Pressure is a fundamental concept in physics, representing the force exerted by a substance per unit area. It describes how a force spreads across a surface, explaining why a sharp knife cuts easily, as the force is concentrated over a small area. While often discussed in terms of positive values, “negative pressure” is a commonly used term that can be confusing. This article clarifies what “negative pressure” truly means and explores its relevance in various real-world situations.
What “Negative Pressure” Really Means
The term “negative pressure” does not imply a pressure value below absolute zero, which is physically impossible. Instead, it is a relative term indicating a pressure lower than a specific reference point, most frequently atmospheric pressure. Consider elevation: a location might be “negative” 100 feet if it is 100 feet below sea level, even though its absolute height above the Earth’s center remains positive.
Pressure always exists, even in what is considered a vacuum, though it can be extremely low. When a gauge measures “negative pressure,” it typically displays gauge pressure, which is the difference between the measured pressure and the surrounding atmospheric pressure. For instance, a reading of -5 PSI (pounds per square inch) means the pressure is 5 PSI below the current atmospheric pressure, not an absence of all pressure.
How Low Can Pressure Go?
The theoretical limit for pressure is absolute zero, a perfect vacuum where no particles exist and the pressure is precisely zero. Achieving a perfect vacuum is not possible in practice, as it would require removing every single atom or molecule from a space. Even in outer space, a truly perfect vacuum does not exist, with sparse particles and residual radiation present.
Scientists distinguish between a “partial vacuum” and a “hard vacuum.” A partial vacuum is a space where some air has been removed, resulting in pressure lower than atmospheric pressure. Most everyday applications of “negative pressure” involve a partial vacuum. A hard vacuum represents an extremely low-pressure environment, approaching the theoretical perfect vacuum, but still containing some sparse particles.
Where We Encounter Low Pressure
Low pressure environments are common in daily life and specialized settings. When you drink through a straw, you reduce the air pressure inside the straw by sucking, allowing the higher atmospheric pressure outside to push the liquid up into your mouth. Suction cups adhere to surfaces because pressing them down expels air, creating a lower pressure inside than the external atmospheric pressure, which then holds the cup in place. Similarly, vacuum cleaners operate by a motor spinning a fan to create a low-pressure zone, causing air and debris to rush in from the higher-pressure surroundings.
Low pressure environments are also found in:
- Hospital isolation rooms, which use negative pressure to contain airborne pathogens by ensuring air flows in from outside.
- Cleanrooms in industries like pharmaceuticals and electronics, which can use negative pressure to contain hazardous materials.
- Vacuum packaging, which removes air to extend food shelf life by inhibiting microbial growth and oxidation.
- High altitudes, where fewer air molecules result in naturally lower pressure.
- The center of weather systems like tornadoes and hurricanes, which are characterized by low pressure.
Effects of Low Pressure Environments
Exposure to low pressure environments can have various physical and physiological effects. For inanimate objects, a significant pressure differential can lead to implosion, where the higher external pressure crushes a container with lower internal pressure. Conversely, objects with sealed internal pressure, like certain packages, might burst if placed in an extremely low-pressure environment, as the internal pressure becomes much greater than the external pressure.
For living beings, low pressure can cause physiological challenges. At high altitudes, reduced atmospheric pressure means less oxygen is available for breathing, leading to symptoms like shortness of breath. Rapid decompression, such as for divers ascending too quickly or aviators in unpressurized aircraft, can cause decompression sickness (“the bends”), where dissolved gases in the body form bubbles. In the extreme vacuum of space, bodily fluids would begin to boil due to the lack of external pressure, a phenomenon called ebullism, though human skin and tissues offer some resistance. Controlled low pressure also has benefits, as seen in medical applications like negative-pressure wound therapy or industrial processes like vacuum drying and preservation.