Suction describes the practical effect of creating a partial vacuum, which is simply a low-pressure area relative to the surrounding environment. This low-pressure state does not actively “pull” objects, but instead allows the higher-pressure air outside to push objects toward the low-pressure region in an attempt to equalize the forces. The perceived “force” of suction is ultimately a result of the everyday pressure exerted by the atmosphere pushing everything around us. By using common household items, we can manipulate this natural pressure differential in several distinct ways to achieve a functional partial vacuum. Understanding the underlying physics of pressure, volume, and temperature allows for the intentional creation of suction using simple, accessible methods.
Generating Suction Using Temperature Changes
One effective way to create a partial vacuum involves manipulating the temperature of the air within a contained space. This method relies on the fundamental relationship between gas temperature and volume, which influences the pressure inside a sealed container. Heating the air causes the gas molecules to move more rapidly and spread out, resulting in a temporary expansion of volume if the container is open. To generate the suction effect, the container must be heated, sealed, and then allowed to cool rapidly.
A common demonstration involves placing a small heat source, such as a lit match, inside a glass jar resting on a smooth, flat surface. The flame quickly heats the air inside, causing the air molecules to expand and some to escape past the jar’s opening. The primary mechanism creating the vacuum is the thermal expansion and subsequent contraction of the gas inside the container, not the consumption of oxygen.
As the heat source is extinguished and the jar is sealed against the surface, the remaining, trapped air rapidly cools. As the air temperature decreases, the gas volume contracts, which proportionally lowers the internal pressure. This drop creates a pressure differential between the inside of the jar and the higher atmospheric pressure outside. The external air pressure then pushes down on the jar and the surface, effectively holding the jar firmly in place due to the resulting suction.
Generating Suction Using Fluid Displacement
The use of liquids, most often water, provides a practical method for generating a low-pressure state, particularly through the action of siphoning. A siphon is a bent tube or hose used to transfer liquid from a higher container to a lower container, moving the fluid uphill over a barrier without the aid of a pump. This process relies on gravity pulling the fluid down the longer side of the tube and the atmospheric pressure pushing down on the surface of the liquid in the upper reservoir. The entire tube must be filled with fluid and free of air bubbles to establish a continuous fluid path.
To initiate the flow, the tube is typically filled entirely with liquid, and both ends are submerged or sealed. Once the outlet end is placed at a lower elevation than the starting container, the weight of the liquid in the longer, descending side begins to pull the entire column of fluid downward. This pulling action creates a reduced pressure zone at the highest point of the siphon, which is the actual suction generated by the fluid displacement.
The higher atmospheric pressure pushing down on the fluid surface in the upper container then forces the liquid up the shorter, inlet side of the tube and into the low-pressure area. This continuous force maintains the flow until the fluid levels in both containers equalize or the inlet end of the tube is exposed to air. The maximum height a siphon can lift water is approximately 10 meters at sea level, a limit directly related to the magnitude of the surrounding atmospheric pressure.
Generating Suction Using Mechanical Action
Another accessible method for creating suction involves physically manipulating the volume of a sealed or partially sealed space, often accomplished with common tools like a plunger. This mechanical action directly applies Boyle’s Law, which states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional. By physically increasing the volume of a contained space, the pressure of the gas within that space is lowered, thus generating a partial vacuum.
A standard toilet or sink plunger works by first establishing a seal over a drain opening with its flexible cup. Pressing the plunger down expels air, and then pulling the handle upward rapidly increases the volume inside the cup, which is sealed by the rubber rim. This rapid volume increase causes a significant decrease in the internal pressure, creating a strong suction effect that pulls material toward the low-pressure area. The higher atmospheric pressure on the exterior surface of the plunger then pushes it firmly against the smooth surface.
The same principle can be applied using flexible, everyday containers such as a plastic bottle. Compressing the bottle forces air out and sealing the opening while compressed minimizes the internal volume of air. When the pressure is released and the container attempts to return to its original shape, the internal volume increases. Because the container is sealed, this volume increase lowers the internal pressure, causing the higher external atmospheric pressure to exert a force strong enough to cause it to adhere to a flat surface.