We can use compasses to investigate
magnetic fields. A compass needle gives
us the direction of the magnetic field
at a point. The needle acts almost like a
vector in vector fields. It points
tangential to the field line at its
position. The compass needle demonstrates
direction exactly like a vector would
except it doesn't have magnitude - sadly compass needles don't shrink or
grow based on the strength of the
magnetic field it's in.
We know what the magnetic field around a magnet looks
like from previous experiments,
but magnetic fields are a type of vector
field and iron filings don't show the
direction of the magnetic field, so we'll
use a compass to find the direction of
the magnetic field around the magnet.
To
draw the magnetic field around the
magnet, we need to know the direction that the magnetic field points in. We can use
a compass to identify the direction at
different points. Remember that the red
arrow of a compass points north. So at a
point directly below the magnet the
compass points up and directly on top of
the magnet the compass also points up.
Slightly to the side you can see the
compass starts changing directions. Now
if we do this for lots of points around
the magnet, we'll get the magnetic field.
So if we connect these vectors we'll get
the field line representation of the
magnetic field around this magnet.
The
field lines around a magnet found with a
compass has the exact same shape as the
field lines indicated by iron filings
with this picture though, we also get the
direction of the magnetic field.
The magnetic field lines always come out
from the North Pole of the magnet and
point towards the South Pole of the
magnet. If you think back to how we
analyze the electric field around
charged particles by looking at how a
positive particle will behave, this is
similar. In this case we pretend that
there is a North Pole of a magnet in
isolation and see how that pole behaves.
Monopole means single isolated magnetic
pole. So if there was a North Pole by
itself it would be a monopole - mono for
one . Dipole means pair of equal and
opposite charge poles . So that's a magnet
with both a North and South Pole. The
magnetic field around a bar magnet looks
very similar to the electric field
around two oppositely charged particles.
The positive charge and the North Pole
of the magnet behave similarly. Same for
the negative charge and South Pole.
The main difference is that charge can exist
independently of each other , so it is
possible to have a single positive
charge or single negative charge without
the other. While magnets are dipoles, theoretically
monopole magnets could exist but we
haven't found any yet. The magnetic field
around multiple interacting magnets is
more complicated. We could do the same
thing and use a compass to draw out the
vector field and then draw in the field
lines to find the magnetic field, but
that's rather tedious, so instead I'll
just show you what the magnetic field
around two magnets look like. In image
one the magnets are placed end-to-end
with the North Poles facing each other.
The field lines come out from the North
Pole and go to the south. In image 2 the
magnets are still placed end-to-end
except this time the North and South
Pole are facing each other. One of the
field lines coming out from the North
Pole of the left magnet goes into the
South Pole of the right magnet. However
one of the other lines comes out of the
North Pole of the left magnet and goes
into the South Pole of the left magnet.
So it doesn't matter if the field line
goes into the left magnet South Pole or
the right magnet South Pole. In image
three we have two magnets parallel to
each other with the top magnet South
Pole being closer to the bottom magnet
North Pole and vice versa. And again the
field lines come out of the North Pole
and go into the South Pole and just like
an image - it doesn't matter which
magnet the field line comes out of or
goes into. It is possible for the field
line to go to a different magnet than
the magnet it came from. And here we have
two parallel magnets, except with the
South Poles and north poles facing each
other. And again, the field lines come out
from the North Pole and go into the
South Pole. Looking at these images we
know from real-life experience that the
magnets in image 1 & 4 would repel each
other
and the magnets in image 2 & 3 would
attract each other. The field lines in
image 1 & 4 go back to the magnet from
where it came from, while in image 2 & 3,
the field lines can come from a
different magnet than the one it goes to.
Magnets have many real-life applications
aside from being really fun to play with,
they are used to seal fridge doors store
data and computers, used in every motor and so much more.
