All of the rules in this video stem from one major concept: high to low. Think about gravitational force taking something from a higher place and bringing it down to a lower place. If a ball rolls down a hill, there’s also a potential energy difference; the ball has high potential energy at the top of the hill and low potential energy as it reaches the bottom. In short, the high-to-low phenomenon indicates differentials in energy.

The four rules are as follows:

1. High pressure goes to low pressure
2. High temperature goes to low temperature
3. High voltage goes to low voltage
4. High humidity goes to low humidity

Those four rules illustrate that things in nature tend towards equalization (equilibrium).

In an A/C system, the compressor generates a differential in pressure between the low (suction) side and the high (head) side. Without those differences to generate motion throughout the refrigeration cycle, we wouldn’t have air conditioning as we know it.

We are also constantly manipulating temperature in HVAC/R work. We want to transfer heat from one place and move it to another. (In the case of residential A/C, we move the heat from the inside of the house and move it outside the house by making the heat travel to the cold refrigerant inside the HVAC system.) It’s also worth noting that temperature refers to the average speed of molecules, so temperature is a measure of molecular movement and intensity.

In the case of humidity, we can take a cloth and notice the behavior of two air masses of varying humidity concentrations. An air mass with a higher concentration of humidity will attempt to move some of that humidity to the side of the cloth with lower humidity via diffusion.

If we have a neutral side of power that’s sitting at 0v to ground and another side that has 120v AC or DC power, a path between them would result in the electrons moving from the side of higher voltage (120v) to lower voltage (neutral).