Yes- magnetic fields can't penetrate a
superconductor. They can't even pass through
the hole in a loop made of superconductor.
One of the basic laws of physics is that any
changing magnetic field creates an electric
field, which exerts forces on charged
particles such as electrons. Voltage is a
measurement of electric field strength, and as you
know, applying voltage to a conductor causes
electrons to move. This is electricity.
So, a changing magnetic field in
a conductor creates electricity in the
conductor. The effect even works when a
conductor (such as a loop of wire) encircles a
This is how generators work- rotating
magnets spin next to or inside loops of wire, and
electricity comes out of the wires. This is
also why you can fry sensitive electronic devices
by swiping a magnet past them. This even
explains why solar storms can cause
Hot gas and charged particles from the Sun
change the shape of the Earth's magnetic
field. This field is weak, but the gaps
between major power lines can be hundreds of miles
across. The bigger the loop, the higher the
voltage- so even a small change in Earth's weak
field can cause voltage surges which shut down the
Now on to superconductors.Superconductors
are materials which conduct electricity without
any loss or dissipation- obviously a useful
property! Some common metals (such as lead) can
become superconductors, but all known
superconductors lose their superconducting ability
if they're not kept extremely cold, which is why
you don't see them used everywhere.
In a loop of superconductor, electricity can
even run around chasing its own tail- forever.
This would never happen in an ordinary conductor
because of the problem of maintaining a
voltage, but superconductors don't need
voltage to conduct.
Ohm's Law states that
voltage=current*resistance, or V=I*R. In
superconductors R = 0 and I cannot
be infinite, so therefore V must also be 0.
If you put a voltage on a superconductor,
electricity will always flow in a way which
cancels out that voltage.
A changing magnetic field generates
voltage, so the electricity must flow in a way
which cancels the change in the magnetic field.
You know from the electromagnet that electricity
creates a magnetic field. The superconductor
automatically creates the right pattern of
electricity to generate a counter-field which
cancels any change in the magnetic field piercing
Now, if the superconductor
becomes superconducting without any magnetic field
present, no field will be able to pierce it as
long as it remains superconducting. Put the
superconductor over a magnet, and its counter-
field can even become strong enough to
levitate the superconductor by magnetic
repulsion. That's why you see those pictures of
the superconductor floating in a cloud of mist.
Conversely, if the superconductor is
cooled below its critical temperature while a
magnetic field is present, the superconductor will
retain that field unchanged. If the original
magnet is removed, the superconductor just becomes
an electromagnet and regenerates the field.
Such interesting magnetic effects aren't
foolproof, though. High magnetic fields induce
high electric currents, which will remove the
superconductor's superpowers just as surely as
high temperatures will. So, a super-strong
magnetic field will penetrate a superconductor,
but first it must make it lose its
Super-cool temperatures can also transform
helium into a superfluid. If your orange juice
was a superfluid, it would crawl up the side of
your glass and escape. The rare
mineral magnetite has its own magnetic
field, so when it's in a repulsive configuration
it can keep out a magnetic field weaker than its
own. This is not a perfect shield like a
superconductor, because inside the magnetite the
influence of the opposing field can still be felt.
Iron and niobium can also become
magnetized, so they can keep out magnetic fields
in the same way magnetite does. Iron and
niobium can also distort and even amplify
magnetic fields, but the effects depend on the
shape of the metal. You can make an electromagnet
by winding wire around a stick, but an effective
electromagnet uses a steel nail as a magnetic
amplifier. The nail not only allows the
field to pass through it, but it also makes it
stronger. However, a large steel plate seems
to block magnetic fields completely.
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