. I have read recently that achieving absolute zero is impossible. Why is this the case? What will happen to objects at this temperature (i.e., solid, liquid, and gas)? — BC, Ottawa, Ontario
Absolute zero can't be reached for the same reason that any perfect order is impossible. It's just too unlikely to ever happen. For an object to reach absolute zero, every single bit of thermal energy and every aspect of disorder must leave the object. If the object is a crystalline material, then its crystal structure must become absolutely perfect. This sort of perfection is essentially impossible. Reducing the temperature of an object towards absolute zero requires great effort and ends up creating a great disorder elsewhere. The closer the approach to absolute zero, the more disorder is created elsewhere. To reach absolute zero, you'd have to create infinite disorder elsewhere. For something to think about, imagine trying to make you lawn absolute perfect. The more perfect you tried to make it, the more gardeners you'd need and the more food, money, and services would be consumed. The lawn would grow more and more perfect but everything else would grow more disordered. And still you would never have a truly perfect lawn.
. Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water that is medium to very hard? I want enough to use in a steam iron regularly? — HD, Kintnersville, PA
There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts. The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange
sodium ions for the calcium, magnesium, and iron ions because sodium ions don't have such bad effects on soaps and detergents. Adding salt to hard water, as you propose to do, won't exchange sodium ions for the other ions. It will only add more metal ions to the water and the water will remain hard.
Second, a steam iron shouldn't use hard water because when hard water boils away as steam, it leaves behind all the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for your iron. The best way to demineralize water is to distill it.
. How long will the magnetic data last on a VCR tape before it becomes no longer useable as read data? — KR, Urbana, IL
As long as the tape is kept cool and dry, its magnetization should remain stable for years. However, there is the problem of magnetic imprinting from one layer of tape to the adjacent layers on a spool. With time, one layer transfers some of its magnetization to those adjacent layers. In a videotape, this imprinting leads to a gradual appearance of noise in the video images. As long as you're willing to tolerate a little video "snow," this imprinting shouldn't be too much of a problem. You can reduce its severity by occasionally winding and rewinding the tapes. But I don't see any real reason why a tape won't be reasonably useable for decades.
. I am interested in finding out if and what materials affect magnetic fields. — HLD, Jacksonville, FL
Magnetic fields are associated with lines of magnetic flux, invisible structures that stretch between north and south magnetic poles or that curve around on themselves to form complete loops. Unless a material has its own north or south magnetic poles, it can't terminate the magnetic flux lines and can have only small effects on magnetic fields. The few materials that do affect magnetic fields substantially are ones such as iron or steel that are intrinsically magnetic and that can easily develop strong north and south magnetic poles. These magnetic materials can significantly shift the paths of the magnetic flux lines. If you put an iron or steel box in a magnetic field, the flux lines will tend to travel through the walls of the magnetic box. As a result, there will be few magnetic flux lines inside the box and almost no magnetic field. This effect is used to shield sensitive equipment such as the picture tubes in televisions from magnetic fields.
. Don't microwaves penetrate metal at all? — DR, Tampa, FL
If the metal is a good conductor, then the microwaves don't penetrate more than a fraction of a millimeter. That's because the microwave electric fields push on the metal's mobile electrons and those electrons immediately rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal responds to the fields and it shields the rest of the metal from the microwaves.
. How can we clean the microwave oven? - PTW
Since the cooking chamber of a microwave oven doesn't get hot, there is no way to make a "self-cleaning" microwave oven. Instead, you have to clean it by hand with a sponge and perhaps a little soapy water. As long as you get the soap or any other cleaning agents out, you can clean the cooking chamber just as you'd clean the top of a stove.
. If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? — IF, Bakersfield, CA
According to a reader, most microwave oven capacitors have fuses in them so that when they fail, they usually become open (they lose all of their ability to store separated charge and behave as a simple open circuit). You'd need a capacitor checker to find this open circuit within the capacitor.
. Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW
Not one that came from the microwave oven. Microwaves are all around us and are completely innocuous. Your body emits weak microwaves all the time, as part of its thermal radiation! Like light, microwaves don't remain still in objects so you can't eat one that was put in the food by the oven.
. Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? - PTW
You can put a microwave oven anywhere that it's stable and where it has adequate ventilation. A microwave oven has a fan and vents through which it gets rid of its excess heat. You mustn't block the vents or the oven will overheat.
. How does ultrasound detect cracks or imperfections in metal? Is this to do with density or is it just reflecting off surfaces? — PA, Essex, UK
Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting sound and detecting the reflected sound, it's possible to image the imperfections. The same technique is used in medical ultrasound to image organs or fetuses, and even to image the insides of the earth.