Impedance For Mechanics

In ham radio we’re always talking about impedance, especially when discussing antennas. The concept is used everywhere else in radio, too. There are several mathematical ways of calculating it and electrical ways of measuring it. (MFJ anyone?) If you ever drove a stick shift car or a 10- speed bike, you know a lot about impedance. You could take a course on AC Circuits from WA3VEE and learn many things about it from an expert. For now, though, let’s apply some simple thinking about what’s going on physically with this thing called Impedance.

Impedance is a way of expressing the relationship between Force, Flow and Time.

Force can be voltage. It can be pounds of mechanical force. It can be torque.

Flow is a quantity of something moving through a path. Stuff in motion. It can be gallons per minute (gpm). It can be revolutions per minute (rpm). It can be coulombs* of electron flow per second (amperes).

Time is, well, time. (Seconds, milliseconds, microseconds, etc)

*Eggs come in dozens. Sheets of paper come in reams. Electrons come in coulombs– a really big number. You can look it up.

When force and flow happen at the same time, it’s called Power. Work is happening. Heat is happening. The ratio of Force and Flow happening at the same time is Resistance. In other words, how much force is required to achieve a given flow. In electricity, the unit is Ohms (volts per amp).

Mechanically, this might be a steady push against a big box, sliding across the floor. Electrically it might be the voltage across a light bulb filament. The voltage is forcing current through the filament making it hot. Power happening over a duration of time is called Energy.

Sometimes force and flow do not happen at the same time because there is some kind of energy storage going on. In the mechanical world we have springiness and we have inertia.

A long spring is easy to pull with your finger, at first. Lots of motion (flow) without much force. The farther you pull on it, the harder it gets. The force per unit of motion starts increasing. Your rate of motion isn’t changing but the force is building up. As you do this you’re storing your own energy into the spring. Finally, you can’t pull any harder and you stop. The force is still there. There is no motion but there is stored energy. Your finger gets tired and you let go. Wham! The energy is released into force and motion again. If the force is “voltage” and the motion is “current”, you’ve just described Capacitance. In a capacitor the stored energy is in the form of an electrical charge. If you short circuit a charged capacitor, you get the sparky blast of releasing stored energy. The opposition to changes of voltage across a capacitor is called capacitive reactance. Capacitors are “springy”.

The complement of springiness is inertia. Pretend you have a dead car that you need to move. Let’s disregard friction for the moment. You push the car. It doesn’t want to move. You keep pushing. It starts moving. You keep on pushing and it moves faster and faster. You stop pushing but the car wants to keep rolling. You have injected your energy into that mass. Oops! The car rolls itself into another car, releasing the energy back into force and motion. And a lawsuit. In electricity we call this Inductance. In an inductor the stored energy is in the form of a magnetic field. The opposition to changes of current through the inductor is called inductive reactance. The current flow in an inductor acts like a moving mass. The inductor opposes the flow of current at first but, once current is flowing, it wants to keep it flowing.

Now let’s get “complex”. This time when we push the car we’ll also bring in the friction. The car is hard to push in general (resistance). It’s hard to accelerate or decelerate its mass, too. (reactance). That combination of resistance and reactance is called Impedance. It has a resistive component and a reactive component. The reactive component can be either capacitive or inductive. An impedance with no reactive component is called a pure resistance or a resistive impedance. An impedance with no resistive component is called a pure reactance. In real life with real hardware, there is no “pure” component. All resistors have some capacitance and inductance. All capacitors have resistance and inductance. All inductors have resistance and capacitance. It is a matter of what frequency these “parasitic” effects become evident or troublesome.

Preview: When inductive reactance and capacitive reactance interact, a wonderful thing can happen called Resonance. (It also might be horrible thing if it’s in the wrong place!) But that will wait for the next article called, “Resonance-When reactances tango!”.


Chuck / NA3CW