Download 1 Major Misconceptions on Chapter “Electromagnetism

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Major Misconceptions on Chapter “Electromagnetism”
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Misconceptions on “Magnetic Force and Fields”
The greatest confusion in the study of magnetism arises here.
Students learn that opposite magnetic poles attract and similar poles repel;
then they observe the north pole of a magnet rotate until, by magnetic
attraction, it points toward the North Pole of earth. This seems to be a
contradiction.
Most students are aware that Earth has a magnetic field but tend to
think of a monstrous bar magnet in the centre of Earth inclined at a small
angle to Earth’s axis. Point out that for this analogy to work, the south pole
of this magnet would have to be at the North Pole of Earth.
What must be stressed is that the proper names of the poles of a
magnet are the “north-seeking pole” and the “south-seeking pole.”
Therefore, it is natural that the north-seeking pole of a magnet should seek
the northerly direction. The magnetic pole of the Northern Hemisphere has
the same magnetic properties as the south-seeking pole of a bar magnet.
As a result, there is really no contradiction in law of magnetic poles.
2. Misconceptions on “Magnetic Materials”
Some common misconceptions with magnets are as follow:
 The magnetic field around a bar magnet has something to do with the
amount of charge on the magnet. Remember: The magnet is neutral
and has nothing to do with a net positive or negative charge. (The
north pole of a magnet will not attract or repel an ebonite rod).
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 All substances with domains can be magnetized if the domains are
lined up. Remember: Most substances do not have domains, so they
cannot be magnetized.
Figures in this section show the domain boundaries as being square or
rectangular. This is done for the sake of simplicity; n reality, the boundaries
are irregular in shape and size, and the magnetic orientation may be any
direction (not just up, down, left, or right).
3. Misconceptions on “Oersted’s Discovery”
Students often do not understand the three-dimensional nature of the
magnetic field around a long, straight conductor. They sometimes miss that
the field gets weaker as the distance from the wire increases, or they do
not draw the magnetic field lines farther apart to represent this.
Students often have trouble picturing the true nature of the field around
a long, straight wire so careful observations during any activities and close
attention to the diagrams can help with this.
4. Misconceptions on “The Magnetic Field of a Coil or Solenoid”
Students often have trouble with three-dimensional nature of the field, since
it is always drawn as if it is two-dimensional. Give special attention to the
three-dimensional nature of the field during any instruction, demonstrations,
activities, or discussion.
5. Misconceptions on “Conductor in a Magnetic Field –The Motor
Principle”
Students have more trouble with the right-hand rule than with any of the
others. It is requires a lot of practice by the students.
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6. Misconceptions on “Application of The Motor Principle
Many of the students will have trouble following the diagrams of the
different devices at first. Allow hands-on activities to help the students with
this. Many misconceptions can be cleared up with the construction of a
motor.
7. General misconception on Electric and Magnetic fields
Since electric and magnetic fields are very different in their properties,
students often assume that the fields are separate entities. In fact, the
fields are not separate entities, but two views on a single entity electromagnetic field where electric and magnetic fields are inseparable. For
example, a charge at rest creates only the electric field. Nevertheless, a
moving observer may also register a magnetic field in this case (since
motion is relative). The opposite is also true: a magnet at rest creates only
the magnetic field, but a moving observer may also register an electric field.
Therefore, presence of electric and magnetic sides of the field does depend
on an observer. The fields are rather two different points of view on the
same entity (electro-magnetic field).
8. Misconception: in electromagnetic induction, lines of magnetic
fields must cut through wires of the coil.
In fact, there is no such requirement, it is enough for the magnetic field to
go just through a region inside of the coil, and all wires of the coil may stay
outside of the magnetic field. This and similar examples may create severe
difficulties in understanding of the nature of the induction, in particular, how
the magnetic field away from the wires may act on the electrons in the
wires. (Answer: The magnetic field act on the electrons in the wires via
loops of the electric field, created by the changing magnetic field).
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9. Misconception: in electromagnetic induction, either magnetic
field must change or the coil must rotate (or both).
In fact, there is no such requirement. For example, it will be an induced
voltage in a straight wire, moving through a uniform magnetic field. Such
and similar examples may cause severe difficulties in understanding of the
induction phenomena since in the reference frame, attached to the wire,
there is no magnetic force that may act on the electrons, v=0. (Answer: the
moving observer, following the wire, will register an electric field that is
pushing the electrons in the wire.)