Physics

Linear dilation


As for gases, one of the effects of temperature variation is the variation of dimensions in solid and liquid bodies. This variation is what we call Thermal expansion.

It applies only to solid state bodies, and consists of considerable variation of only one dimension. As for example in bars, cables and wires.

When we consider a homogeneous bar, for example, of length at an initial temperature . When this temperature is increased to a (>), it is observed that this bar has a length of (>).

Thus it can be concluded that the linear expansion occurs proportionally to the temperature variation and the initial length . But when analyzing bars of equal dimensions but made of a different material, their variation in length would be different, because dilation also takes into account the properties of the material with which the object is made, this is the proportionality constant of the expression. , call of coefficient of linear expansion (α).

Thus we can express:

The unit used for α is the inverse of the temperature unit, as: .

Some usual values ​​of linear expansion coefficients:

Substance
Lead
Zinc
Aluminum
Silver
Copper
Gold
Iron
Platinum
Glass (common)
Tungsten
Pyrex Glass

Bimetallic blade

One of the most commonly used linear dilatation applications in everyday life is for the construction of bimetallic blades, which consist of two plates of different materials, and therefore different welded linear expansion coefficients. When heated, the plates increase their length unevenly, causing this welded blade to bend.

Bimetallic blades are mainly found in electrical and electronic devices, as electric current causes conductors to heat up, which cannot be warmer than they were built to withstand.

When the blade is bent it has the purpose of interrupting the electric current, after a rest time the conductor temperature decreases, making the blade return to its initial shape and rehabilitating the passage of electricity.

Graphic representation

We can express the linear dilation of a body through a graph of its length. (L) depending on the temperature (θ), thus:

The graph must be a straight line segment that does not go through the origin as the initial length is not zero.

Considering an angle φ as the slope of the line in relation to the horizontal axis. We can relate it to:

Because: