Inspiring experiments exploit strong attraction of magnets

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M AGNETISM
<strong>Inspiring</strong> <strong>experiments</strong> <strong>exploit</strong>
strong attraction of magnets
The extra strength of neodymium iron boron magnets
provides opportunities for several <strong>experiments</strong>
(Featonby 2005). Here is a further selection that
should encourage you to think of even more.
Single-wire motor
Astrong neodymium disc magnet with a small cylindrical
magnet is placed below a 1.5 V cell (figure 1).
A copper wire frame is then bent into the shape
shown, with a loop at the bottom, which makes contact
with the cylindrical magnet. The size of this
loop needs to be slightly bigger than the smaller
magnet so that it is able to move freely, but still make
good electrical contact. The top of the wire frame
rests on top of the cell to complete the circuit.
The magnet holds the cell balanced in the vertical
position as well as being part of the circuit. The
current will, in this case, flow outwards from the
cylindrical magnet along and up both sides of the
wire. It is the forces on the horizontal lower wire
that provide the couple to rotate the whole frame.
The copper frame used was obtained by stripping
some 13 A mains cable containing 1 mm diameter
copper wire, and this frame is approximately
0.09 × 0.11 m. It is worth trying different-sized
frames to achieve a good effect. The key is to keep
the moment of inertia of the frame small.
Simple motor with single magnet
Asimilar arrangement has been described in a recent
issue of Physics Education, but details are repeated
here with some additional information. The strong
neodymium disc magnet holds the nail onto the bottom
of the cell, but it is still free to rotate (figure 2).
In this case it is the current flowing through the
magnet that generates the force that rotates the
magnet/nail arrangement. Avariety of magnet sizes
can be used to demonstrate this. I have used discs
from about 10 mm in diameter.
I have also successfully used both flexible and
rigid copper wire, and also a strip of aluminium foil
to make the connection between the top of the
Figure 1. Single copper wire rotating motor.
1.5 V
dry cell
steel nail
(2 inch)
neodymium
button
magnet
connecting wire
magnetic field
(within magnet)
current
steel nail
neodymium magnet
Figure 2. A simple motor made from a
neodymium button magnet holding a 2 inch steel
nail onto the bottom of a 1.5 V cell.
battery and the magnet. The nail must be attached
to the exact centre of the rotating magnet to produce
steady motion.
Rolling wire and kicking wire
During many years of teaching I used to demonstrate
the force on a conductor by rolling a thick
piece of brass rod down two brass curtain rails
(figure 3). This was possible using an Alnico magnet
held above the wire, producing a vertical field.
The disadvantage of this arrangement is that the magnet
has to be moved to keep the wire moving.
Asimilar experiment was carried out using a large
Alnico U magnet (figure 4). The vertical field of the
magnet is at right-angles to the current and the subsequent
motion. The problem with this experiment is
292 P HYSICS E DUCATION July 2006

Figures 3 and 4. The rolling wire (top image): a
demonstration of the force on a conductor
achieved by rolling a thick piece of brass rod along
a pair of brass curtain rails using an Alnico
magnet held above the wire to produce a vertical
field. The kicking wire (bottom): a large Alnico U
magnet is used to produce a vertical field at rightangles
to the current and to the subsequent motion.
the expense of the large Alnico magnet, but a similar
effect can be obtained with magnadur magnets.
With neodymium iron boron magnets a kicking
wire can be set up for less than £5 by mounting
a single round neodymium magnet in a hole in a
block of wood and using a trapeze-style copper
frame (figure 5).
Double magnet roller
We can use neodymium disc magnets as both the
wheels of a roller and the source of the magnetic
field. In this set-up (figure 6) two discs are placed
carefully on the smoothed, flat end of a nail with its
head cut off. It is important that the nail axle is
aligned exactly with the centres of the magnets to
produce a good run. The strength of the neodymi-
July 2006
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Figure 5. Kicking wire set-up using a neodymium
magnet and a copper wire trapeze frame. The
wire carries the current across the top of the
magnet and the force causes the frame to move.
nail shaft
aluminium foil
neodymium magnet
magnetic field
inside magnet
current
Figure 6. Rolling motor (shaft of nail) with two
neodymium disc magnets, with explanation.
ums enables them to be fixed with like poles facing
each other. This is essential to produce a turning
force on both wheels in the same sense. The tracks
are two strips of aluminium foil, stuck to the bench
with overhanging ends for electrical contacts. The
current flows down the aluminium and up through
one magnet, along the nail and down through the
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Figure 7. Another simple motor that uses a
neodymium magnet with a bolt through its centre.
Figure 9. ‘Loudspeaker’with cardboard box.
other. The forces arise within the magnetic wheels
and the arrangement accelerates down the track.
Other motors
In figure 7 a ring magnet has a bolt through its centre
and lies on a copper circuit board. The magnet makes
contact outside a circle etched in the board and a pin
though the centre enables contact to be made there.
In figure 8 a strip of aluminium foil rotates rapidly
on top of the magnets. Contact is made to the
central part of the rotating foil with some pencil lead,
while the foil touches the outside of the magnets.
Loudspeaker
The strength of the neodymium magnet can be
shown with a simple ‘loudspeaker’(figure 9). Acoil
of thin copper wire is taped to the surface of a shoe
box with the leads connected to the earphone output
of a portable hi-fi. Simply bring up a neodymium
magnet and the forces on the coil move it to produce
sound from the box. Varying the position of
the magnet and hence the strength of the field at the
coil greatly affects the volume of the sound.
Figure 8. A motor arrangement using a piece of
aluminium foil rotating rapidly on top of magnets.
Figure 10. A simple
but effective loudspeaker
unit that uses
a piece of aluminium
foil over a large
neodymium magnet
and passes the
output from a radio
through the
aluminium foil layer.
Even simpler is to lay a small piece of aluminium
foil over a large neodymium magnet and pass the
output from a radio through the foil (figure 10). The
sound from the radio is clearly heard. This set-up
can also be used as a simple microphone.
You may by now be thinking that neodymium magnets
are the best thing since sliced bread, but with
their strength come disadvantages. Neodymium iron
boron is susceptible to corrosion if unprotected,
though this will not be of great concern to us because
commercially available magnets are coated.
Curie temperature
Of more concern is the Curie temperature. At only
120 ºC neodymium magnets cease to be magnetic,
although this loss can be recovered by remagnetization.
The comparable temperature for ferrite
magnets is 250 ºC and for Alnico it is 550 ºC. The
loss in magnetism in neodymium is irreversible at
320 ºC compared with 860 ºC for Alnico.
This low Curie temperature for neodymium can
be demonstrated simply by using a tealight candle
as a heat source (figure 11). A small neodymium
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Figure 11. Demonstration of the Curie temperature.
ring magnet is suspended on a swing cradle made
of thick copper wire, which is in turn suspended on
a frame of the same copper wire. This small magnet
is attracted to a larger, fixed neodymium magnet
(25 × 5 mm). When the candle is lit, it heats up
the small magnet, which then loses its magnetism
and falls back with the swing in the vertical position.
This, however, takes it out of the flame and
enables it to cool, be remagnetized and attracted
once more to the larger magnet. Here it is again in
the flame, so it heats and loses its magnetism again,
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and the cycle repeats. The overall size of my demonstration
can be seen from the size of the tealight
used. Considerable testing is required to produce a
reliable demonstration.
Safety warning
Neodymium magnets are dangerous because of their
strength and the fact that they shatter easily. They
should not be given to children to use unsupervised.They
should be kept away from electronic
apparatus and the magnetic strips on bank cards.
References
Featonby D 2005 Phys. Educ. 40 505–8
Neodymium magnets are available from
www.powermagnetstore.co.uk
www.sci-toys.com/scitoys/electro has
instructions about how to construct a number of
<strong>experiments</strong> using neodymium magnets
www.wondermagnet.com also has some
interesting <strong>experiments</strong>
www.amasci.com/neodemo.html includes some
neodymium demonstrations
David Featonby, North East Teacher Network
coordinator.