Thursday, March 11, 2010

What is Heat?

What is Heat? Interestingly to build on last week’s blog, heat is kinetic energy. But we will address in particular a bit later. First let’s think about what everything is made of.

All substances are made up of atoms, of which only about 92 are of available in nature. From there, atoms combine to form molecules. Molecules are chains or collections of atoms. These chains or collections come in various structures and shapes and this determines to a great degree the way we perceive, interact, or can manipulate them at a macroscopic – real world – level.

A good example of this is water. Often we see expressions of the atomic structure of molecules, such as water which is expressed as H2O. This signifies that the molecule water consists of 2 atoms of Hydrogen and one atom of Oxygen. A water molecule is dipolar, which means that there is an opposite charge on each end of the atom. The Hydrogen’s come together at one end and the Oxygen at the other. Since the Oxygen atom has a slightly higher electronegativty, it causes this end of the water molecule to have a slightly negative charge, and the Hydrogen end to a manifest a positive charge. This charge can be seen if one takes a comb that has a slight negative charge on it from running it through hair, and holds it near a narrow stream of falling water. The electronegative electrons left behind on the comb will attract the slight positive charge of the hydrogen atoms in water and cause it to bend or be attracted toward the charged comb.

Another example that is unique to water and dictated by its atomic structure is its being known as the “universal solvent.” This means that water has the amazing ability to break down many other substances and act as a solvent. Again this is a by product of its dipolar structure. As an example, typical table salt or NaCl which consist of the separately toxic substances of Sodium and Chlorine is easily dissolved in water. After juggling an electron, the electropositive charge of the sodium binds well with the electronegative charge of the chlorine atoms, creating a stable ionic bond in the absence of water. When water is introduced the dipolar nature of water is insidious. The positive charge of the two hydrogen atoms pulls the negative charge of the salts chlorine and the negative charge of the waters oxygen attracts the sodium. And through a divide and conquer method repeated billions of times the divisive charges of the water atoms overwhelms the ionic bond of the NaCl breaks it down. Most any substance whose electromagnetic bonds can not overcome the small but abundant dipolar H2O atoms are soluble in water.

Ok, so now that we have addressed a couple examples of how atoms although extraordinarily small can have and macroscopic effect we can address heat briefly.

What is heat?

In ordinary substances the molecules that constitute a substances structure are constantly in motion. They continually bounce off each other and off other and other materials they come into contact with. This motion is universal and constant, and the hotter a material gets the faster on average these molecules and atoms move and bounce off each other. Conversely the colder something is the slower or less vigorously the molecules move, that is until one reaches zero Kelvin where theoretically the motion of all the molecules ceases. So how can we observe this in real life? Well, when one rubs their hands together, the friction speeds up slightly the molecules in the hand, and they intern propagate their motion by bouncing off adjacent molecules and ultimately the average speed of the molecules in your hand increases, and thereby the heat or warmth increases. And how fast on average to molecules move in a substance? Well interestingly it works out at room temperature to be about the speed of sound. And remember that although a molecule is moving fast, it can not move far without bumping into another molecule and bouncing off. Because of this each individual molecule does not move far over time but rather experience a slow random walk.

So again, what is heat, well heat is kinetic energy. Heat is the individual motion of a molecule running into another molecule, imparting its kinetic or motion energy on the adjacent molecule. So when a pot of water is heated up, the molecules average speed increased. Each individual molecule on average gains a little more speed, and from this a little bit more kinetic energy. Then if one takes their hand which is approximately 99 degrees and in which the individual molecules are moving slower on average then in the boiling water, and places it in the water, the individual molecules of the water collide with the skin molecules and impart their acceleration and energy to your hand. This energy or acceleration of the molecules in ones hand is perceived as heat, or in the case of boiling water, enough energy or speed is imparted to result in damaging the skin cells. So heat is kinetic energy.

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