Final answer:
The black hand of the duplex air gauge on a car indicates the tire pressure relative to atmospheric pressure. Tire gauges are designed to read zero at atmospheric pressure and show positive for pressures above it, defining the concept of gauge pressure.
Step-by-step explanation:
The black hand of the duplex air gauge on any type of car indicates that air pressure in the tire compared to atmospheric pressure. When you use a tire gauge, if your tire is nearly flat, the gauge will read nearly zero. This is because tire gauges are designed to show a reading of zero at atmospheric pressure. Only when the pressure in the tire is greater than atmospheric pressure will the gauge show a positive reading.
Gauge pressure is an important concept that refers to the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, and thus a tire gauge will indicate a positive reading when the tire's pressure exceeds that of the surrounding environment. When we talk about the pressure in a tire, it’s really the gauge pressure that we are referring to.
Manometers are tools that utilize this principle and can help to understand the gauge pressure concept better. They can measure the difference in pressure between the fluid column and atmospheric pressure, which operates similarly to tire gauges but can be used in various applications.