Electric Toothbrush
school level
general theme
special theme
theoretical
level
practical
level
provision
preparation
experiment
secondary
electricity
electromagnetic
induction
diode
2
2
weeks
aprox. 1 hour
5 min
An electric toothbrush helps to demonstrate the principle of the contactless charging of a rechargeable cell and the solid state operation of an electric motor, respectively.

Materials
  • electric toothbrush
  • power supply 10V~/5A~
  • 5 wires (30 – 50 cm long)
  • inductor coils, fitting into each other
  • low voltage d.c. motor
  • diode (e.g. 1N4007)
  • oscilloscope (optional)
  • an iron core for the coils

Preparations
In order to clarify the relationship between the original and the model, make sure to take the toothbrush apart so that you can show its relevant elements and their functions (fig.1 and 2).
The power supply of the toothbrush also serves as its electric charger. After opening the case you can see a small transfomer, which is applied to a printed circuit board, and the coil, which is soldered to the printed circuit board. You needn´t pay too much attention to the other components, resistors etc. which are not really necessary for the understanding of the phenomenon..
The opening of the toothbrush itself might cause some difficulty as it is watertight when closed.In this case, you can either use a saw or a knife for cutting the toothbrush open along its welded joint carefully. After that you can pull it apart.
If you wish to avoid possible slipping, it is advisable to clamp the toothbrush in a vice.
Now you can see a small coil which is connected with a rechargeable cell by a diode and a wire. The electric motor in the upper part of the toothbrush can possibly be used for the further model
It is most likely that the rechargeable cells of defective toothbrushes are leaking. If so, avoid any kind of contact.

Experiment / Function
Before starting to set up the model, make sure to place the components on the table in a manner that allows the pupils to understand their combination in the model later.
Connect the primary coil with the power supply. Connect the scondary coil with the core with the electric motor by help of a diode. Now push the secondary coil with the core into the primary coil, and the motor will start running because of electromagnetic induction (fig.3 and 4).
On an oscilloscope you can make the d.c. current visible which is necessary for the charging of a rechargeable cell. At this point, the rectifier-function of a diode could also be explained.
The coils are getting warm after some time, which, however, is completely harmless, considering the shortness of the experiment. Yet, you might make use of this effect and let the pupils estimate the efficiency factor of the apparatus.

Estimation
It also makes sense to let the pupils estimate the life of an electic toothbrush with a rechargeable cell. Considering that one charging provides 20 minutes of usage and that the cell can be charged a thousand times, you can clearly see that after 333 hours the toothbrush cannot be used any more. Until then you will have brushed your teeth 6666 times, if you brush them twice a day for three minutes each time, as recommended. The toothbrush will be nine years old then.
Without difficulty, results like these can be transferred to other machines with rechargeable cells, like razors or cordless phones.

Methodical Use
This experiment helps to explain electromagnetic induction as well as, especially, the principle of the transformer. Apart from that, the function of the diode can also be explained.

 

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