# Rolling Magnets

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PROBLEM 12. ROLLING MAGNETS

Josko Jelicic1, Milan Markovic2, Marin Lukas1, Damjan Pelc1, Ivan Sudic1 1 V. Gimnazija, Klaiceva 1, 10000 Zagreb, Croatia

2 XV. Gimnazija, Jordanovac 8, 10000 Zagreb, Croatia

Abstract

We present a solution to the problem of a permanent magnet rolling down an incline, investigating two main cases, conducting and nonconducting plane. For the former, more interesting case first – order Maxwellian theory was developed and thoroughly tested experimentally, yielding good agreement, while the latter, much simpler situation was modeled considering Earth field – magnet interactions. Plate boundary effects on the conducting plate are considered qualitatively and only demonstrated experimentally due to mathematical difficulties.

1. Introduction.

The phenomenon of a magnet slowly sliding down a metallic incline is very familiar, being one of the most dramatic demonstrations of eddy currents and the force they produce. Many studies have been dedicated to it, and we can say that it is well understood in the framework of the Maxwellian electromagnetic theory. However, this effect is by far not the only amazing consequence of eddy currents induced by moving magnets, and in this research we will study a closely related phenomenon: the rolling motion of a magnet on an incline. In

contrast to the aforementioned matter of a magnet sliding down a metallic plane, in this casethe only influence on the moving magnet is the magnetic field of the Earth. This field can provide a torque causing the magnet to change direction so as to set its magnetic moment in a direction parallel with the direction of the field. This means that, even on a nonconducting

plate, the magnet will stray to the north, following a curved trajectory; thus the first part of our research will focus on the behaviour of a magnet rolling down an insulating plane under the influence of the Earth field. In the second, by far more interesting and fascinating part, we will add a conducting plate and try to explain the effects of the currents induced therein. Both cases have a number of common parameters, primarily the properties of the rolling magnets (radius, thickness, magnetization) and plane inclination.

As the main actors of our investigation were magnets, we had to be very careful when

choosing them. We decided to work with three different disc magnets made of Nd2Fe14B, a permanently magnetic sintered ceramic material having a magnetization field of 1.4 T and a density of 7500 kg/m3 (measured). These magnets have excellent magnetic properties (large magnetization, stability) while being readily available, which made them ideal for our research.

2. The Nonconducting Incline.

In this first, simpler case, we will explore the effects of the Earth magnetic field on a magnet as it rolls down a nonconducting plate. Unless the plate is so set up that the magnetic moment of the magnet points north all the time, the torque the Earth field is causing will deflect the magnet towards the North direction. As a very simple experiment may reveal, the

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curvature of the resulting trajectory will mainly depend on the plane inclination, the initial angle between magnet and Earth field direction and the parameters of the magnet, such as mass and dimensions. This makes good physical sense; the larger the plane inclination, the larger the downward – pulling gravity component will be, increasing the downward velocity component of the magnet and decreasing trajectory curvature. Also, the larger the initial angle

between the magnetic moment and the Earth field, the larger the torque will be, resulting in anincreased curvature. The mass influence is also quite understandable – for heavier magnets the influence of inertia will be greater and the curvature smaller. To obtain an exact analytical representation of the trajectory we need to write the equations of motion for a circular magnet (of radius R, thickness D, density ρ and magnetization M) under the influence of gravity and the magnetic field of Earth. To faciliate this task we will have to introduce some approximations. First of all, in order to always have the magnet in rolling motion we will limit the considerations to small inclination angles, which means that we may neglect the influence of the vertical component of the Earth field and consider only the constant horizontal component. Due to the homogeneity of this field we can approximate our magnet with a dipole of magnetic moment V M= , where V is its volume. Second, we must define our

coordinate system. We will see that the simplest calculations are obtained if the xy - plane coincides with the plane of the incline and the z – axis is perpendicular to that plane; this means that initially the magnet is seated on the x – axis, its magnetic moment pointing along the y – axis. The rolling acceleration can easily be found:

ϑ ϕ sinsin 3

2 g a −=

where g is the acceleration of gravity, φ the angle of inclination and θ the angle between the magnet's direction of motion and the x – axis. This acceleration we know to be equal to the second time derivative of the magnet path, s&& , with ϑ sindsdx = and ϑ cosdsdy = . These facts will enable us to obtain the trajectory equation. The angle θ will be obtained from the

second influence on the magnet, the Earth field torque: ( ) β ϑ µ ϑ −−= sin E B I &&

Where I is the moment of inertia of the magnet, µ its magnetic moment, B E the horizontal component of the Earth field and β the angle between this field and the x - axis. We see immediately that the further treatment of these coupled equations of motion becomes very simple if 0≈ β , while if this is not the case matters get a lot more complicated; therefore we will first make a simple calculation for this special case. After inserting (2) into (1) and integrating we see that

ϑ ω

d g

ds 2 0

sin

3

2 =

where the parameter 20ω stands for I

B z µ , and we have taken that the initial velocity of the

magnet equals zero. The trajectory equation can simply be obtained by noting that

ϑ cot−= dx

dy and combining this with relation (3):

C d g

y +−= ∫ ϑ ϑ ω cos

sin

3

2 2 0

where C is a constant of integration we will determine from the fact that the y – coordinate of the magnet position is initially equal to zero, while the x – position is an initial x0. In the end

one obtains the trajectory equation

(1)

(2)

(3)

(4)

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( )

−−

−−= 1

sin9

4 1

sin

3

2 2 0

22 0

2 0

x x g

g y

ϕ

ω

ω

ϕ

Taking a closer look at this relation we see that the trajectory is in fact a portion of the circle

( ) ( ) 2220 r r y x x =−+− with radius 2 0

sin32 ω

g ! This is a fact well substantiated

experimentally. However, this derivation was made in the limiting case 0≈ β ; a complete treatment would call in for numerical techniques, necessary for solving the nonlinear coupled system (1) and (2) in the case 0> β . The calculation is rather cumbersome but in our opinion less physically appealing – the considered special case seems to catch all the main features of the effect. Rewriting the radius as

D R B

g r

z

ρ µ

π 4sin

6 =

where R is the magnet radius, D the thickness and ρ the density of the magnet, we see that it

increases as the inclination angle grows, as we have predicted before from elementary considerations; also, we see that the radius strongly depends on the inertial properties of the magnet, increasing with increasing magnet radius and density, in agreement with our previous conclusions. We might say that altering the angle β would have no significant effects on these dependencies; it would merely change the path a bit due to the more complicated geometry involved.

The measurements on a nonconducting plane were quite simple to conduct, aside from some matters requiring greater attention. The main problem was insulating the magnet from all parasite fields due to wiring and the many iron parts in a laboratory. To minimize this influence our incline construction used no metal parts and was set as far away from iron

pieces as possible. The incline itself (a wooden plate with a sheet of white paper glued to the measuring side) was standing on a wooden box so as to be separated from the wiring and metal tubing or constructions in the floor. In order to obtain the magnet trajectory we soothed the magnet we worked with so it left a trace on the paper glued to the wooden incline. The measurements were repeated for several inclination angles, yielding the predicted circular trajectories (fig. 1.).

3. The Conducting Incline.

3.1. Theory.

This second case is, as we have mentioned in the beginning, by far the more interesting and complicated situation. This is due to the fact that a moving magnet, sliding or rolling, ca

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