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Chapter 19 - Temperature, Thermal Expansion and Gas Laws

Atomic Theory of Matter

To introduce the concept of temperature we need to start from the atomic theory, which considers matter to be composed of atoms.

A useful unit when discussing the mass of atoms is the atomic mass unit [$u$]


Atoms and molecules are in continual motion, the amount of motion is proportional to the temperature, which we can think of as a measure of energy.

Brownian motion, named after Robert Brown who observed it as random movements of pollen or dust suspended in water was explained by Albert Einstein in 1905 as a product of the thermal motion of molecules or atoms. In 2010 the instantaneous velocity of a particle in Brownian motion was measured by a group at University of Texas using optical trapping techniques.

We can consider temperature to be a measure of the energy contained in the motion of the microscopic constituents of a material.

Temperature Scales

To be able to measure temperature we need a scale. The temperature scale in common usage in the United States, Fahrenheit, $\mathrm{^{o}F}$ is based on what may now seem to be fairly arbitrary reference points.

Most of the world uses the Celsius scale, $\mathrm{^{o}C}$, for everyday measurements. This is based on dividing the difference between the freezing point and boiling point of water in to 100 degrees and fixing $0\mathrm{^{o}C}$ as the freezing point.

For thermodynamics an absolute temperature scale, in which 0 is the complete absence of thermal energy, is appropriate. This scale is the Kelvin Scale. In this scale water freezes at $273.15\mathrm{K}$. Note that we do not use a degree symbol for temperatures in Kelvin.

To convert between Farenheit and Celsius you can use the fact that water freezes at $32\mathrm{^{o}F}$ and boils at $212\mathrm{^{o}}F$ to deduce that:




Thermal Equilibrium

If two objects with different temperatures are brought in to contact with one another thermal energy will flow from one to another until the temperatures are the same, and we then say that the objects are in thermal equilibrium.

The zeroth law of thermodynamics states that:

“If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.”

This law may seem obvious but this is only because we already have some expectation from our everyday experience that temperature is a general quantity that we can measure with a thermometer. This law allows to declare this formally, and turns out to be important, in conjunction with the First and Second Laws (we'll talk about these later!), for establishing the true definition of thermal equilibrium.

Thermal Expansion

Most, but not all, materials expand when heated. The change in length of material due to linear thermal expansion is

$\Delta l=\alpha l_{0}\Delta T$

$\alpha$ is the coefficient of linear expansion of the material, measured in $\mathrm{(^{o}C)^{-1}}$

The length of the object after it's temperature has been changed by $\Delta T$ is

$l=l_{0}(1+\alpha\Delta T)$

A material expands in all directions, and if we are interested in the volume changes of a rectangular object, that is isotropic, meaning it expands in the same way in all directions, then

$\Delta V = \beta V_{0}\Delta T$

$V_{0}=l_{0}w_{0}h_{0}$ → $V=l_{0}(1+\alpha\Delta T)w_{0}(1+\alpha\Delta T)h_{0}(1+\alpha\Delta T)$

$\Delta V=V-V_{0}=V_{0}(1+\alpha\Delta T)^{3}-V_{0}=V_{0}[3(\alpha\Delta T)+3(\alpha\Delta T)^{2}+(\alpha\Delta T)^{3}]$

If $\alpha\Delta T << 1$ then $\beta \approx 3\alpha$

Coefficients of thermal expansion can be found here or your textbook.

Some Thermal Expansion demos

A Bimetallic Strip combines invar with either copper or brass (ours is brass). The very different coefficients of thermal expansion of the two materials means it bends when heated. Invar is very useful, because of it's low coefficient of thermal expansion. Another material with a very low coefficient of thermal expansion is fused quartz.

Does a hole expand or contract on heating?

Negative Thermal Expansion

Some materials in certain temperature ranges do not expand with temperature. An example is water, which actually decreases in volume as it's temperature is increased from $0\mathrm{^{o}C}$ to $4\mathrm{^{o}C}$. This has an important effect, it explains why the surface of lakes freeze, which helps prevent the flow of heat out of the lake. If the thermal expansion was normal between $0\mathrm{^{o}C}$ and $4\mathrm{^{o}C}$ the circulation effect would continue below $4\mathrm{^{o}C}$ and the whole lake would freeze, starting from the bottom.

Thermal Stress

What happens when an object wants to expand, but can't because it is fixed?

If an object is trying to expand by a length $\Delta l$ due to thermal expansion, but doesn't because something is holding it in place, that something must be exerting a force.

The force can be found by considering the relationship between stress and material deformation, which we discussed when we introduced elastic moduli.

$\frac{F}{A}=E \frac{\Delta l}{l_{0}}$

And as

$\Delta l=\alpha l_{0}\Delta T$

$\frac{F}{A}=\alpha E \Delta T$

These stresses can be reduced by the inclusion of expansion joints in bridges, roads and pipes.

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