Matter and Radiation
1. Matter & Thermodynamics: Temperature, Heat, Entropy
Thermodynamics deals with the science of heat and heat exchange. This branch of physics was developed by examining heat flow in a manner very similar to fluid flow in mechanics. One way of monitoring the heat exchange is to measure changes in temperature, which is a measure of how "hot" or "cold" an object is. When you heat objects, they typically get hot (unless they undergo a phase transition). This means that heat has flown into the object. So, in this sense, temperature is a measure of how much heat the object has. This statement is not always true! This is because heat is a form of energy, and when an object is heated it may use some of that energy to change its internal consistency (for example store the heat as potential energy ) or do work, i.e. use up the heat. For example, when you heat the tip of a match by striking it against a rough surface the phosphorus gets hot (its temperature goes up), then it explodes. This explosion is a form of using the energy from "heat" in order to react with oxygen. Once the reaction takes place, all the stored (potential) energy gets used up. Then the tip has reached even a higher temperature than it would have gotten from just direct heating that it received from rubbing against the rough surface. Things can get complicated rather quickly, when dealing with thermodynamics from a macroscopic perspective! So, let us take the atomistic point of view, which allows us to get a more clear picture of things!
There are five known phases of matter. Of these three phases are familiar to all of us as part of our daily experience: solid, liquid, gas. In addition to these "common" phases we now know of two others distinct phases: plasma and Bose-Einstein condensate. In all these phases, except for the plasma phase, we are dealing with a collection of neutral atoms that arrange themselves in different forms for each phase. Plasmas are collections of ions, which are atoms that are missing electrons (positive ions), or have extra electrons (negative ions). In solids atoms are in fixed positions relative to each other. They can wiggle about, but keep the same relative position. In the case of fluids the constituent atoms have no fixed relative positions, but as in the case of solids, they are relatively close to each other. In the gas phase the atoms not only have no fixed relative positions, but also they moving very fast and are mostly very far from each other. In all these cases temperature is a measure of movement of the atoms. Motion in the solid phase is restricted to small oscillations, because the atoms have to retain their relative positions. But in the liquid and gas phases the atoms can literally move about. So, in these cases temperature is mostly a measure of how fast the atoms are moving from one place to another.
When we are dealing with a collection of atoms the energy of this collection is not only determined by the energy of the contribution individual atoms, but also by the composition of this collection. So, some how we need to also take into account the composition/form of our collection of atoms. The measure that is used in thermodynamics to take this into account is called Entropy. To be more precise, entropy is a measure of "order" of the atoms. The more ordered the form is, the lower its entropy. In this sense solids have less entropy than the liquids, because in the solids the atoms occupy more or less fixed positions whereas in liquids the atoms can move randomly. Gases (or vapors) have more entropy than liquids because the arrangements of the atoms in the gas phase is more random than it is in the liquid phase. So, as water molecules in the clouds are condensed into liquid as rain and then freeze on our cold winter roads the entropy of their collection gets more and more reduced.
2. Electromagnetic Radiation: waves, wavelength, frequency
We often speak of electricity and magnetism as two separate phenomena. But they are the effects of a single force, called electromagnetic force. When an electric charge moves it not only produces an electric current, but it also sets up a magnetic field. When a charge accelerates, i.e. its speed changes with time, then it produces a propagating electromagnetic field. This propagating field can interact with other electromagnetic fields and is called an electromagnetic wave. This electromagnetic wave (i.e. the traveling electromagnetic field) is what we call electromagnetic radiation. In late 1860s it was Sir James Clerk Maxwell who succeeded in combining four empirical laws of physics dealing with electromagnetics into one that described the motion of a wave; the electromagnetic wave. He showed that visible light is in fact such an electromagnetic wave. We now know that the spectrum of this type of wave includes many other seemingly unrelated forms of radiation, from gamma-rays, to X-rays, to the IR, to the visible light, etc.
A wave is a traveling disturbance. In water waves the actual water droplets don't travel the length of the ocean, the disturbance - up and down motions - does the traveling. Similar to water waves, electromagnetic waves are traveling disturbances. The wavelength of the wave is the distance that the wave travels in order to produce one complete disturbance. In water waves at the beach, for instance, wavelength is the actual distance from a peak to a peak, or from a valley to a valley. In the case of the electromagnetic waves the longer the wavelength, the weaker the effect of the wave. This is very anti-intuitive! We think of the "height" of the wave as a measure of its strength, as in the water waves. In the case of electromagnetic waves the "height" is a measure of "how much there is", or its intensity. But the strength of it is dependent on size of its wavelength. The more "ripples" pass a point, the shorter is the wavelength, and the stronger the wave. A long wavelength wave has fewer "ripples", so we say that it has a low frequency. Similarly, a high frequency wave has short (small) wavelength.
For more further information you could read more of my Web pages on:
Oscillations and waves and Electromagnetic Waves
An object, can change its temperature by three separate processes: conduction, convection, and radiation. Of these, the first two require the object to have direct contact with other things. It has to touch another object (solid or liquid) in order for it to conduct heat. When the object loses heat by contact with vapor or gas molecules, then we talk about convectional heat exchange. However, any object can radiate even in perfect isolation from other objects and in perfect vacuum. This is because atoms and molecules that make up objects operate though electromagnetic forces. So, they can change their energies by absorbing or emitting electromagnetic radiation. We will see later in the course that a given atom or molecule's absorption or emission of radiation is limited to specific frequencies (or wavelengths). So, typical objects cannot absorb radiation of frequencies not appropriate for the atoms that make up the object. A black body is an idealized object that can absorb any frequency radiation; i.e. it is a perfect absorber.
Maxwell's theory describes electromagnetic radiation as the flow of a continuous disturbance, very similar to the flow of ocean waves. But in 1900 Max Planck suggested that electromagnetic radiation is more like tsunami waves! The energy travels through space as a chunk, rather than a continuum flow. Planck's theory has lead to a totally new concept: electromagnetic radiation can not only be thought of as traveling waves, but equivalently as traveling particles. These particles are called photons that carry energy and momentum. They can "collide" with other particles as do, say, pool balls. Their energies however is not dependent on their speed, which has a fixed value, but on their "frequency". But more on this oddity later!
Questions on Matter & Radiation
Last Modified: September 10, 2007 Malekis@union.edu
