. Why don't soap bubbles float forever (I believe they fall when not influenced by wind currents) if you have the "same" air both inside and outside the bubble?
The average density of an air-filled soap bubble is just slightly higher than that of the surrounding air. That's because the soap film itself is denser than air and because the air inside the bubble is very slightly compressed, thus having a slightly higher density than the surrounding air. Because the bubble's average density is slightly higher than that of the surrounding air, the bubble will slowly sink in still air and will eventually reach the ground.
. How much natural pressure is around us when we are on the ground? Does this pressure decrease in higher places? Why don't people in aircraft explode because the pressure is lower?
Near sea level, the air around us has a pressure of about 100,000 newtons per square meter or 15 pounds per square inch. That means that each square meter of surface on your body is exposed to an inward force of 100,000 newtons or that each square inch of your body is exposed to an inward force of 15 pounds. Your body is thus exposed to enormous inward forces. However, you don't notice these forces because your body is composed of solids and liquids that resist compression ferociously. To see that this is so, try to squeeze a sealed bottle of soda or to squash a coin by stepping on it. It's very hard to shrink the volume of a solid or liquid by squeezing it.
The origin of the large pressure around us is the weight of the atmosphere overhead. The air near you is supporting the weight of several miles or kilometers of air overhead and the weight of this air is squeezing the air down here. When you ascend a mountain, the amount of air overhead decreases and so does the pressure of the air around you. Your body becomes less tightly squeezed by the air around it. However, you don't explode because releasing the pressure on you doesn't change your volume very much. Solids and liquids don't expand very much when the pressure on them is released.
. Could you explain the microscopic model of temperature in a gas? — DD, SC
Thermodynamics imposes a severe constraint on the meaning of temperature by observing that when two objects are at the same temperature, no heat flows between them when they touch. That constraint leads to the follow possibility: in a gas composed of independent particles, temperature must be proportional to the average internal kinetic energy per particle. By internal kinetic energy, I mean that we are excluding any kinetic energy associated with the movement of the gas as a whole. And by average per particle, I mean to add up all the internal kinetic energies and divide the sum by the number of particles. With this definition of temperature, two bodies of gas that have the same temperature won't exchange heat when they touch. It turns out to be a good definition of temperature and the one that we use in general.
. How do you figure out the weight lifting ability of a hot air balloon? — BK, Meraux, LA
The air surrounding an object pushes upward on it with a force equal to the weight of the air the object displaces. The observation is called Archimedes' principle. If the object weighs less than the air it displaces, the object will experience a net upward force and will float upward. Since hot air is less dense and weighs less than cold air, a balloon filled with hot air can weigh less than the air it displaces. To determine the net upward force on the balloon, you subtract the total weight of the balloon (including the air inside it) from the weight of the air it displaces.
At room temperature, air weighs about 12.2 newtons per cubic meter (0.078 pounds per cubic foot). But air's density and weight are proportional to its temperature on an absolute temperature scale (in which absolute zero is the zero of temperature). At 200° F, air weighs about 20% less than at room temperature, or about 9.7 newtons per cubic meter (0.062 pounds per cubic foot). Thus each cubic meter of 200° F air inside the balloon makes the balloon 2.5 newtons lighter than the air it displaces (or each cubic foot of that hot air makes it 0.016 pounds lighter). If the balloon's envelope, basket, and occupants weigh 4000 newtons (900 pounds), then the balloon will have to contain about 1600 cubic meters (56,000 cubic feet) of hot air in order to float upward.
. Why does a helium balloon in a car seem to defy Newton's laws? When you accelerate forward suddenly, the balloon moves forward and when you brake, the balloon moves back. Is that because the air inside the car compresses when you accelerate? — CT, Charlottesville, VA
Since the air in the car is denser than the helium balloon, the air's motion dominates the helium balloon's motion. When your car accelerates forward, the air's inertia tends to move it toward the back of the car-the accelerating car is trying to leave the air behind. The balloon moves forward in the car to give the air more room near the back of the car. When you stop suddenly, the air in the car continues to coast forward and accumulates at the front of the car. Again, the balloon moves backward in the car to give the air more room at the front of the car. You'll see exactly this same effect if you watch an air bubble in a bottle of water as you drive the bottle around in a car.
. Why does a can of Diet Coke float on water while a can of regular Coke sinks? Does that have to do with density and Archimedes' law? —AB, Riverside, CA
Because regular coke contains large amounts of dissolved sugar, it is much denser than water or than Diet Coke (which has far less dissolved material). Although even Diet Coke is denser than water, as is the aluminum in the can, a can of Diet Coke contains enough gas bubbles to lower its average density to just below that of water. According to Archimedes' law, an object with an average density less than that of the liquid in which it's submerged will float upward. A can of regular Coke has an average density that's greater than that of water, so it sinks.
. If air in a rigid 80 cubic foot scuba tank is pressurized to 3000 psi, giving the diver a certain amount of breathing time, then why does bottom time decrease with depth? I know about external pressure, but how does the pressure affect air inside
the tank? - RJ
The deeper a scuba diver goes, the greater the water pressure and the more the water presses in on the diver's chest. To be able to breathe, the air in the diver's mouth must have roughly the same pressure as the water around the diver's chest. That way, the diver will be able to use chest muscles to breathe the air into the diver's lungs. But the pressure of the air in the diver's mouth is proportional to its density and thus to the number of air molecules contained in each liter of air. At great depths, the diver must breathe dense, high-pressure air and this air contains a great many air molecules per liter. Since the scuba tank contains only so many air molecules, these molecules are consumed more rapidly at great depths than they are at shallow depths. The scuba regulator automatically controls the density of air entering the diver's mouth so that the air pressure is equal to the surrounding water pressure. That way, the air is easy to breathe. The deeper the diver goes, the more air molecules the regulator releases into each of the diver's breaths and the faster the air in the scuba tank is consumed.
. Why doesn't a helium balloon pop when it reaches the ceiling?
The buoyant force lifts the helium balloon upward—the denser air flows downward to fill the space vacated as the balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon's skin is gentle enough and spread out enough that it doesn't injure the rubber. The balloon simply comes to a stop and remains suspended until enough helium diffuses out of the balloon to cause it to descend.
. How do balloon pilots navigate around countries that forbid overflights? — LS, Ashland, OR
The speeds and directions of the winds vary considerably with altitude. For example, while surface winds near the sea blow toward shore on a hot summer day, high-altitude winds blow away from the shore at the same time, completing a huge circulation loop. Unlike a sailboat, which is at the mercy of the surface winds, a balloonist can adjust the balloon's altitude to search for winds heading in the desired direction. The balloonist makes these altitude adjustments by changing the balloon's weight and volume so that it sinks or rises.
. How do thermals affect the atmosphere and air currents? — RM, Praire Farm Schools, Wisconsin
Thermals are air currents in the atmosphere. When sunlight and exposure to warm ground raises the temperature of surface air, that air expands—its molecules travel faster and bounce against one another more vigorously, so they push themselves farther apart. This expanded air weighs less per cubic foot or meter than cooler air, so the cooler air around it lifts it upward in a rising current of warm air—a "thermal." The air can't simply accumulate way up overhead forever, so cooler air descends to take its place. The overall result is rising warm air and descending cool air. These air currents are part of giant circulation loops or "convection cells" that also include surface winds and high altitude winds.