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I'm trying to understand, why magnets attract certain metals but not objects made out of paper, plastic etc.

And the answer I got is "in paper, electrons cancel each other". What does it mean?

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    $\begingroup$ possible duplicate of Is aluminium magnetic? $\endgroup$ – DumpsterDoofus Feb 19 '14 at 20:14
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    $\begingroup$ The phenomenon is called ferromagnetism, and it's due to a complicated interaction between electrons in the material. See for example the answers to physics.stackexchange.com/q/95909 or physics.stackexchange.com/q/90400 $\endgroup$ – John Rennie Feb 19 '14 at 20:17
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    $\begingroup$ Magnets do affect things like paper and plastic, but not as strongly as you might think. Organics tend to be composed of atoms which have filled shells, which by the Pauli exclusion principle must have paired electron spins, and thus no strong magnetic properties. $\endgroup$ – DumpsterDoofus Feb 19 '14 at 20:17
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First of all what is magnetism. Maxwell says that:

$$ \nabla \cdot \vec B = 0 $$

Where $\vec B$ is the magnetic field.

This says that the divergence ($\nabla \cdot$) of the magnetic field is zero. Which means that you can visualise a magnetic field as a large tank of fluid (lets say water) permeating all of space. This "water" can flow but not expand or contract. I.E. no bubbles or hoses pouring water in.

Magnetism is the flow of this water. Inside materials there are electrons. Within atoms these electrons move around in circles effectively acting like lots of tiny little pumps. Because of the way inner electron shells work half of these pumps will be pointing in the opposite direction to the other half, and only the outer unfilled electron shells contribute.

There are three types of arrangement for these pumps. Ferromagnetic Diamagnetic and paramagnetic.

Within a material these pumps may like to align in the same direction as each other, in random directions or in the opposite direction of each other. When they are aligned in random directions or opposite direction to each other then there is no net flow out of the material as it just goes around and around in tiny circles.

Ferromagnetic creates the strongest type of magnetism and is caused by all of the pumps wanting to align in the same direction giving a large flow of water out of the material.

In a paramagnetic material the pumps only like to line up with each other if there is already a large existing water flow. So if there is already a strong magnetic field, a paramagnet will become slightly magnetic and be attracted to the field. Think of an iron nail being attracted by a magnet, the iron is not magnetic until it is in the field of a magnet.

In a diamagnetic material the pumps only like to line up with each other if there is already a large existing water flow, however in these materials they like up line up in the opposite direction to the direction of the flow. So if there is already a strong magnetic field, a diamagnetic material will become slightly magnetic and be repelled by the field. This is a very weak effect but can be used to levitate organic materials since they are mainly made of water which is diamagnetic.

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All matter is made out of elementary particles , these combine to form protons and neutrons, among other composite particles, and protons and neutrons form the nucleus of what we see as an atom: a nucleus surrounded by electrons. The electron is the elementary particle with which matter we encounter every day, the very keyboard I am using , we have most interaction with, because electrons form shells around the nucleus so that the atoms are generally neutral, unless they lose or gain electrons, as in rubbing a cat and static electricity. Being the outer level of an atom their ordering is important in defining the properties of the atoms , including the magnetic ones.

Electrons are like tiny magnets and carry a magnetic field. In the filled inner shells around the atoms the magnetic fields of the electrons cancel each other due to the way the shells are formed. It is only the outer shells where the magnetic field of the electron might not be canceled by other electrons for some atoms on the periodic table of materials, or some molecules ( combinations of atoms), and long range magnetic forces can be built up. In ferromagnetic materias ( see the other answer) the individual atoms build up a strong magnetic field because the form the outer electron shells have allows for a build up of the magnetic properties of the electron. In paper , which is composed of organic molecules, the shells are closed and the electrons cancel each other's magnetic field almost completely.

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