Noon Edition. Home Archives About Contact. What Is Heat? By William Orem Posted April 22, Media Player Error Update your browser or Flash plugin. The Theory But there's a problem. Another simple experiment you can do shows heat can't be a liquid.
Kinetic Energy Is The Answer! Become an Indiana Public Media supporter. Learn More ». Thermal energy can be otherwise understood as the total microscopic kinetic and potential energy of a system. The second law of thermodynamics is a complex topic that requires intensive study in the field of thermodynamics to truly understand. However, for the purpose of this article, only one small aspect needs to be understood and that is the fact that heat will always flow spontaneously from hotter substances to colder ones.
This simple statement explains why an ice cube doesn't form outside on a hot day or why it melts when dropped in a bowl of warm water. Imagine the aforementioned ice cube dropped into a bowl of warm water—the ice must gain heat thermal energy from the water in the bowl see preceding paragraph.
Adding thermal energy leads to an increase in the kinetic energy of the ice molecule, and thus an increase in temperature. This is known because temperature is in fact the measure of the average kinetic energy of the molecules. Furthermore, the ice will continue to gain thermal energy causing its molecules to move faster and eventually break their intermolecular bonds or melt. In conclusion, the transfer of heat or thermal energy will typically change the temperature of the substance, but not always!
For example, at the moment when the ice in the bowl turns to water those water molecules will be at the exact same temperature as when they were ice. Heat , you will recall, is not something that is "contained within" a body, but is rather a process in which [thermal] energy enters or leaves a body as the result of a temperature difference. So when you warm up your cup of tea by allowing it to absorb J of heat from the stove, you can say that the water has acquired J of energy — but not of heat.
If, instead, you "heat" your tea in a microwave oven, the water acquires its added energy by direct absorption of electromagnetic energy; because this process is not driven by a temperature difference, heat was not involved at al!! We commonly measure temperature by means of a thermometer — a device that employs some material possessing a property that varies in direct proportion to the temperature.
The most common of these properties are the density of a liquid, the thermal expansion of a metal, or the electrical resistance of a material. The ordinary thermometer we usually think of employs a reservoir of liquid whose thermal expansion decrease in density causes it to rise in a capillary tube. Metallic mercury has traditionally been used for this purpose, as has an alcohol usually isopropyl containing a red dye. Mercury was the standard thermometric liquid of choice for more than years, but its use for this purpose has been gradually phased out owing to its neurotoxicity.
Although coal-burning, disposal of fluorescent lamps, incineration and battery disposal are major sources of mercury input to the environment, broken thermometers have long been known to release hundreds of tons of mercury. Once spilled, tiny drops of the liquid metal tend to lodge in floor depressions and cracks where they can emit vapor for years. Temperature is a measure of the average kinetic energy of the molecules within the water.
You can think of temperature as an expression of the "intensity" with which the thermal energy in a body manifests itself in terms of chaotic, microscopic molecular motion. This animation depicts thermal translational motions of molecules in a gas. In liquids and solids, there is vary little empty space between molecules, and they mostly just bump against and jostle one another.
You will notice that we have sneaked the the word " translational " into this definition of temperature. Translation refers to a change in location: in this case, molecules moving around in random directions. This is the major form of thermal energy under ordinary conditions, but molecules can also undergo other kinds of motion, namely rotations and internal vibrations. These latter two forms of thermal energy are not really "chaotic" and do not contribute to the temperature.
Energy is measured in joules , and temperature in degrees. This difference reflects the important distinction between energy and temperature:. Temperature is measured by observing its effect on some temperature-dependent variable such as the volume of a liquid or the electrical resistance of a solid. In order to express a temperature numerically, we need to define a scale which is marked off in uniform increments which we call degrees.
The nature of this scale — its zero point and the magnitude of a degree, are completely arbitrary. Although rough means of estimating and comparing temperatures have been around since AD , the first mercury thermometer and temperature scale were introduced in Holland in by Gabriel Daniel Fahrenheit. Fahrenheit established three fixed points on his thermometer. Zero degrees was the temperature of an ice, water, and salt mixture, which was about the coldest temperature that could be reproduced in a laboratory of the time.
When he omitted salt from the slurry, he reached his second fixed point when the water-ice combination stabilized at "the thirty-second degree. Normal human body temperature registered Belize and the U. In , the Swedish astronomer Anders Celsius devised the aptly-named centigrade scale that places exactly degrees between the two reference points defined by the freezing- and boiling points of water. For reasons best known to Celsius, he assigned degrees to the freezing point of water and 0 degrees to its boiling point, resulting in an inverted scale that nobody liked.
After his death a year later, the scale was put the other way around. The revised centigrade scale was quickly adopted everywhere except in the English-speaking world, and became the metric unit of temperature. In it was officially renamed as the Celsius scale.
When we say that the temperature is so many degrees, we must specify the particular scale on which we are expressing that temperature. A temperature scale has two defining characteristics, both of which can be chosen arbitrarily:. In order to express a temperature given on one scale in terms of another, it is necessary to take both of these factors into account.
If you remember this, there is no need to memorize a conversion formula; you can work it out whenever you need it. Near the end of the 19th Century when the physical significance of temperature began to be understood, the need was felt for a temperature scale whose zero really means zero — that is, the complete absence of thermal motion.
This gave rise to the absolute temperature scale whose zero point is — This was eventually renamed after Lord Kelvin William Thompson thus the Celsius degree became the kelvin.
In the Scottish engineer and physicist William J. Rankine proposed an absolute temperature scale based on the Fahrenheit degree. The Rankine scale has been used extensively by those same American and British engineers who delight in expressing energies in units of BTUs and masses in pounds.
The importance of absolute temperature scales is that absolute temperatures can be entered directly in all the fundamental formulas of physics and chemistry in which temperature is a variable.
Perhaps the most common example, known to all beginning students, is the ideal gas equation state. As a body loses or gains heat, its temperature changes in direct proportion to the amount of thermal energy q transferred:. The proportionality constant C is known as the heat capacity. The greater the value of C , the the smaller will be the effect of a given energy change on the temperature. It should be clear that C is an extensive property— that is, it depends on the quantity of matter.
For this reason, it is customary to express C in terms of unit quantity, such as per gram, in which case it becomes the specific heat capacity , commonly referred to as the "specific heat" and has the units J K —1 g —1. Thus if identical quantities of heat flow into two bodies having different heat capacities, the one having the smaller heat capacity will undergo the greater change in temperature.
You might find it helpful to think of heat capacity as a measure of a body's ability to resist a change of temperature when absorbing or losing heat. Note: you are expected to know the units of specific heat. The advantage of doing so is that you need not learn a "formula" for solving specific heat problems.
The specific heat of water is 4. From the definition of specific heat, the quantity of energy. How can I rationalize this procedure?
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