Kind: captions
Language: en
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only a few elements can be permanent
magnets iron is one copper is not but if
you pass an electric current through any
metal it becomes a
magnet an
electromagnet but how does this work
well strangely enough it's a consequence
of special relativity special relativity
is the fact that in our universe length
and time aren't absolute they're
perceived differently by Observers
moving relative to each other hence
relativity for example if you measure
carefully enough you'll find that time
passes slower for observers moving
relative to you hey Derek when did you
last shave 6 hours ago actually it was 5
hours 59 minutes and 59.9
9999999999 seconds and moving objects
are also contracted in their direction
of motion you're looking slim only in
your frame of reference so when an
object is moving relative to you it
actually takes up less space than when
it's not moving and even though this
effect is obviously way tinier than
we've shown length contraction is what
makes an electromagnet work picture a
copper wire it consists of positive
metal ions swimming in a sea of free
negative electrons now the number of
protons is equal to the number of
negative electrons so overall the wire
is neutral so if there were a positive
charge or positively charged cat nearby
it would experience no force from the
wire at all and even if there were a
current in the wire the electron would
just be drifting in One Direction but
the density of positive and negative
charges would still be the same and so
the wire would be neutral so no force on
the kitty but what if the cat starts
moving imagine for Simplicity that the
cat is moving in the same direction as
the electrons with the same velocity
well now in my frame of reference the
wire is still neutral and so there
should be no force on the cat but
consider the same situation in her frame
of reference in the cat's frame of
reference the positive charges in The
Wire are moving so according to special
relativity their separation will be ever
so slightly contracted also from this
perspective the electrons aren't moving
so they'll be more spread out than
before remember objects take up more
space when they're not moving than when
they are these two changes together mean
there's a higher density of positive
charges in The Wire so it's no longer
neutral it's positively charged which
means that the positively charged cat
will feel a repulsive electric force
from The Wire but in my frame of
reference this seems mysterious there's
no force on a stationary charge chared
cat but a moving cat is somehow repelled
from this neutral wire how do you
account for that Force well we say it is
the magnetic force and that's mainly
because a wire with current in it
deflects nearby magnets so really what
this experiment shows is that a magnetic
field is just an electric field viewed
from a different frame of reference in
the cat's frame of reference it is
repelled from The Wire due to the
electric field created by the excess
positive charges produced by the effects
of length contraction in my frame of
reference the cat is repelled from a
neutral wire due to the magnetic field
created by the current flowing in The
Wire so whether you see it as an
electric or a magnetic field just
depends on your frame of reference but
in either case the results are the same
so an electromagnet is an everyday
example of special relativity in action
now that might seem crazy since
electrons drift through wires at about
00000000000000 1% the speed of light so
how can special relativity have anything
to do with it well the truth is there
are enough electrons in a wire and the
electric interaction is so amazingly
strong that even the minuscule effects
of length contraction can produce
significant charge imbalances that
produce a noticeable Force so special
relativity explains electromagnets but
what about permanent magnets yeah I mean
there can't be electrical currents
flowing around inside lumps of rock can
there click here to go to minute physics
where we'll explore magnetite compasses
and all the Wizardry of permanent
magnets
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