Physics Teaching in Engineering Education PTEE 2011
MAGNETIC FIELD LINES AND ELECTROMAGNATIC INDUCTION
K. HÄRTLEIN, L. OROSZ, P. PACHER
Department of Physics, Budapest University of Technology and Economics
H-1111 Budapest, Budafoki út 8., Hungary
E-mail: pacher@eik.bme.hu
A simple device has been built to demonstrate that magnetic field lines do not exist and so
there is no “line cutting” as well, often used in the explanation of electromagnetic induction. It
consist of two coaxial disks: one of them is a strong permanent magnet producing the axial
magnetic field, the other one is a conducting disk with sliding electrical leads. When the
magnet is at rest and the conducting disk is turned, induced emf appears between the rim and
axle of the disk. But when the conducting disk is stationary and the magnet is rotated, no
induced emf can be measured, demonstrating the fact that the rotation of the magnet has no
effect on its magnetic field. Although there is no “line cutting” when both disks are rotated
simultaneously, induced emf can be detected between the axle and the rim of the conducting
disk.
Keywords: magnetic field lines, line cutting, electromagnetic induction
INTRODUCTION
Magnetic field lines are used to visualize the magnetic field. When drawn, the distance
between them is an indication of the strength of the field. Various phenomena have the effect
of displaying magnetic field lines as though the field lines were physical phenomena. For
example, iron fillings placed in a magnetic field line up to form lines that correspond to field
lines.
Students are often taught that the magnetic flux through a given surface is proportional to
the number of magnetic B field lines that pass through the surface and electromagnetic
induction is also explained by means of cutting magnetic field lines (lines of forces). All these
make the impression, that field lines are real objects.
DISCUSSION
The unipolar dynamo or Faradays disk [1] is described in most general physics textbooks.
It is a DC generator comprising an electrically conductive disc rotating in a plane
perpendicular to an external axial static magnetic field.
Suppose that the magnet producing the external field is removed and the conducting disk
is replaced by a conducting magnetized disk, which supplies its own magnetic field parallel to
the axes [2]. Most student, even physics teachers and electrical engineers will immediately
say that such a device (the so called one - piece Faraday generator) will not work, although –
as it was found by Faraday in 1851 [3] - the device does generate emf when the disk is
rotated. The false imagination of the magnetic field - as the lines of forces were tangible
things attached to the magnet - deceives us!
A simple device has been built and used to demonstrate in class that magnetic field lines
do not exist. Field lines exist in our mind only, and therefore there is no “line cutting” as well,
thought to be responsible for electromagnetic induction.
Physics Teaching in Engineering Education
PTEE 2011
The device consist of two coaxial disks, one of them is a strong permanent magnet
producing the axial magnetic field, the other one is a conducting disk with sliding electrical
lead. The disks can be rotated independently.
When the magnet is at rest and the conducting disk is turned, the device works as a
conventional Faraday disk and induced emf appears between the rim and axle of the disk. The
result corresponds to our expectations.
When we ask the students: “What do you expect, if the conducting disk is stationary and
the magnet is rotated?” the majority answers: the same result. And they are astonished when
the experiment is done and face the result: there is no induced emf. This experiment
demonstrates that the rotation of the magnet has no effect on its magnetic field. There are no
magnetic field lines attached to the magnet as we usually imagine how the magnetic field
looks like!
In the third experiment both disks are rotated simultaneously. Although in this case there
is no “line cutting”, induced emf is detected between the axle and the rim of the conducting
disk. This proves, that induced emf has nothing to do with the so called “line cutting”.
CONCLUSIONS
The concept of magnetic field lines is a useful tool to visualize the magnetic field and
helps us to have an impression about the structure of the field. The simple experiments
described above confirm the students that even in classical physics such picturesque notions
cannot be taken too seriously. For the correct results one should turn to the mathematically
formulated equations of laws of nature.
References
[1] M. Faraday, Philos. Trans. R. Soc. 1832. p. 125. Also Faraday’s Diary, ed. By T. Martin
(Bell, London, 1932), Vol. I. p. 384, paragraph 117.
[2] M.J. Crooks, D. B. Litvin, P.W. Matthews, R. Macaulay, J. Show, Am. J. Phys., Vol. 46.,
No. 7 (1978)
[3] M. Faraday, Philos. Trans. R. Soc. 1852. p. 25. Also Faraday’s Diary, ed. By T. Martin
(Bell, London, 1932), Vol. V. p. 403, paragraph 11 371.