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• Example of an experiment sequence:
Exp. no.
Pole piece separation a [mm] Pole shoe width b [mm]
1
2
3
4
5
6
4.1.3 Experiment evaluation
•
The conductor swing is considered as a simple
mathematical pendulum, i.e. the weight of the
braided copper strands is neglected and the cop-
per wire is seen as a point mass (m = 6.23 g). The
effective pendulum length s is somewhat smaller
than the length of the copper strands, due to the
fact that these do not fold cleanly at the upper
edges, when the conductor swing is deflected. The
length s is thus the result from the theoretical
point where the elongation of the linear copper
strands intersects with the verticals (cf. Fig. 2). In
approximate terms: s = 200 mm.
•
The resulting force on the copper strand F
prised of the Lorentz force F
and is inclined at an angle ϕ because the copper
strand is (virtually) subject to no lateral forces.
Consequently it is true that:
F
ϕ
L
=
tan
F
G
⇔
c
s
=
F
mg
L
c
−
1
s
•
In the above experiment sequence the pole pieces
in experiments 4 - 6 were rotated by 90° in com-
parison to experiments 1 - 3. As such the conduc-
tor length which protrudes into the magnetic field
changes. During the evaluation however, the true
pole piece size may not be used as the basis be-
cause the magnetic field "bulges out" beyond the
edges (see Fig. 3).
Fig. 3: Bulging effects at the edges of the pole pieces
10
10
10
10
10
10
is com-
K
and the weight F
L
(1)
2
Deflection c [mm]
50
50
50
20
20
20
•
The resulting effective length within the magnetic
field is approximately:
b
= b + a
w
•
The evaluation of the experiment series using
Equations 1 and 2 yields the following:
Exp. no.
Effective
conductor
length b
1
2
3
4
5
6
G
•
The result is also depicted in Fig. 4. It is immedi-
ately discernible that the Lorentz force is propor-
tional to the current. An evaluation of the linear
gradients shows furthermore that the Lorentz
force is also proportional to the effective conduc-
tor length. Consequently it holds true that:
∝ b
Ι
F
L
w
Fig. 4: Lorentz force as a function of the current flowing in the conductor.
Square symbols: b
= 60 mm, rhombuses: b
W
4.2 Induced eddy currents
•
The experiment set up is depicted in Fig. 5. The
pole gap amounts to approximately 10 - 30 mm
and is varied. If both pendulums are jointly dis-
placed by the same angle and then released, the
7
Current Ι [A]
15
0.57
30
1.20
45
1.87
15
1.16
30
2.36
45
3.57
Current Ι
Lorentz
force F
L
[mm]
[mN]
w
60
4.60
60
9.27
60
14.1
30
4.60
30
9.27
30
14.1
= 30 mm
W
(2)
[A]
0.57
1.20
1.87
1.16
2.36
3.57
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