Properties of Materials: Thermal Expansion, Stickiness, and Magnetism


By Robert Hazen, Ph.D.George Mason University

Properties of materials depend on the type of atoms and the way they are bound together by a chemical bond. Alchemists of the Middle Ages tried, but failed, to turn metals like lead into gold. But modern chemists have learned to turn these and other elements into a remarkable variety of materials that have transformed society itself.

A 3D rendition of a molecular structure.
Properties of materials depend on the chemical bonding between atoms. (Image: NanamiOu/Shutterstock)

Thermal Expansion

One of the physical properties of materials is thermal expansion. This is the tendency of materials to change their dimensions when they’re heated. Thermal expansion is absolutely dependent on the behavior of the atoms and how they’re bonded together. Most materials will expand slightly when they’re heated. 

In these materials, like a bar of metal, for example, you have atoms side by side. As you heat up the material, each of those atoms starts vibrating more and more vigorously, and starts colliding and bumping into its neighbors. Naturally, this applies a force, and that force tends to make the whole material expand slightly, so the metal becomes larger and larger as you heat to higher and higher temperatures.

An image of two parallel metal bars in a roof structure
A bar of metal tends to expand when it’s heated. (Image: Beekeepx/Shutterstock)

This is normal behavior and almost all materials do that.

Learn more about the atomic theory.

Properties of Materials and Stickiness

The properties of materials are the result of their atoms and their bonds, and the close ties between atomic-scale structure and physical properties are especially evident in the property of stickiness.

There are certain materials that need to be sticky. By the same token, some surfaces need to be non-sticky. This property is the direct result of bonding, right at the material surface. 

An image of various kinds of tapes.
Tape and glue have atoms at the surface that want to bond to other elements. (Image: Ver2ver/Shutterstock)

Sticky substances refer to a whole group of atoms and materials where atoms at the surface want to bond to other elements. The electrons aren’t quite properly distributed; they’re just waiting for another material to come along and satisfy their bonding needs. So tapes and glues have this property of a not-quite-satisfied bonding, and therefore they’re sticky. 

There are lots of good adhesives out there because there are lots of materials that aren’t quite bonded in the way they’d like to be.

Learn more about electromagnetism.

Magnetism Properties of Materials

Magnetism is another vital property of materials. Permanent magnets are those materials that possess the ability to attract iron in other magnetic materials. What is the property of a permanent magnet that allows it to do so?

An electromagnet—nothing more than a coil of wire through which you pass current—creates a magnetic field, because electrons are moving around in a loop, over and over again. The electrons circulating in that loop create what is called the dipole field, with a north and a south magnetic field.

Atoms with electrons in orbit around the nucleus are, in effect, tiny little magnets, or at least they can be. In many atoms, the different paths of the electrons cancel each other out, so the atoms themselves aren’t magnetic; but in a few elements—like iron, and nickel, and cobalt—the electrons amplify each other. They align in the same sorts of orbits, and you have little atomic magnets. Each atom becomes a dipole magnet.

If you take some iron, melt it, and crystallize that iron in a magnetic field that’s imposed from the outside, all those atoms may align themselves. So you have one continuous array of atoms, all with their little dipole fields aligned—that gives you a permanent magnet, in which there’s a large external magnetic field – the kind of thing you use in a refrigerator magnet. 

The effectiveness of a magnet can be destroyed simply by heating it because that ruins this alignment of atoms. It can also be destroyed just by beating it with a hammer because in case you’re randomizing these orientations, each atom then loses the alignment that led to the collective property of the magnetic behavior.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

When Magnet Is Subjected to a Magnetic Field

What happens to a magnet when it is subjected to a magnetic field? You have a magnet, and you apply a magnetic field to it. Think about this: every one of those little atomic magnets then experiences the magnetic field, and as a result, the atoms have to shift slightly in position. 

When you apply a magnetic field to a magnet, or when you change the shape of the magnet, you actually change the magnetic field; you change the shape of the whole material as it’s being subjected to that magnetic field. This becomes a very important technology.

You can actually use magnets to create a microphone. As you talk into a speaker, you have a diaphragm. The diaphragm moves; as it moves, it can change the magnetic field near a magnet. The magnetic field then is modified by the magnet itself and signals are pulsed through the wire.

This is the principle behind many microphones and many speakers.

Common Questions about the Various Properties of Materials

Q: What is the property of thermal expansion?

One of the physical properties of materials, present in almost all materials, is thermal expansion. Materials tend to increase their dimensions due to an increase in temperature.

Q: What is the property of stickiness?

Stickiness is one of the properties of materials resulting from the atomic bond between the surface atoms of a material and other elements. Surface atoms in sticky materials are not well distributed and are waiting for other elements with which they can satisfy their bonding needs.

Q: What is the property of magnetism?

Magnetism is one of the properties of materials. It is caused by the constant movement of electrons in a loop. The circulating electrons in the loop create a dipole field with north and south magnetic fields. A refrigerator magnet is one example of a magnetic material.

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