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Name: Simon Quellen Field
Location: Los Gatos, California, United States

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Saturday, April 15, 2006

Impedance matching

Levers do it.
Pulleys do it.
Ramps, transformers, gears, megaphones, and wheelbarrows do it.
Even screws do it.

Match impedance, that is.

Impedance is the opposition to the flow of energy.

If you try to lift your refrigerator, you will experience an opposition to the flow of energy. The refrigerator will just sit there, and you will get tired. The ability of your muscles to lift the weight is not matched to the weight.

There are a number of ways you can lift a 500 pound refrigerator by matching the impedance of your muscles to the impedance of the load. You could push the load up a ramp. You could use a lever, or a block and tackle, or a hydraulic jack, or a screw jack. Each of these devices allows you to trade lifting the 500 pound load for lifting a smaller load, say 50 pounds. You generally trade off time, pushing 50 pounds for ten seconds instead of 500 pounds in one second. The same amount of energy is expended, but at a much lower power level.

When impedances are mismatched, energy put into the system is reflected back. If you jump on a see-saw with a refigerator on the other end, you will bounce back off as if you were on a diving board. But if you move the fulcrum closer to the refrigerator, you can jump onto the see-saw, and your end will move down, lifting the heavy load at the other end.

You can line up a row of billiard balls, and hit the row with the cue ball, and the last ball in the row will shoot off down the table. But if one of the balls is made of steel, the cue ball will simply bounce off of it, and most of the energy will be reflected.

We can match the impedances to get the steel ball to move. We put a row of balls in front of it, each one made of a slightly lighter weight material than the last, until the ball nearest us is almost the same mass as the cue ball. Now the speeding cue ball will stop dead when it hits the row of balls, and the steel ball will slowly move off down the table, having absorbed all of the energy.

When you shout to a friend who is underwater in a swimming pool, the sound from your voice bounces off the water, and very little sound energy gets to your friend's ears. But take a traffic cone and put the narrow end of it into the water and shout into the large end. Now your friend can hear you, because the low pressure sound waves over a large area are converted into high pressure waves over a small area, and the water moves from the high power sound. Here we are not trading time. Instead, we are trading a large area for a smaller one.

An electrical transformer also matches impedance. It takes high voltage, low current energy, and matches it to a load the needs low voltage, high current. It also works the other way around. Without the transformer, most of the energy is reflected back to the source, and little work gets done.

A water nozzle is an impedance matcher. So is cupping your hand behind your ear.
A telescope is an impedance matcher. So is a magnifying glass, or a winding mountain road, or the gears on your bicycle.

Now that you are aware of impedance matchers, you will start to see them everywhere.


Blogger Hrishikesh said...

That is a very interesting perspective towards impedance matching! As an electrical engineer in the making, I must confess, I'm pretty darn impressed! This is something I'm definitely going to share with my classmates.

April 27, 2006 6:05 AM  
Anonymous Anonymous said...

WOW!!!! That was really wonderful. You should write a text book.

July 12, 2007 2:56 PM  
Anonymous Anonymous said...

Don't forget the gears on a bike or a car!

August 24, 2007 2:23 PM  

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