Why are diamagnetic materials repelled when placed in magnetic field and why are paramagnetic materials attracted when placed in magnetic field?
Okay... I write this as an inspiration by Feynman..!
These "WHY?" and "How's such a thing possible?" are quite related in a way that they make the question ambiguous. The question author wouldn't be satisfied for sure, hearing a physicist who try to answer such hating questions. But, truly speaking, these questions are very very good (In fact, +1'd it). But, the physicist can't really provide a satisfying answer. Well, this guy really hates it. He explains what's the problem with "WHY" to the interviewer..!
The reason I say this is because, here's a similar example.
- You can ask, why a glass falls down and breaks..? And one says, due to gravity. If someone is satisfied, it's OK. (But, going deeper) into two possibilities -
- Why does the glass break? It's brittle (or) Why is this gravity pulling this glass down..? Because it's always attractive. This is also OK. But, if someone goes still deeper...
- Why is it brittle? (or) Why is this gravity always attractive..? - The answerer will definitely go nuts trying to put his numbers into you. Because, the physicist knows that it's an observed phenomenon. He can't explain for sure why it's like the way it is..!
This is because the physicists always try to explain something on the way it occurs, or roughly how this can be explained with his numbers and Greek symbols. He can't surely telly "WHY". That totally goes to an omnipotent being (if such a thing exists) which has created these things. Now, this goes philosophical.
But, we can still correct your question by saying, "How does this phenomenon occur?" or "How it's been theorized by our physics fellas?"
To answer this question in a sentence - "All materials are diamagnetic". It's their magnetic permeability $\mu$ (a number) which determines whether their diamagnetic property is thrown out or their paramagnetic property exceeds it. Thanks to quantum mechanics which helped in relating these paramagnetic and diamagnetic properties to electron pairing and especially their spins.
Well, there's a lot and lot more than just that...
You can look at the permeability of a material as a factor which scales the "energy" of the magnetic field lines. So if two materials with different permeabilities are placed within an inhomogeneous magnetic field the lowest potential energy can be achieved if the material with the higher permeability would move higher into the magnetic field (denser magnetic field lines) assuming that the two materials would which places (such as air and a solid material like aluminium).
Assuming that these values from wikipedia are correct, then copper surrounded by air would be repelled by a magnet, however it would be attracted if it would be surrounded by water.