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This text is meant to accompany class discussions. It is not everything there is to know about the basics of torque and the two conditions of equilibrium. It is meant as a prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)
Questions Click for the questions that go with this reading
Thermal Energy


The energy associated with a collection of unorganized molecules.

If you push on one molecule in a collection and the whole collection moves then it is called an organized collections of molecules. All solids are organized collections of molecules.

oragnixed collection

If you push on one molecule in a collection and only the molecule(s) you touch move,then collection is called an unorganized collection of molecules. All fluids and gasses are unorganized collections of molecules.




Temperature can be defined many ways. We will define it as, "The tendency to spontaneously release energy." The higher the temperature, the more energy that is released. We feel this and interprets it as "hotness" of the body.

When two bodies touch each other and exchange thermal energy, it is called thermal contact. When two or more bodies are in thermal contact with each other and the energy given off and received is equal, then the bodies are said to be at the same temperature. This is called thermal equilibrium.


Temperature Scales


A temperature scale is a way of dividing up increments of energy. There are several scales: Centigrade, Celsius, Kelvin, Farinheight, Rankine. Centigrade and Celsius are the same. Centigrade is no longer used by the scientific community. Although there are several scales we will only use two scales, Celsius and Kelvin. The Celsius and Kelvin scale have the same increments. Celsius temperatures are reported as "Degrees Celsius," °C. But Kelvin temperatures are reported as Kelvin -without the degree symbol. As you will see proved later, 2 Kelvin corresponds to an level of energy. Kelvins are a energy scale like Joules. You would not say, "the energy of that car is 500 Joules." Simularly when using Kelvins, you would say, "the temperature of that car is 500 Kelvin. If the temperature changes from 100K to 200 K, then the energy of the body with these temperatures doubles since the Kelvins are a energy scale they double. If a body's temperature changes from 100 °C to 200 °C, then this does not correspond to doubling the temperature. This is because °C does not correspond to an energy level of zero. A temperature difference of 1 degree Celsius is equal to the temperature difference of 1 degree Kelvin.

Temperature Scales Comparison


Zeroth Law of Thermodynamics

Zeroth Law Image

Place thermometer in a beaker of room temperature water. Eventually the "mercury" will stabilize and be at the same temperature as the water. The thermometer and water are said to be in thermal equilibrium. The water is the same temperature as the air because it has sat around long enough in the air. Therefore the thermometer is in thermal equilibrium with the air. This is the zeroth law of thermodynamics. If body "A" is in thermal equilibrium with body "B," and body "B" is in thermal equilibrium with body "C," then "A" is in thermal equilibrium with "C."

This law like the transitive property of mathematics.

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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)