Life is all about circuits. In hydraulic circuits, a pump forces a fluid to flow, such as water through pipes or soda through a straw.
In pneumatic circuits, a pump forces airflow down a path, such as the air conditioning system in your house or the air hose that you use to inflate your tires.
We all know intuitively that pumps produce a pressure and rate-of-flow of something.
A path must exist for the flow to follow, whether intentional (electrical traces on a circuit card) or unintentional (a bolt of lightning through a dwelling). Where there is flow, there will also be opposition.
On earth, something always “fights back” when we try to move anything from point A to point B. This opposition to flow is called impedance. A power source (pressure and flow) and a path form a circuit.
A circuit is complete when the flow ends up where it started - back at the power source.
This may be in the same physical location, such as the blood in your veins returning to the heart, or the current from a battery returning to the other terminal.
Alternately, the flow will stop when it reaches the same “potential” as the pressure reference of the source - a concept that’s a little harder to grasp but is actually just another way of saying the same thing. The circuit must be complete for flow to take place.
We can make the flow do something for us on its trip back to the source. This is the very heart of technology, harnessing the energy in the world around us to do useful things that make our lives easier.
Source Be With You
In electrical circuits, the pressure of the source (voltage) produces a rate-of-flow of electrons (current), against an opposition (impedance).
The paths are the wires and circuit board traces that we create in our electronic gadgets.
Nature is always ready to create additional paths if we provide the correct conditions. These “parasitic” paths for current flow can cause us much grief - hum and noise in the best case, loss of life in the worst.
Figure 1: An analog meter is a plus when looking for open and short circuits and intermittent problems. This popular meter sources enough current to test for “pin 1” problems on electronic components.
Most audio and acoustic measurements are voltage measurements because pressure is the easiest parameter to measure in most circuits - just like hopping on the bathroom scales and reading the number.
Current is a bit more difficult, because the flow must pass through the meter. This is why doctors measure your blood pressure rather than the rate of blood flow, as a measurement of the latter could make you quite uncomfortable!
Once the pressure and rate of flow are known, the impedance can be calculated. Ohm’s law provides the relationship between the three parameters in electrical and acoustical circuits, and analogous laws do the same for hydraulic and pneumatic circuits.