Magnetic Torch - (EU

Logo designed by Armella Leung,
www.armella.fr.to
Krzysztof Pawłowski
Center for Theoretical Physics, Polish Academy of Science
Warsaw
Magnetic Torch
Faraday's Law? Oh .. "The relationship between change in magnetic flux linking...", „A
student should be acquainted with CONTENTS of definition ...". We shall present concept
on how to reconcile program requirements with effective teaching of age-old students’
nightmare - one of Maxwell's laws. Of course, as usual it can be illustrated by a simple
and effective experiment. This time, we shall show how to build your own flashing
flashlight alias bicycle dynamo.
1. Needed materials
The following are needed for the
experiment:
Two neodymium magnets with a
diameter of about 15mm (sold in
many online shops - cost a few
Euros or less),
Plastic tube of diameter greater
than magnet diameter, for
example, syringes, test-tubes,
effervescent tablet package etc.,
Film wrapping,
Diode,
Copper wire,
gum for fastening wire to the testtube (piece of bicycle inner tube
seen on the drawing),
paper knife,
bit of cotton wool.
2. Implementation
Rys. 1. Zestaw elementów niezbędnych do wykonania
doświadczenia.
Wind around 300 turns of wire on the test-tube we, as shown in Figure 2. The task may be
facilitated by a piece of inner tube or rubber, which secures the wire and prevents it from
slipping out of the tube. The wire should be wound such that both its ends have a length of at
least 40 cm.
After winding all the coils, scrape insulation off both ends - Figure 3.
Fig. 2 Wind 300 turns of wire on the test-tube, secure
wire against slipping using inner tube
Fig. 3. Using paper knife, scrape insulation off the wire
ends.
To help observe luminous diode, it’s possible to make a casing for it. First, cut off the bottom of
film box, as in Figure 4.
Next make two small holes on film box cap. Wire ends stripped of insulation are pushed through
these holes and wrapped around the diode ends. Diode can be installed 'permanently' in the
flashlight by pressing its tips on film box cap. Such prepared diode is shown in Figure 5. The
cap with diode is pressed on film box with cut-off bottom.
Fig. 4. Cut off the bottom of film box.
Fig. 5. Fix the diode on film box cap.
Throw magnets connected by the poles Into the test-tubes with wound coils and ... ready.
Now it’s enough to shake the test-tube with wound coils. The diode should flash - as in Figure 6.
For the toy to last longer, place a little cotton wool in the tube before and after throwing in the
magnet. The cotton wool cushions the hitting of the magnet on the test-tube wall, protecting it
from damage.
Fig. 6. Shake the test-tube with magnet inside - diode starts flashing
3. Explanation, discussion and exercises
What happens when shaking the test-tube? The magnet inside is moved, this automatically
causes the movement of the magnetic field. Change of magnetic field causes current to be
generated in the wire.
It is worth noting that the diode does not light up before shaking the test-tube, that is, for current
to be induced magnetic field must change. This should shield us against a typical mistake
among the students, often forgetting about word "changing" in Faraday’s law.
Will the flashlight work if the magnet is outside of the test-tube?
For example, a magnet dropped in the direction perpendicular to the flashlight axis fails to
cause the diode to glow. But if dropped so that it falls close to the test-tube, parallel to its axis,
the flashlight should shine for a moment.
In both cases, the field varies in the same way. The main difference consists in change in
magnetic flux linking the frame.
In the first case, the magnetic flux is practically zero all the time, changing only slightly. If there
was an infinitely thin frame and a magnet was dropped so precisely that it all the time moved in
the direction of the frame and in the same plane as frame, such a flux would be practically equal
to zero. Such a situation is schematically illustrated in Figure 7.
Fig. 7. If a magnet is moving beside towards the frame (e.g., a magnet is approaching the frame, but is
moving in its plane), the magnetic field flux is close to 0.
In the latter case, changes in the magnetic flux may be large. This situation is illustrated
in Figure 8.
Fig. 8. If the magnet is moving in the direction of the frame, but not in its plane, then the magnetic field
flux can be large.
In summary - to generate electricity in the frame
• there must be a changing magnetic field, for example, a moving magnet,
• ultimately it is not the magnetic field itself, but its flux, for example a magnet must be
moving in a certain direction in relation to the frame, which determines whether current
will be induced.
All properties are hidden within the Faraday's law of induction:
Ε = − dφ/dt,
Where E is electromotive force, i.e. voltage induced in the frame conductor due to change of
magnetic field, φ - magnetic field flux linking the surface of the frame. The denotation dφ/dt is the
rate of magnetic flux with time – quantity, which ‘measures’ rate of change of magnetic flux. The
formula help calculate not only the value of induced voltage, but also the direction in which
electrons are moving in the frame.
Curriculum
Core curriculum: Electricity and magnetism.
Lesson topics: Faraday’s Law. Alternator.