What is rewinding the electric motor

Understand the fundamentals of the electric motor clearly - Determine the dependencies of the magnetic field of the finite coil

Understand the fundamentals of the electric motor clearly - Determine the dependencies of the magnetic field of the finite coil

cover photo: Experimental setup to investigate the dependencies of the magnetic field of a finite coil

Brief description
The dependence of the magnetic field of a finite coil on current strength, number of turns and coil length should be investigated experimentally in order to understand the importance of the coil in the electric motor.
Electricity, magnetism
Classification in the curriculum
Suitable for: Grade 9-10, sec. I
Basic concept: Interaction, energy
Implementation form Demo experiment, group experiment
Number of experiments in this sub-category 1
Claim of the structure heavy
Surname: Michael Meyer
Contact: @
University: Humboldt University of Berlin
Supervisor: Dr. Ulrike Gromadecki-Thiele
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The electric motor has become an indispensable part of the modern technological world. It is more or less obviously built into almost every electrically operated device. The possible uses of electric motors are as diverse as their technical implementation options.[1] For this reason, school lessons in grade 9/10 can only be about making the basic functional principle of the electric motor understandable: the interaction between two magnets. In the simplest case of technical implementation, this is the interaction between a rotatably mounted bar magnet (rotor) and the magnetic field of a finite coil (stator).

In the present experiment, the focus is on the finite, current-carrying coil as a controllable element of the electric motor. In terms of content, the dependence of the force effect of their magnetic field on the magnitudes of the current strength becomes , Number of turns and winding density Developed. Experimenting is methodically practiced using an analog spring dynamometer.

For this is the knowledge of the magnetic force law

necessary, whereby the magnetic sample symbolizes on which the magnetic field of the coil works. The magnetic field of the finite coil is generally through


described. This equation is only for the teacher to keep in mind and can be approximated in a form that is manageable for schoolchildren

from which the dependencies to be examined result.

Didactic part

The electric motor is a technical device that the students often use without reflection in their everyday lives. This unconsciousness can be taken up as an introduction and overcome by showing the students the various places where the electric motor is used - in the form of pictures or tangible devices. From this the question can be developed why the electric motor is or can be used in so many ways, which in turn leads to the problem of how it works. By examining the inside of an electric motor, the students can quickly identify its main components: permanent magnet and coil (s). While the students are familiar with the permanent magnet from their everyday lives, they should not be able to see why it interacts with the coil. This ultimately makes a closer examination of the current-carrying coil necessary.

At the end of the unit, the pupils should get an impression of the connection between the magnetic field of the coil, which can be regulated by the variables examined, and the mechanical power of the electric motor in the form of the speed, which in turn results from the interaction between rotor (permanent magnet) and stator ( Coil). It is precisely this energy conversion - electrical into mechanical energy - that makes the diverse use of electric motors as drive units so attractive.

By using the electric motor, the treatment of the magnetic field of the finite, current-carrying coil can be embedded as a physical phenomenon within a context-oriented lesson. According to Heinz Muckenfuß, this “meaningful” context awakens[3] the interest of the students and motivates them to deal with the subject matter. Furthermore, in this way, they are able to "implement the technical implementation of scientific findings with their personal or social significance"[4] clear.

The experiment presented here can open the teaching series "Movement through electricity - electricity through movement", in which, according to the Berlin framework curriculum for secondary level I, double year 9/10, the electric motor is explicitly dealt with and interpreted as an energy converter. In addition to acquiring new knowledge about the magnetic field of the finite, current-carrying coil, the previous knowledge of the students can also be recorded and repeated in the experiment, which is necessary for understanding the experiment in particular and the functioning of the electric motor in general. According to the Berlin framework curriculum, secondary school students from the two years 7/8 should have a basic knowledge of magnetostatics about the properties of a permanent magnet (magnetic poles, magnetic field) and magnetic interaction (repulsive or attractive effects of magnetic poles of the same name or of different names) as well as the magnetic effect of electric currents must be known.

The experiment presented is a demonstration experiment that is based on two things Ways to be used:

  1. By observing the deflection of the dynamometer as the various variables change, the experiment can serve to give the students a qualitative To give an idea of ​​the dependence of the strength of the magnetic field of a coil. The focus is on imparting physical knowledge.
  2. The experiment also offers the possibility of quantitative Evaluation. By means of the force meter, measurement data about the force exerted by the magnetic field of the coil on the magnetic sample can be logged as a function of the various variables and presented in tables and diagrams. The approximate equation can then be derived from these be derived. In addition to imparting physical knowledge, the focus in this case is on the methodical aspect of the competency acquisition of knowledge, i.e. in particular logging, representing and mathematicizing.

While the first variant derives the physical content from the description of the observation, the second variant gives the students the demanding task of interpreting the measurement results in order to formulate a law in mathematical form. This also requires an interdisciplinary transfer of mathematical knowledge about linear functions into the physical context. Therefore, both variants can also be understood as a form of performance differentiation - based on an entire class or individual students. However, variant b has the advantage that the pupils practice critical analysis of information. This is a competence that you need in all areas of life in the face of a flood of information in the modern media society.

Experiment instructions


The following materials are required for the experimental setup:

Fig. 1: Overview of the materials used
Fig. 2: Circuit diagram for the experiment setup
Fig. 3: Back of the resistor used in the construction (16)
(1) 1 large tripod base
(2) 1 small tripod base
(3) Coils, different number of turns and coil length
(4) 1 table, height adjustable
(5) 1 ammeter (3 A -)
(6) 1 power supply unit (20 V -)
(7) 1 power plug
(8) 1 spring dynamometer (1 N)
(9) 1 stand rod (approx. 100 cm)
(10) 1 stand rod with stand socket (approx. 20 cm)
(11) 1 tripod hook
(12) 1 small round bar magnet
(13) 1 switch
(14) 1 universal clamp
(15) 5 connecting cables (1 red, 1 blue, 3 black)
(16) 1 resistor, continuously adjustable (approx. 25 Ohm)
String and tape

According to the cover photo first becomes the stand rod (9) in the tripod base (1) attached. The tripod hook will then (11) with the smaller stand rod (10) connected, which in turn is attached to the previously set up stand rod with the help of the stand sleeve.

Now you can use the tripod hook (11) the dynamometer (8) and on these in turn the round bar magnet (12) to be hanged. For this it is necessary beforehand on the magnet (12) to attach an eyelet formed from the cord using adhesive tape. With this setup, make sure that the spring dynamometer (8) securely in the tripod hook (11) hangs to avoid falling off while experimenting.

The desk (13) is in the smaller tripod base (2) attached. The aim of this construction is to ensure that the switch-on and switch-off process can be easily observed by the students. A coil (3) is now according to the circuit diagram in Fig. 2 about the sliding resistance (16), the demonstration measuring device (5) and the switch (13) to the power supply unit (6) connected. In accordance with technical conventions, the red connecting line leads to visualize the flow of electricity (15) from the + - output of the power supply (6) away and the blue connecting line (15) to the - - entrance. Since it is unclear where the current begins to "flow back", the other components of the circuit (Fig. 2) the black connecting cables (15) used.

Then the bobbin is put on the table like this (4) placed so that an opening points upwards and so under the suspended spring dynamometer (8) placed that the suspended round bar magnet (12) about 1 cm into the inside of the bobbin. It is advisable to do this at the end of the magnet (12) Measure and mark 1 cm and then the immersion depth with the height-adjustable table (4) (either by eye or with the help of a ruler placed on the spool). Finally, it is important to ensure that the polarity of the magnetic field of the coil (3) is oriented so that the sample magnet (12) is also pulled into the inside of the bobbin.

Depending on the size of the resistance (16) and the resistance of the coil used, it is possible that the current strength is not zeroed. This can be corrected by adding an additional resistor (in series) or by changing the pointer of the ammeter (5) is set to zero.


The basic procedure is as follows: The power supply unit (6) is set to 15 V and the circuit via the switch (13) at the maximum set resistance (16) closed. Now the resistance becomes (16) slowly reduced until the desired amperage is reached.

A series of measurements can easily be recorded by using the value displayed on the dynamometer (8) is read by the teacher or a student and announced out loud, whereupon the other students write down this value. Alternatively, the measured values ​​can also be recorded centrally on the board.

When recording the measured values, however, the experimenter ensures that the test magnet is immersed at a constant depth (12) pay attention to the inside of the bobbin. equation shows the dependence of the magnetic field on the distance to the middle of the spool length. This means that the force exerted by the magnetic field inside the coil increases (3) on the sample magnet (12) exercises the deeper it dips into the inside of the coil. The measurement results would be falsified. However, the immersion depth can be adjusted by adjusting the table height (4) correct it easily. With every change in the structure - different amperage, number of turns or coil length - the table height must be adjusted so that the test magnet (12) immerses constantly approx. 1 cm into the inside of the bobbin.

Annotation: That when trying on the immersion depth of the test magnet (12) Paying attention to the inside of the coil can be relatively unexpected for the students, because from the equation this cannot be seen. This irritation can, however, be used as an opportunity to talk to the students about approximations and modeling in physics. Because the equation on which the experiment is based is a model because it is derived from considering an infinitely long coil. The students should have prior knowledge that the strength of the magnetic field depends on the distance to the object generating the magnetic field. This can therefore be linked to and the distance dependency generalized as a general property of magnetic fields.

Results and optional evaluation options

The pupils observe that, for example, when the current strength increases, the test magnet (12) is drawn deeper and deeper into the coil. If the current strength is reduced again by increasing the resistance, the spring force of the dynamometer pulls (8) return the magnet to its original position. Qualitatively, the dependence of the strength of the magnetic field on the current strength would be shown.

By using coils with different numbers of turns and lengths and, associated therewith, different density of turns, it can also be shown that the strength of a magnetic field also increases with a greater number and density of turns or decreases with a greater length. By using coils with cores made of different materials, the dependence of the strength of the magnetic field on the magnetic permeability of the core materials used can also be investigated. Here, however, is the interaction between the sample magnet (12) and core material to be observed.

If the experiment is treated and evaluated quantitatively, the results in the Tab. 1 to 3 series of measurements listed and those in the Fig. 4 and 5 shown measurement curves. In the test runs of the Tab. 1 and 3 were coils with or. used. In the test run of the Tab. 2 was with an amperage of worked. It is advisable to take at least three values ​​from the spring dynamometer for each setting (8) to record. The measured force was plotted over the variable to be examined using qtiplot - a program for analyzing and visualizing data - so that the respective linear dependency can be clearly read. This was further clarified with a best-fit straight line.

Fig. 4: Graphical representation of the measured values ​​from Tab. 1
I. [A]F. [mN]<F> [mN]
NF. [mN]<F> [mN]
L. [mm]F. [mN]<F> [mN]
Fig. 5: Graphical representation of the measured values ​​from Tab. 2

The yellow line in Tab. 2 is in Fig. 5 marked as outliers. A coil from another manufacturer was used here. As can be seen, this has massive effects on the measurement, because theoretically this value should also lie on the straight line within its uncertainty range. It is therefore with the use of different coils strict towards it respect, think highly ofthat only coils one Manufacturer are used.

Only when evaluating the dependency on the coil length has a graphical evaluation proven to be impractical, since within the scope of the teaching aids available only two coils of different lengths were available for a certain number of windings (Fig. 6).[7] The following mathematical evaluation can be used here:

Fig. 6: The coils used to investigate the length dependency of the coil field

According to the equations and the proportionality applies

Of course, this must also apply to the individual force-length pairs, so that the relationship equation


With the appropriate medium forces <F> out Tab. 3 and according to Gaussian error propagation



and thus equality within the uncertainty areas. This confirms the linear relationship between the strength of the magnetic field of a coil and the reciprocal of the coil length.

Optionally, in classes with a strong mathematical understanding, this experiment can also be used to determine the strength of the test magnet used. For this purpose, a straight line of best fit must be placed in the respective measurement curve. This can be done by eye or it becomes the linear compensation function of the qtiplot program. The advantage of using the computer program is that the slope of the straight line A is indicated automatically and with a high degree of accuracy. However, this can also be calculated approximately by hand; e.g. between the smallest and largest value according to the general form

As an example in the following the count is made Fig. 3 treated:

From the comparison of the compensation function with equation and identifies the current strength as a variable , so results for the increase

and thus for the field strength of the sample magnet used (12)

The same procedure can be used for all three in Tab. 1 to 3 a value for the field strength of the sample magnet used (12) obtained, compared with each other and possible deviations from each other or to a reference value in the class discussed. This exercise strengthens the students' interdisciplinary thinking and illustrates the usefulness of graphical representations in physics. It also stimulates thinking about measurement uncertainties and sources of error in the experimental setup.

Measurement uncertainties

The ones in the Fig. 4 and 5 The uncertainty areas shown resulted from the force values ​​as the sum of the standard deviation of the individual measured values ​​(6 mN) and the reading error of the spring dynamometer, which was estimated with half a scale division (10 mN). The uncertainty range of the current intensity also resulted from the reading error of the ammeter (5), which was estimated to be 10 mA.

Deviations in the measured values ​​can result from the following:

  1. The immersion depth cannot be kept completely constant. Ultimately, it is done through a sense of proportion using the table (4) corrects what can be done more or less precisely with aids (e.g. a ruler). This has an influence on the acting force.
  2. The input voltage varies. E.g. the resistance (16) reduced in order to adjust the current strength, but also the total resistance of the circuit is changed, which in turn affects the voltage source (6) acts back and changes the input voltage.
  3. With equation it is an idealization. Real observations do not have to completely coincide with this.

safety instructions

  • When trying to do this, make sure that the coils are not operated at higher currents, as indicated on them. A strong temperature increase inside the coil and its possible damage would be the result.
  • Particularly when changing the coils, make sure that the generator is switched off every time. Although there is only a low voltage, the students should still be trained to use electrical devices carefully and safely.
  • When working with the adjustable resistor (16) make sure that on its back (Fig 3) the wires are exposed. With the power supply switched on (6) A current flows through it. Avoid touching it.
  • In order to ensure that the experimental set-up does not tip over, it must be ensured that the extensions of the stand base (1) with the alignment of the horizontally attached stand rod (10) match (see section construction).

Literature and Notes

  1. ↑ For more in-depth information, see Meschede, Dieter (ed.): Gerthsen Physik, 25th edition, Berlin Heidelberg 2015, pp. 437-441, Handbuch des Physikunterrichts. Secondary level I, Vol. 6: Electricity II / Electronics, ed. v. Rainer Götz, Helmut Dahncke and Fritz Langensiepen, Cologne 1996, pp. 133-139 and Rossmann, Axel: Structure and simulation of technical systems, Vol. 4: Electrical machines and transformers, Berlin 2014.
  2. ↑ For the exact derivation see Demtröder, Wolfgang: Experimentalphysik 2. Elektrizität und Optik, 6., revised. u. act. Ed., Berlin Heidelberg 2013, pp. 93f.
  3. ↑ Muckenfuß, Heinz: Learning in a meaningful context. Draft of a modern didactics of physics lessons, Berlin 2006.
  4. ↑ Berlin framework curriculum physics for secondary level I, grades 7-10, p. 32. To be found at https://www.berlin.de/sen/bildung/unterricht/faecher-rahmenlehrplaene/rahmenlehrplaene/ [last access: 05.08.2016 , 1.30 p.m.].
  5. ↑ The idea for the experiment was taken from Physikalische Schulversuche, Vol. 9: Elektrizitätslehre II, ed. v. Sprockhoff, Georg, 8th edition, Berlin 1988, p. 49. On p. 67-71 there are further possibilities of force measurement (using a beam balance, magnetometer, magnetic pointer).
  6. ↑ GDR teaching material reels were used when recording the measured values ​​presented here. With various plug-in options, three different numbers of windings could be implemented in one device. However, these windings were not distributed over the entire length of the device. It was no longer possible to determine the length over which the respective windings were distributed. Therefore, not only the number of turns was varied, but effectively the number of turns density, which was plotted in Fig. 5.
  7. ↑ If more than just two pairs of coils are available, a graphic evaluation can of course also be carried out.