# What is the difference between diamagnetism and superconductivity?

Basically, What is the difference diamagnetism and superconductivity?

As far as I understand, diamagnetism comes from the fact, that all electrons in a solid, when exerted by an external magnetic field, create so-called Eddy Currents which oppose the external magnetic field, right ?

And in a superconductor, the supercurrents, are actually are "normal" current in the material, which opposes the magnetic field.

But is that the whole story, or am I simplifying it too much ?

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There doesn't have to be eddy currents to obtain diamagnetic. In fact you can have eddy currents in iron plate, which is ferromagnetic. Diamagnetics can be insulators, so there don't have to be any currents at all. In some cases it's energetically beneficial to align so that magnetic field is lowered. There are always diamagnetic and paramagnetic tendencies struggling, and in some materials one is just stronger than the other.

Magnetic field can enter diamagnetic and is only slightly ejected (most of the times). It means that inside it magnetic field is lowered.For reasons described above diamagnetism is usually quite small. In case of superconductors magnetic field can't enter at all. That is provided it's small enough and superconductor is classical type one. Magnetic field inside superconductor is zero. You can think of superconductors as ideal diamagnetics. But keep in mind that superconductors are more than that.

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So if it isn't the Eddy Current that counter the external electric field in a diamagnet, what is it then ? – Denver Dang Jan 21 '14 at 11:47
It's quantum mechanic effect, in pure classical theory there can't be neither diamagnetism nor paramagnetism. In harsh aproximation diamagnetism can be thought as being due to magnetic field caused by electrons orbiting nucleus. These electrons can be thought as current loop, which is not entirely good description but it works. – Jarosław Komar Jan 21 '14 at 12:09

A more formal answer than what Jaroslaw Komlar perfectly describes.

Diamagnetism is a property of matter, superconductivity is a state of matter.

Diamagnetism describes how a material reacts when plunged into a magnetic field. Paramagnetism says almost the same thing. The difference between para- and dia-magnetism is just the sign of the response of the material with respect to the magnetic field. A diamagnetic is repelled by a magnetic field, whereas a paramagnetic is attracted by a magnetic field. The other way around : a diamagnetic material tends to repel the applied magnetic field, whereas a paramagnetic material tends to favour the applied magnetic field. The difference between dia- and para-magnetism is measured as the response of the magnetization $M=\chi H$ for a change of the magnetic field $H$, in terms of the (magnetic) susceptibility $\chi$. When $\chi < 0$, the material is said to be diamagnetic, when $\chi>0$, the material is said to be paramagnetic.

Clearly different now, superconductivity is the state characterised by a perfect diamagnetism below a given temperature (at some normal pressure condition, for weak magnetic field, ...)

You could have chosen as well to characterise the superconducting state with the voltage $V$ response to an applied current $I$, when the susceptibility is usually called the resistance: $V=RI$. For positive resistance, you have a Ohm metal, if you imagine that negative resistance could exist (only non-linearities allow for fictitious negative resistance to exist, so we already outside he scope of the linear response theory as discussed here) you may say you have an amplifier. Superconductor is the state characterised by a zero resistance.

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