in this video we give a brief qualitative introduction to permanent magnets in magnetic
field so in this picture we have a typical bar magnet it has two poles labeled as north and south
and there are magnetic field lines surrounding the magnet pointing from north to south
in addition the magnetic field points from south to north within the bar magnet the magnetic field
surrounding that bar magnet actually originates from the orientation of iron atoms that are in
the magnet the direction in which the current of the electrons flows around the atom determines
the direction of the magnetic field that it produces so as this electron orbits around
this iron atom it generates this dipole magnetic field around it and it's the sum of all of those
contributions through the entire permanent magnet with all the iron atoms on average lined up in the
same direction that creates the magnetic field of the permanent magnet when we're talking about
individual magnetic dipoles so like individual iron atoms it simplifies the presentation to
communicate the orientation of that magnetic field contribution with one single arrow so that's what
i'm going to do from now on in this side we get into a little more detail about the magnetization
of a piece of iron or other ferromagnetic material in ferromagnetic materials like
iron the orientation of the atoms might start out random you see in this first picture but the atoms
respond very strongly to any external field on the material and the atoms will align in the same
direction if they're exposed to a magnetic field but there's a catch different neighborhoods will
orient in slightly different directions that's what we're looking at in the second picture here
i have a magnetic field put across this piece of iron and the iron atoms respond to this by
aligning more or less in the same direction as that magnetic field but there are slight
discrepancies in the direction what happens is these little neighborhoods form where all the
atoms are lined up in exactly the same direction but when they run into other neighborhoods that
are lined up in a slightly different direction of boundary forms and the regions that form
the material here are called magnetic domains while the alignment with the field isn't perfect
this dramatic level of reordering of the magnetic dipoles in the material makes the ferromagnetic
material much more reactive to a magnetic field than a non-ferromagnetic material so after
exposing this piece of iron to a strong magnetic field we can remove the magnetic field and we'll
have a permanent magnet because on average we've aligned the iron atoms in the same direction
now one interesting note here is that we could demagnetize this magnet if we simply heated it
to a high enough temperature and that's called the curate temperature for the material for iron
that's about 770 degrees celsius so if we heated this piece of metal up that high random thermal
motion within the material would reorient all the iron atoms in random directions and it would no
longer be magnetized there are a couple properties of bar magnets that you should be aware of first
if you break a bar magnet in half you don't get an isolated south pole in an isolated north pole
you get two tiny bar magnets and the reason why it goes back to that microscopic view of what's
going on inside the permanent magnet it's because iron atoms are all lined up in the same direction
and if you snap this thing in half you still have two chunks of iron with iron atoms lined up in the
same direction so you get two permanent magnets each of them having a south and a north pole
second the force between two bar magnets originates from the interactions of the
field generated by one with the currents swirling around the iron atoms in the other
and it turns out the fringing of the field so that actual curving of the field is critical to
explaining how this force arises right now we're just trying to deal with it qualitatively and say
i end up with a repulsive force if i have like poles pointed towards each other
so in this top picture i have two north poles pointing towards each other and the force
between the magnets is repulsive in the second picture south poles pointed towards each other
and the force between the magnets is repulsive third picture i have a north pole on one magnet
getting close to the south pole of the other and they attract each other the earth itself generates
a magnetic field and in fact magnetic fields play a major role in the behavior of most planets
provided they still have enough interior heat to possess a liquid portion in their core
the field of the earth is generated by swirling currents in the liquid outer core which is mostly
made of iron and nickel so as a combination of the rotation of the earth along with the convection
of the hotter liquid metal rising from lower in the core to up higher then ends up creating
these swirling currents and we already know that magnetic field is generated by swirling currents
so we shouldn't be surprised to find out that there's a large scale magnetic field around the
earth the field generated by the earth looks a lot like the field of a bar magnet with an interesting
complication the magnetic south is very nearly aligned with the geographical north of the earth
so in this diagram i showed geographical north and south as straight up and straight down and the
magnetic north and south are slightly misaligned with that with the magnetic south pole being very
close to the geographical north pole so how does a compass work a compass has a small magnetic needle
balanced at a point to reduce friction the north pole of this needle will point to the magnetic
south pole of the earth another way of saying it is that the needle aligns itself with the magnetic
field lines at whatever point you're standing on the surface of the earth with that needle pointing
to the magnetic south pole of the earth it's pointing very close to the geographical north pole
of the earth but the alignment isn't perfect and if you're using a compass to do survey you have to
look up the declination for the region that you're working in and it tells you the offset between
the angle to the magnetic south pole versus the geographical north pole to wrap things up we look
at the common symbol for magnetic field we usually use a b for this it has a vector over it because
it is a vector field meaning the magnetic field that anywhere in space has direction
the units of magnetic field are given by teslas that's the s-i unit or gauss teslas are way bigger
than gauss and the conversion is that one tesla is 10 to the fourth gauss so just a couple quick
qualitative examples a neodymium magnet can easily have a field of one tesla at the surface compared
to that the magnetic field of the earth is very weak magnetic field strength at the surface of
the earth is around a half a gauss depending on where you stand and remember a gauss is
only one ten thousandth of a tesla if you find the physics content on zack's lab helpful click on the
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