. Does blowing on or waving a developing Polaroid picture actually speed up its development process? — PS, Columbus, OH
The speed of the development process is determined by the diffusion of molecules within the developing film and by the rates at which they react with one another. Both processes, diffusion and chemical reactions are temperature sensitive. If blowing on or waving the film manages to increase its temperature, then diffusion and reactions will both speed up and the development time will decrease.
. I have read about how black holes can emit X-rays and radiation. If they absorb light, why do they emit these other things? — BA, Fairbury, IL
A black hole is surrounded by an imaginary surface called the event horizon. Nothing at all can escape from within this surface-not light, not X-rays...nothing! However, as matter falls into the black hole, and before it reaches the event horizon, the matter can emit any type of radiation it likes. The X-rays and radiation emitted "from a black hole" are actually coming from the area surrounding the event horizon, not from within that surface. As matter pours into a black hole, it often heats up so hot that it emits incredible amounts of radiation of all types so that black holes appear as very bright objects.
. Is the jet stream flowing in the same direction in the southern hemisphere as it is in the northern hemisphere? — LS, Ashland, OR
The jet streams flow eastward in both hemispheres. Their directions of flow are determined by the Coriolis effect, in which high-altitude winds that are heading away from the equator veer eastward because of their angular momentum on the spinning earth.
. How do you demagnetize a magnet?
A permanent magnet was magnetized when it was first made out of metal. It did have microscopic regions of magnetic order—magnetic domains—but those regions all pointed in random directions and the magnet didn't have any overall magnetic poles. To give it poles, it had to be magnetized. It was placed in a very strong magnetic field so that its domains grew or shrank until most of them were aligned with the magnetic field. The magnet acquired overall magnetic poles for the first time. When the field was removed, the domains remained as they were and the magnet permanently retained its new magnetic poles.
If this same magnet were reversed and then placed in that strong magnetic field again, it would become remagnetized in the opposite direction from before—its domains would grow or shrink until most of them were aligned with the magnetic field again. The magnet's north poles would become south poles and vice versa. Finally, if the magnet were wiggled back and forth in that strong magnetic field and gradually removed from the field, its domains would grow or shrink almost randomly. The magnet's magnetic domains would become randomized and it would end up with no overall north or south magnetic pole at all. It would be demagnetized.
. How do batteries go dead if you are only sending current through them?
As current flows through a battery, from its negative terminal to its positive terminal, the battery does work on that current. It must pull positive charges away from the negatively charged negative terminal and push them toward the positively charged positive terminal. An alkaline battery needs 1.5 joules of energy to transfer each coulomb of positive charge in this manner. This transfer operation consumes the stored chemical potential energy inside the battery and eventually causes the battery to go dead. Just because you don't see anything moving in the wires or in the battery doesn't mean that something substantial isn't occurring inside the battery—it undergoes electrochemical reactions whenever current is flowing through it.
. If you reverse one of the batteries in a string, does that reversed battery recharge?
Yes. While the other batteries in the string will pump positive charge from their negative terminals to their positive terminals, the reversed battery will extract energy from the positive charge as it flows from that battery's positive terminal toward its negative terminal. The charge will lose energy and the battery will gain energy. Some of the battery's additional energy will go into recharging the battery—converting its used chemicals back into their original forms. But some types of batteries are better at recharging than other. Those that aren't meant to be recharged may turn most of this energy into thermal energy and thus waste it.
. Why are batteries different sizes (e.g., AAA, AA, C, and D) if they all have 1.5 volts?
Those different alkaline battery sizes are chemically equivalent, which is why they all produce the same voltage rises for currents passing through them from their negative terminals to their positive terminals. The same chemical reactions allow each of these batteries to pump the charges, giving each coulomb of positive charge about 1.5 joules of energy—a voltage rise of 1.5 joules-per-coulomb or 1.5 volts. Where these batteries differ is in how many charges they can pump each second—their maximum currents—and in how many charges they can pump before running out of chemical potential energy—their total stored energy. The bigger cells (C and D) can handle far more current than the smaller cells (AAA and AA) and they also contain more stored energy.
. What do you feel g-forces when you ride on a roller coaster? - F
Whenever you accelerate, you feel a gravity-like sensation "pulling" you in the direction opposite your acceleration. What you feel isn't really a force—it's really just your own inertia trying to keep you going in a straight line at a constant speed. In other words, your inertia is trying to keep you from accelerating. For example, whenever you turn left in a roller coaster, your inertia opposes your leftward acceleration and you feel "pulled" toward the right. This "pull" of inertia is sometimes called a "fictitious force" but you should remember that it isn't a force at all, no matter how "real" it feels. Perhaps the most striking effect of acceleration occurs during your trip around a vertical loop-the-loop. When you are arcing around the top of the loop-the-loop, you are accelerating downward so quickly that you feel an enormous "fictitious force" upward. This "fictitious force" has a stronger effect on you than the real force of gravity, so you feel as though you are being pulled upward. The result is that you feel pressed into your seat, even though your seat is actually upside-down.
. Why do some parts of a house get dustier than others? — BC, North Reading, MA
Dust particles are tiny bits of rock, ash, and organic matter that have been ground into fine pieces by the wind and wear. Although these particles are denser than the air that surrounds them, they have trouble falling through the air because as soon as they move faster than about a snail's pace, they experience considerable air resistance or drag forces. A dust particle has trouble falling through the air because the upward drag force it experiences while descending even a few millimeters per second is enough to balance its weight so that it stops accelerating downward. Because dust particles have so much trouble descending through air, they tend to be swept along with moving air. That's why areas of your home that have large air currents tend to accumulate relatively little dust—the dust is swept along with the air currents and doesn't have time to descend all the way to the floor or furniture. But in areas of your home with fairly still air, the dust can slowly settle out so that it coats all the surfaces.
. How does a vacuum flask operate? — MA, Altamonte Springs, FL
A vacuum or "Dewar" flask is a double-walled vessel with a vacuum between the two walls. It's effectively a bottle inside a bottle, with their only contact at their mouths. Since there is no air in between their bodies, no heat can flow from one bottle to the other by either conduction or convection. The only way in which heat can move between the bottles is via thermal radiation. By coating the surfaces of the two bottles with highly reflective materials such as aluminum, even thermal radiation can be almost prevented from transferring heat between the bottles. Since there is virtually no way for heat to flow into or out of the vacuum flask, except through its mouth, the flask can keep hot things hot or cold things cold for extremely long times.