### Re: What is the differnce between electromagnetic and mechanical waves?

Area: Physics
Posted By: Paul Noel, Nursing and Business, Individual
Date: Wed Mar 19 16:40:12 1997
Message:

Thank you for your question Salman Akbar

I hope you will be able to understand that what you have asked is a very fundamental question about matter and energy and as a result the answer I give will be subject to some theoretical understanding of matter. Theories are not proved. In this case we have pretty good data to suspect that the theories are right but they are still just good guesses. My answer will also be somewhat incomplete because of this.

Also I hope you will feel free to look more deeply into this matter in the future as the issue of how matter transmits energy is one very fundamental question which will contain many answers to things we want to do in the future.

The answer in very short has to do with energy. All energy transfers are electromagnetic, so strangely enough mechanical waves are electromagnetic waves in terms of energy. We all know that they look much different.

I must say that nobody has yet come up with a true explanation of what electromagnetic waves are. We have a lot of guesses.

Matter is connected together by electromagnetic bonds. Exactly how these work is not well understood but we have some working models. Basic Atomic theory indicates that electromagnetic forces bond matter together and these bonds are elastic. They can stretch or bend.

Imagine if you will a set of 2 heavy steel balls connected by a coiled spring. If you pull the balls far apart the spring will pull the balls back together until they bounce off each other if you let them go. The spring in this case is like the bonding force between two or more atoms. The weight of the balls in relationship to each other will determine the rate at which they move and which ball will move more than the other.

As some springs are stronger than others and some are quicker in action than others so bonds don t all act in the same way. There are bonds between matter that are strong like the chemical bonds and others which are weak like the bonds that hold together the powder that makes up a pill when it is pressed in a pill press. Other bonds are even weaker. In the case of mechanical waves the bonds that hold the material together vary. It depends on the type of the material in question.

A laser is a special kind of light which is tuned so that the bonds of the chemicals that glow and form the beam of light can be stretched all together and in a specific way. (Bend or stretch on a specific bond) We can measure the stretch by the wave length of the light from the laser. This powerful stretching adds up to a very bright and powerful light when released. The mirrors that encase a laser merely cause the light to bounce back and forth many times building up force in the light wave by pushing the matter back and forth. The wave length determines the color of the light and tells us the length of the stretch in the bond and which bond stretched. A lithotripsy machine used to break up kidney stones inside of a living person uses the fact that this laser light can bounce matter very hard. These hard bounces cause the shock waves in matter that converge on the stone and shatter it.

You may have seen a similar thing in a sloshing water tank or in a bathtub where you slosh back and forth adding one wave on top of the other until the wave gets very high. The ends of the tub act just like the mirror in a laser. Mechanical waves such as in water tend to be the product of very long distance bonds that bond the water loosely together. Thus when a shock or push is added it takes some time for the energy to transmit and the matter around gets squished up and together. Thus the bond force gets transmitted very slowly. There are more types of bonding than just chemical bonding.

Mechanical waves of this type which involve the compression or motion of matter vary in speed and method of movement depending on the media involved. A steel ball transmits a wave very fast because it is solid and very strongly bonded. Water transmits the wave slower and you see the wave more easily because the water piles up in a wave you can see. If you had a very fast camera you could see the steel has a similar wave but is must smaller because of the type of wave solids tend to transmit more easily.

Gasses also transmit waves. The speed of a wave traveling in a gas in known as the speed of sound. Since the air has different densities, it has different speeds of sound. This relates to the nature of the bonding of the air together and the mass being moved.

Electromagnetic waves transmit directly to a bit of matter over a distance or may cascade from matter to matter. This is in the form of either light, electrical currents, or magnetism. (J. K. Maxwell wrote that light electricity and magnetism were clearly manifestations of the same force, in 1864 and to date it appears he is correct) In any case these waves eventually move matter somewhere. Since it is energy that bonds matter together any movement of matter involves these waves. Even the gravity accelerations we see in matter have electromagnetic effects.

So generally we assume that mechanical motions are not related to electromagnetism since they appear unrelated to electromagnetism. They still involve the transmission of electromagnetic energy. Exactly what electromagnetism is has not yet been determined. It is more understood as existing than what it is. This is a matter for substantial study in Sub Atomic Physics. We really do not know how matter is linked together but we know it is.

Some Mechanical Wave forms which are determined by which way the matter is moved are Lateral waves: These travel in a line through the medium and generally are expressed as a direct in line push of matter. Transverse waves: These travel in a line by way of displacing matter perpendicular to the wave motion and in water represent a rolling of atoms in the wave.

Ocean Waves are primarily lateral in deep water and become transverse in shallow water. This is why waves appear taller at the shore than out to sea. They are composite having the behavior of both types of waves.

In an earthquake the rocks transmit lateral waves until they meet wet ground or water. The water converts the wave into a transverse wave. The result is that buildings on a bedrock foundation generally are not bothered by earthquake but those on wet soils are bothered.

Some Electromagnetic Wave Forms:
A beam of light is an electromagnetic wave. So is an AC current. A magnet clearly emits electromagnetic waves as that is where we get the term.

In the end all of these waves are nothing but one of the many ways energy transmits through the our world. It is a question of what is being bumped and what it is hooked to by way of an electromagnetic bond.

I hope this answer lets you see:
[1] that there is a lot we need to learn and
[2] that everything is attached in some way and
[3] we are only now starting to understand.

Waves do many things in our world that we don t even think about. In your area they shape the sand on the Saudi Arabian Desert. They form the clouds and determine the wind in how it acts. They even heat and cool us. All of the motion of matter around us is determined by these waves. The fact that the distance at which the wave varies or the speed at which it is moving varies determines what kind of wave we see. I don t know any place you can go to learn about exactly how each wave form transmits its energy to another.

Much of the science of remote detection relies on the more basic understanding of how energy interacts and makes up matter.

Here are a few simple experiments you may be able to do to see what I have said.

• Take a large audio speaker and hook the wires from it to a full wave bridge rectifier (AC to DC conversion device) Then by speaking into the speaker, measure the electricity generated on a meter. This shows that even sound waves transmit electromagnetic energy.
• Reverse the energy flow by applying an amplified signal to a speaker and see the electrical energy converted to sound waves.
• Put some dust or fine filings on a paper which is above the cone of a speaker. When you energize the speaker (of a radio or similar device) you will see the files move. They will form wave patterns. (These sound patterns were studied and the study gave rise to the invention of the telephone. )
• Get a DC motor and spin it with your hand and measure the current it puts out.
• Get a photocell and generate electricity with sunlight. Then remember that electricity drives even the light from your computer screen. One very simple experiment to show you that it is energy that holds matter together is easily done with a plastic soft drink bottle. Use a large bottle because the effect is easier to see. Take the bottle (empty of liquid) and get it outside where the air is hot. Cap the bottle tightly and then take it inside and place it in a freezer. After an hour or so observe the bottle and see that it is somewhat collapsed. Merely return it to the sunlight and you will see that it fills up quickly. Here by mere addition of light energy you can change the size of matter and its bonding.

Another way is to burn any fuel. The fire merely releases the energy that bound the molecules together. The oil your nation sells is nothing but solar energy collected long ago and stored by plants in hydrocarbon bonds.

So the nature of the wave we see is determined by which bond got stretched and how much it got stretched. The displacement of matter depends on its structure of bonds. The mass of the matter determines how much energy it takes to move. This is all very simple relationships like the ones which determine the sound of a note made by a flute. There are many relationships so they tend to confuse but all of them are very simple. Remember that not all interactions are in straight lines. Most happen as odd angles. The result is that the energy gets directed based upon the angle of the bump.

One very curious feature of matter is that for every action there is an equal and opposite reaction. Thus if a wave is sent one way, there is another wave exactly the same size sent the opposite way. This makes everything happen in double sets. (if this does not confuse you then you are truly rare or don t understand at all.) This is why waves bound around the way they do. It is a principal of Quantum Mechanics which is a science which even those who study it find a bit odd. Again it is a worthwhile and very deep study but if you like the study of Physics it is the stuff to excite a lifetime.

How a mirror reflects Light? From Scientific American.

Wave Physics discussed in terms of acoustic energy.

Simple sound experiments.

Carnegie Melon University site for Condensed Matter Physics.

Here is a NASA research page into the matter of your question.

NASA Discussing Radio Detection / Transmission / GPS and more.

Helsinki University of Technology.

USGS Earthquake Laboratory and their links page.

GRAVSAT: The Gulf of Mexico. This is a Remote Observation Company.

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