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    Properties of Temperature, Heat and Their Relationship

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    What is Heat? What is temperature? What are the various properties that account for temperature? What are the various sources of heat? What is the relationship between heat and temperature? How are they different?

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    Many people confuse heat and temperature. Heat is a type of energy. Temperature is a measure of heat.

    Extensive vs. Intensive Properties Heat is referred to as an extensive property. An extensive property depends on the amount of material present. A cup of water at 100o C has contains less heat than a gallon of water at 100o. Temperature, on the other hand, is an intensive property. It does not depend on the amount of substance present. Water boils at 100o C regardless of the amount of water present.

    Thermal Energy

    Heat is thermal energy. Atoms or molecules that are moving very fast have more thermal energy, i.e., more heat than do atoms or molecules that are moving slowly. In fact, if molecules were completely at rest, they would have no thermal energy, and the temperature would be defined as absolute 0. We can see the effect of thermal energy in phase changes. Consider water. When water molecules are bound together in a crystalline state, we call this ice. The molecules vibrate, but they do not have enough energy (heat) to break the bonds that hold them in place. So, water remains in the solid state that we call ice. When the molecules have enough energy (heat ) to break free of the bonds holding them in a crystalline state, water becomes liquid. The molecules are still held together by weaker hydrogen bonds, but the molecules are able to flow past each other. When still more heat is present, the molecules have enough energy to break free of even the hydrogen bonds and water molecules escape to a gaseous state where they are in constant independent motion.

    Notice that heat and cold are not separate properties. What we refer to as cold is simply an absence of thermal energy, or motion.

    There are many misconceptions regarding heat. The most common is reflected in our language. We speak of the flow of heat. Heat is not a substance that flows. If molecules are moving very fast (high heat), they bump into other molecules and there is a transfer of momentum and kinetic energy. Thus, the original fast moving molecules lose a little energy and the molecules they bump into gain energy. The original fast moving molecules have less heat and those bumped into have more heat. So, for instance, if you pour boiling water over ice, some energy will be transferred from the boiling water to the ice. The boiling water will cool down (lose energy as it transfers energy to the ice) and the ice will melt (gain energy as a result of collisions with molecules in the boiling water).

    Temperature does not measure the amount of heat in a system since large bodies with more heat may be at the same temperature as smaller bodies with less heat. Temperature is a qualitative property. We may think of heat flowing from regions of higher temperature to regions of lower temperature (although this is not a completely accurate view of heat). For the same body, temperature rises as thermal energy (heat) increases and falls as thermal energy (heat) decreases. Temperature may be thought of as a measure of the average amount of energy that the molecules of a body possess.
    The kinetic theory of gases relates temperature and kinetic energy. Kinetic energy is given by the equation E = 1/2mv2 where m is mass and v is velocity. Average kinetic energy = 1/2kT, where k is a proportionality constant and T is temperature.

    Summary: Heat is energy. Temperature is a number that is related to the average kinetic energy of a collection of molecules. Temperature may be thought of as a measure of heat, on the average.
    Take a look at the references below. Some include diagrams and animations that you may find helpful in understanding these concepts.

    References: http://zonalandeducation.com/mstm/physics/mechanics/energy/heatAndTemperature/heatAndTemperature. http://coolcosmos.ipac.caltech.edu/cosmic classroom/light lessons/thermal/differ.html http://hop.concord.org/h1/phys/h1p.html http://en.wikipedia.org/wiki/Temperature

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