3B SCIENTIFIC PHYSICS U10371 Mode D'emploi page 8

solid pendulum's swing is rapidly damped (brak-
ing) to a stop, whereas the slotted pendulum un-
dergoes several swings first.
•
Explanation: in the experiments in section 4.1 a
current was flowing through the conductor swing.
This brought about a movement of charges (elec-
trons) in a magnetic field, which evidently led to
a measurable force (the Lorentz force) acting on
the electrons.
Fig. 5: Experiment set up for "induced eddy currents"
•
In this experiment too, charges – free electrons
in aluminum – are set in motion in a magnetic
field, whereby the motion here is of a mechani-
cal nature. Through this motion the Lorentz force
also acts on the electrons, leading to a flow of
electrons, i.e. a current flowing in the aluminium,
which in this experiment flows vertically from top
down or vice versa depending on the motion of
the pendulum.
•
In the solid pendulum there is a kind of "short-
circuit" due to the fact that the induced current
can flow back through the parts of the pendu-
lum outside the magnetic field. This is how an
eddy current arises, which can be very high and
can lead to the build up of heat in the aluminium.
The pendulum energy is initially converted into
electrical energy and then into heat.
•
In the slotted pendulum eddy currents cannot
build up because the slots isolate the aluminium
area outside the magnetic field from the area
inside the field. Indeed the electrons here are also
initially pushed in one direction or the other but
once a great many electrons have collected at the
2) Grimsehl, Physik II, Ernst Klett Verlag Stuttgart, 1955
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany • www.3bscientific.com • Technical amendments are possible
top or bottom of the pendulum they repulse each
other with the result that the voltage generated
is in equillibrium with the Lorentz force and cur-
rent does not flow. Thus the pendulum energy is
not converted into heat.
4.3 Dia- and paramagnetism
•
The experiment setup corresponds in principle
with Fig. 5. But now instead of suspending the
pendulum we suspend either the aluminum or
the glass rod in the magnetic field (prior to this
please smooth out any twisted threads, see Sec-
tion 3). The glass rod first starts to turn one way
and then the other while the aluminum rod only
very slowly (due to induced eddy currents, see
last section) into its final position. After some time
has elapsed the rods settle in the positions shown
in Fig. 6.
Fig. 6: Glass rod (left) and aluminium rod (right) in the magnetic field
•
By loosening the knurled screw which holds the
magnets and slowly turning the magnet it can be
demonstrated that the orientation of the rods re-
main aligned relative to the magnets and that
consequently the position cannot be attributed
to the rest position emerging mechanically
(caused by twisted threads).
•
Explanation: although neither glass nor alu-
minium are magnetic, both rods align themselves
in the magnetic field. The decisive variable here
is the relative permeability µ
factor by which the flux density of of the mag-
netic field is multiplied within the material con-
cerned, as compared to the flux in a vacuum.
Surprisingly – and in contrast to dielectric con-
stants – the relative permeability can be greater
or smaller than 1. In aluminum = 1.000023
in glass = 0.99999. Thus in aluminum the flux
density is amplified and the rod turns in the field
direction. This effect is referred to as paramag-
netism. In glass we have the opposite effect. The
rod rotates out of the field and the effect is called
diamagnetism.
8
This specifies the
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