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Heat is a transfer of energy. Heat is not contained in a body, therefore, but is instead a condition of energy moving from one body to another. The amount of heat to change the temperature of a specific substance one degree Celsius is its specific heat capacity. Heat balance occurs when the amount of heat coming into an object equals the amount of heat leaving it.
All matter is made up of atoms. The greater the energy level in the atoms and molecules of objects, the greater their movement. Objects that have rapidly moving molecules are hot, and those with slow molecules are cold. Temperature is a measurement used to indicate the internal energy level of an object.
Temperature indicates the energy transfer that will occur when a hot body comes in contact with a cold body. For example, two identical pieces of iron cut from the same bar are placed into either boiling water or a freezer for an hour, then placed on top of one another on an insulated board. The hot iron would have a higher internal energy level than would the iron from the freezer. The heat energy would flow from the hot iron to the cold iron. This transfer of heat would continue until both bars were the same temperature.
To measure the heat flow into an object, the specific heat capacity of a subject is used to measure how much energy it will take to make it hotter or colder. Scientists and engineers define the term specific heat capacity as the amount of energy (or heat) that it takes to raise the temperature of one gram of water one degree Celsius. Denser materials take more heat to raise their temperature one degree than do less dense materials.
Heat capacity as the capacity of a body to store heat is measured in units of joules per Kelvin. The extensive quantity of a body expresses the size of a body, such as an above-ground swimming pool versus a cup of water. Specific heat capacity is found by measuring the capacity of a material by its mass. The mass-specific heat capacity is an intensive quantity, which means that it is a measure that is not dependant upon the mass or type of material of an object.
Heat is transferred from one body to another in one of three ways. Conduction is the transfer of energy (heat) from molecules bumping into one another. If one end of a silver rod is put into a flame, the other end of the bar will soon warm, because the molecules conduct energy from one end of the bar to the other by bumping into each other.
Convection is the transfer of energy in liquids or gases. Because the molecules are too far apart to be effective conductors of energy, the energy is moved by currents in the fluid gas or liquid. A space heater warms surrounding air, causing it to rise as cooler air is drawn to the heater.
Radiation is the third way that heat is transferred. If the body is a metal, such as a cherry-hot iron or a glowing fire, it sends out infrared waves of energy.
Heat balance occurs when the energy output of a system equals the energy input in a specific place or specific system, creating an equilibrium. In natural systems of the earth, heat balance is very important. The sun’s rays warm the earth and oceans. This produces warm water, evaporation of water into vapor to become clouds, and heated air from the warmed earth. The effect is temperatures that cause rising air and water vapor. The rising air creates a low pressure area into which cooler air, which is heavier, moves. Globally, this is a massive form of heat flux seeking a heat equilibrium or balance that is never finally achieved. If it were, the atmosphere would stagnate from a lack of circulation.
Heat as balance and capacity are also important factors in life. For example, sleeping bags are designed to trap and hold air around the body. If the insulation in the sleeping bag is effective, heat is not transferred from the body to the surrounding colder atmosphere. A heat balance is achieved, and the sleeper is warm all night despite the cold.
- American Institute of Chemical Engineers, ed., Ideal Gas Law, Enthalpy, Heat Capacity, Heats of Solution and Mixing (American Institute of Chemical Engineers, 1984);
- Yunus Cengel, Introduction to Thermodynamics and Heat Transfer (McGraw-Hill, 1996);
- Hans Fuchs. The Dynamics of Heat (Springer-Verlag, 1996);
- Donald W. Rogers, Einstein’s Other Theory: The Planck-Bose-Einstein Theory of Heat Capacity (Princeton University Press, 2005).