When talking about temperature, people give a very broad and general definition of what temperature is. This definition is that temperature is the measurement of hot and cold. However, what is truly hot and cold. Instead of describing temperature this way it should be described as the following. Temperature is the measurement of the kinetic energy within the molecular bond between atoms and particles. The colder the molecules become the lower the kinetic energy, while the warmer they become the more kinetic energy is built up between the molecular bonds.

Temperature and Phase

Temperature as you know has a direct effect on whether a matter’s phase is currently a solid, liquid, or gas. The phase of matter directly relates to the kinetic energy within the molecular bonds and the bond strength. When matter is in a solid phase this means that the molecular bonds between molecules are strong and hold the molecules and atoms closely together. There is however still molecular vibration between the bonds due to kinetic energy. On the other hand when matter is in a liquid phase the molecules and atoms are held together through loose bonds. These bonds are weak enough to cause the fluid molecules to flow over each other when subjected a shear stress of any magnitude. However, when a normal compressible force is applied to the bond the fluid bonds will act as a solid, which is why most fluids are considered incompressible. Finally, gases have enough kinetic energy between molecules and atoms that the molecular bond is not strong enough to hold the atoms together but instead the molecules will bounce around their container randomly. Gases flow like a liquid would flow, but because of their desire to fill their container they are compressible unlike a liquid. Now, all of this relates to temperature because temperature is the measurement of kinetic energy between molecules and can be used to determine when phase changes will occur. The figure below shows the three phases of matter at the molecular level.

Temperature Scales

The two temperature scales that most people have heard of are the Celsius scale and the Fahrenheit scale. The Celsius scale is the temperature scale that is used to measure temperature in the standard international system of measurement, or SI system. At 0 degrees C water will start to freeze, and at 100 degrees C water will boil. Fahrenheit on the other hand is used to measure temperature in the English system of measurement. For Fahrenheit water will freeze and 32 degrees and will boil at 212 degrees. To relate the two scales to each other the following equation would be used.

(Eq 1)  $T(^oF)=1.8T(^oC)+32$

Now, in thermodynamics the thermodynamics scales are used. The thermodynamic scales do not have negative number values, but instead define an absolute zero. At absolute zero there will be no kinetic energy present between molecular bonds. This value has never been reached in real life, but scientists have come close to reaching it. The thermodynamics scales are Kelvin and Rankine. The Kelvin scale is SI thermodynamic scale and can be related to the Celsius scale using the following equation.

(Eq 2)  $T(K)=T(^oC)+273.15$

The Rankine scale on the other hand is the English thermodynamic scale. To relate the Fahrenheit scale to the Rankine scale the following equation would be used.

(Eq 3)  $T(R)=T(^oF)+459.67$

When solving thermodynamic problems the thermodynamic scales should be used.

The Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics is one of the fundamental laws of thermodynamics. It is called the zeroth law because it came after the 1st and 2nd law of thermodynamics when in fact it should have come before. What the Zeroth law says is “If two bodies are in thermal equilibrium with a third body then they are in equilibrium with each other.” In other words all bodies within the system must have the same temperature otherwise there will be have to be some sort of energy transfer between the objects until they reach an equilibrium state.


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