occurs as a result of its width is narrower because the dipole is shifting quicker, but the realm underneath the curve is similar. It’s unlikely {that a} extra lifelike mannequin of the magnet will lead to a unique interpretation of those options. A classy mannequin wants subtle information evaluation to match it, maybe one involving a point-by-point simulation of the whole curve.

## Acknowledgments

I thank @billyT_ for posting the thread quoted above; it seeded this perception. Additionally, the participation of @haruspex and @Charles Hyperlink honed my considering and is vastly appreciated.

## Appendix

The work proven right here was Mathematica-assisted. In equation (4) we first drop ##g## and substitute diminished lengths ##lambda=L/a,~eta=h/a##. Then develop the equation for small values of ##epsilon = zpm lambda/2## to second order. Every growth yielded a quadratic expression ##Aepsilon^2+Bepsilon+C.## Setting the spinoff equal to zero and fixing for ##epsilon## offers ##epsilon=frac{-B}{2A}##. Omitting the gory particulars, the corrections for the minimal on the left and the utmost on the fitting are:

$$start{align} & epsilon_{textual content{min}} =frac{2 eta left(lambda ^2+1right) left[left(lambda ^2+1right)^{5/2}-left(6 eta lambda +lambda ^2+1right)right]}{left(12 eta ^2+1right) left(lambda ^2+1right)^{7/2}+12 eta ^2 left(4 lambda ^2-1right)+12 eta lambda left(lambda ^2+1right)-left(lambda ^2+1right)^2}nonumber & epsilon_{textual content{max}} =frac{ 2 left(lambda ^2+1right) (eta +lambda ) left[6 eta lambda +left(lambda ^2+1right)^{5/2}+5 lambda ^2-1right]} {12 eta ^2 left(4 lambda ^2-1right)+12 eta lambdaleft(7 lambda ^2-3right) +left(lambda ^2+1right) left[12 left(lambda ^2+1right)^{5/2} (eta +lambda )^2+1right]+left(35 lambda ^4-26 lambda ^2-1right)}nonumber finish{align}$$Determine 3 reveals a element of determine 2 within the neighborhood of the utmost. The vertical line is on the corrected worth of ##z/a##. The zeroth order time period of the growth is to its left at ##z/a=5.00##.

I’m a retired college physics professor. I’ve performed analysis in organic physics, principally finding out the magnetic and digital properties on the lively websites of biomolecules and their mannequin complexes. I’ve additionally dabbled in Physics Training analysis.