Three Things They Should Teach in Engineering 101

Lesson Two

I recently taught a class at Utah State University on microprocessors. By the final lecture, we had covered all the material for the semester. One of the students suggested the following lesson. It was something he had heard us discuss at work. (He works at the same company as I do, while he is working on his degree.) He thought his peers could benefit from it and wanted to know the principles in more detail.

As I taught this lesson, I saw more interest in the eyes of the students than was there most of the semester. Although these were junior level students, they had not been exposed to this principle. It obviously struck a chord with them. Also, I never had students thank me for a lecture before, unless some kind of test was involved. They were genuinely glad to learn this lesson:

Physical equivalents of electrical components.

We are going to compare three basic components in electrical circuits to three basic components in mechanicalý circuits, we shall say. There are several reasons to do this.

First, I think the typical person understands the physical world more intuitively than he understands the electrical one. This is because we interact with it using all of our senses, whereas the electrical world is still very magical, even to an educated engineer. This is because much of what happens inside a circuit cannot be seen, felt, or heard. Think about it. You flip on a light switch, and the light goes on. You really don't consider how the electricity caused it to happen. Drag a heavy box across the floor, and you certainly understand the principle of friction.

Second, the rules for both disciplines are exactly the same. Once you understand one, you will understand the other. This is great, because you only have to learn the principles once. We call EE's in our company 'sparkies' and ME's 'wrenches'. If you grok* this lesson, a 'sparky' can hold his own with the best 'wrench' around, and visa-versa.

Third, if you get a feel for what is happening inside a circuit, you can be an amazingly accurate trouble-shooter. The human mind is an incredible instrument for simulation and unlike a computer, it can make intuitive leaps to correct conclusions based on incomplete information. I believe that by learning these similarities you increase your mind's ability to put together clues to the operation and results of a given system, resulting in correct analysis. This will help your mind to 'simulate' a circuit.

Fourth, This will be a building block for lesson three.

The resistor is equal to friction.

Think about what happens in the above example where you drag a box across the floor. A force called friction resists the movement of the box. This friction is related to the speed of the box. The faster you try to move the box, the more the friction resists your work. It can be described by an equation.

friction = force/speed

Furthermore, the friction dissipates the energy loss in the system with heat. Let me rephrase that. Friction makes things get warm. Don't believe me? Try rubbing your hands together right now. Did you feel the heat? That is caused by friction.

The function of a resistor in an electrical circuit is equal to friction. The resistor resists the flow of electricity just like friction resists the speed of the box. And guess what: it gets heats up as it does so. An equation call "Ohm's Law" describes this resistance.

resistance = voltage/current

Do you see the similarity to the friction equation? They are exactly the same. The only real difference is the units you are working in.

The inductor is equal to mass.

Let's stay with the box example for now. First let's eliminate friction, so as not to cloud our comprehension. The box is on a smooth track with virtually frictionless wheels. You notice that it takes some work to get the box going, but once moving, it coasts along nicely. In fact, it takes work to get it to stop again. How much work depends on how heavy the box is. This is known as the law of inertia. Newton postulated this long before electricity was discovered, but it applies very well to inductance. Mass resists a change in speed. Correspondingly, inductance resists a change in current.

mass = force/(speed/time)

inductance = voltage/(current/time)

The capacitor is equal to a spring.

So what does a spring do? Take hold of a spring in your mind's eye. Stretch it out and hold it, then let it go. What happens? It snaps back into position. A spring has a capacity to store energy. When a force is applied, it will hold that energy till it is released. Capacitance is similar to the elasticity of the spring. (One note: the spring constant is the inverse of the elasticity.

spring = (speed *time)/force

capacitance = (current*time)/voltage

A tank circuit.

Take the basic tank or LC circuit. What does it do? It oscillates. A perfect circuit would go on forever at the resonate frequency. How should this appear in our mechanical circuit? Think about the equivalents: an inductor and a capacitor, a spring and mass. In a thought experiment, hang the spring from the ceiling, and a mass from the spring. Give it a tug. What happens? It oscillates, of course -- exactly like the tank circuit does!

A complex circuit.

Let's follow this reasoning for an LCR circuit. All we need to do is add a little resistance, or friction to the mass-spring of the tank circuit. The ME's call this friction component a damper, because it dampens the oscillation. What did you learn that a resistor does to a LC circuit? It dampens the oscillation.

Remember Fourier's Theorems? They were discovered for mechanical systems long before anyone realized that they work for electrical circuits as well. Remember all that higher math you used to know, Laplace transforms, integrals, derivatives etc.? It all works the same in both worlds. You can solve a mechanical system using Laplace methods just the same as an electrical circuit.

Back in the 50s and 60s, the government spent lots of dough using electrical circuits to model physical systems just like described above. Why? You can get into all sorts of integrals, derivatives and other ugly math when modeling real world systems. All that can get jumbled quickly after a couple of orders of complexity. Think about an artillery shell fired from a tank. How do you predict where it will land? You have the friction of the air, the mass of the shell, the spring of the recoil. Instead of trying to calculate all that math by hand, you can build a circuit with all the various electrical components representing the mechanical ones, hook up an oscilloscope, and fire away. If you want to test 1000 different weights of artillery at different altitudes, electrons are much cheaper that gunpowder.

Darren

*If you do not know what 'grok' means, I highly suggest reading Robert Heinlein's Stranger in a Strange Land. I personally rank it as one of the best 10 books ever written.

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