EMC Question of the Week: November 25, 2024

In the water circuit analogy to electric circuits, the quantity analogous to the velocity of signal propagation in a transmission line is the velocity of   

1. the water molecules
2. water molecule vibrations
3. sound in water
4. light in water

Answer

The best answer is “c.” The wave equation describing the propagation of sound is similar to the wave equation describing the propagation of electromagnetic waves in a transmission line. In this analogy, the interactions between the water pressure and volume velocity in an acoustic wave are similar to the interactions between the voltage and current in a transmission line. Because of this, water circuits like the one in the figure can be used to intuitively describe what happens as TEM waves propagate in a transmission line. Lumped transmission line components like capacitors and inductors have analogous components (e.g., pipes with elastic bladders and water wheels) that can be found in water circuits.

In water circuits, the velocity of the wave propagation is much faster than the time-average velocity of the water molecules. Similarly, in transmission lines, the velocity of the wave propagation is much faster than the drift velocity of the electrons. In both water circuits and transmission lines, there can be incident waves, reflected waves, standing waves, matched terminations, signal dispersion, and time-average power flow. In both water circuits and real transmission lines, we can have common-mode and differential-mode propagation with conversion between the two modes occurring due to changes in the system's balance.

While the water circuit analogy can be a valuable tool for understanding how electric circuits and components behave, it is not a perfect. The wave energy in a water pipe is contained within the walls of the pipe. The wave energy in a transmission line is contained in the fields between the two conductors. Because of this, the relative position of the two pipes in a water circuit is relatively unimportant. On the other hand, the relative position of the two conductors in a transmission line plays a key role in determining the wave impedance.

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