. How do Oven Cooking Bags work? I know they are made of heat resistant nylon resin, but can you explain what that means? — HY, Halifax, Nova Scotia
There are two broad classes of plastics: (1) thermoplastics that can melt, at least in principle, and (2) thermosets that can't melt under any circumstances. Thermoplastics consist of very long but separable molecules and common thermoplastics include polyethylene (milk containers), polystyrene (Styrofoam cups), Nylon (hosiery), and cellulose (cotton and wood fiber). Thermosets consist of very long molecules that have been permanently cross-linked to one another to form one giant molecule. Common thermosets include cross-linked alpha-helix protein (hair) and vulcanized rubber (car tires).
Most common plastic items are made from thermoplastics because these meltable plastics can reshaped easily. But different thermoplastics melt at different temperatures, depending on how strongly their long molecules cling to one another. The plastic in an Oven Cooking Bag is almost certainly a thermoplastic form of Nylon, but one that melts at such a high temperature that it doesn't change shape in the oven. It's possible that the Nylon has been cross-linked to form a thermoset, so that it can't melt at all, but I wouldn't expect this to be the case.
. How can we measure magnetic fields or magnetic potentials of solvent atoms that reside interstitially inside solid solutes? — DR, Tampa, FL
You can measure the magnetic fields in which certain atoms reside with the help of nuclear magnetic resonance (NMR). This technique examines the magnetic environment of the atom's nucleus by determining how much energy it takes to change the orientation of the nucleus. Since the nucleus is itself magnetic, it tends to align with any magnetic field—like a compass. The stronger that magnetic field, the harder it is to flip the nucleus into the wrong direction.
. What happens when matter and anti-matter collide? Do they just destroy each other? I thought that matter couldn't be created or destroyed? - S
As Einstein's famous formula points out, mass and energy are equivalent in many respects. In most situations, mass is conserved and so is energy. But at the deepest level, it's actually the sum of those two quantities that's conserved. When matter and anti-matter collide, they often annihilate one another and their mass/energy is converted into other forms. For example, when an electron and an anti-electron (a positron) collide, they can annihilate to produce two or more photons of light. There is no fundamental law that prevents matter from being created or destroyed but there is a fundamental law that mass/energy must be conserved. In this case, the masses of the electron and positron become energy in the massless photons. Overall, mass/energy has been conserved but what was originally mass has become energy. The fact that when matter and anti-matter annihilate, the product is usually energy, makes this mixture attractive as a possible super-rocket fuel. But don't hold your breath; anti-matter is incredibly difficult to make or store.
. How does the light emission of Wint-O-Green Lifesavers work? If you bite them, they give off light, but what are the chemicals involved and how does it work? — KA, Davis, CA
This phenomenon is the result of tiny electric sparks that occur when sucrose crystals in the Lifesaver crack as they are exposed to severe stresses. A separation of electric charge occurs between the two sides of the fracture tip and an electric discharge occurs through the air separating those two sides. The light that you see is produced by this electric discharge.
To understand how this charge separation occurs, we must look at how crystals respond to stress. Many crystalline materials are microscopically asymmetric, meaning that their molecules form orderly arrangements that aren't entirely symmetric. To visualize such an arrangement, consider a collection of shoes: an orderly arrangement of left shoes can't be symmetric because a left shoe isn't its own mirror image—you can't built a fully symmetric system out of asymmetric pieces. Like left shoes, sucrose molecules (the molecules in table sugar) are asymmetric so that a crystal of sucrose is also asymmetric.
Whenever you squeeze a crystal, exposing it to stress, its electric charges rearrange somewhat. In a symmetric crystal, this microscopic rearrangement doesn't have any overall consequences. But in an asymmetric crystal such as sucrose, the microscopic rearrangement can produce a large overall rearrangement of electric charges and huge voltages can appear between different parts of the crystal. The most familiar such case is in the spark lighters for gas grills, where a stressed asymmetric crystal creates large sparks. In a Wint-O-Green Lifesaver, the large build-ups of charge cause small sparks that produce the light you see.
. If the Fermi level is the highest energy level used by an electron, how can electrons shift to conduction levels that are at energies above the Fermi level? — PH
The Fermi level is the highest energy level occupied when all the electrons have as little energy as possible. That situation occurs only when all the electrons are paired two to a level and the levels are filled all the way from the lowest energy level up to the Fermi level. At any reasonable temperature and in the presence of light or other energy sources, some of the electrons will have been shifted out of their normal levels and into levels above the Fermi level. The Fermi level doesn't change when these shifts occur—it's defined before the electrons shift.
. How do you calculate the change in water pressure as the diameter of the hose changes? - JH
When water flows through a hose, it has three main forms for its energy: kinetic energy, gravitational potential energy, and an energy associated with its pressure—which I'll call pressure potential energy. Since energy is conserved, the water's energy can't change as it flows through the hose (we'll ignore frictional forces here, although they really are pretty important in a hose). Let's assume that the hose is horizontal, so that the water's gravitational potential energy can't change. When the water enters a narrowing in the hose, the water must speed up to avoid delaying the water behind it. This increase in speed is associated with an increase in kinetic energy. Since the water's energy can't change, the increase in kinetic energy must be accompanied by a decrease in pressure. If the water then enters a widening in the hose, it slows down, its kinetic energy drops, and its pressure rises to conserve energy! If the hose then rises upward, so that the water's gravitational potential energy rises, the water's pressure must drop to conserve energy. In general, one form of energy can become another but the sum of those three forms can't change.
. How does 240-volt electricity work in house wiring? If each "hot" wire in a circuit from the central wiring panel is at 120 volts with respect to neutral/ground, how are devices that use 240 volts wired? — GK, Ottawa, Ontario
Most homes receive power through three wires: two power wires and one neutral wire. Each power wire is at 120 volts AC with respect to the neutral wire, meaning that its electric potential fluctuates up and down with respect to the neutral wire and behaves as though, on average, it were 120 volts away from the potential of the neutral wire. But the fluctuations of the two power wires are opposite one another—when one power wire is at a positive voltage relative to the neutral wire, the other power wire is at a negative voltage relative to the neutral wire. If you compare the two power wires to one another, you'll find that they behave as though, on average, they are 240 volts away from one another. Thus home appliances that need 240 volts are powered by the two power wires, rather than one power wire and one neutral wire.
. What is a kVA? Can you convert watts to kVA? - M
kVA is the product of kilovolts (kV) times amperes (A) and is a measure of power. In fact, if you multiply the voltage in volts delivered to an electric heater by the current in amperes sent through that heater, you will obtain the electric power in watts consumed by the heater. Thus the heater's power consumption in watts is the same as the product of its voltage times its current, or its kVA. However, there are many devices that don't behave like an electric heater. The heater is purely resistive, while many other devices such as motors are both resistive and reactive. Reactive devices don't obey Ohm's law and may not draw their peak currents at times of peak voltage. Therefore, the power in watts consumed by a reactive device isn't the same as the product of its current times its voltage, or its kVA.
. What is the difference between a single-phase electric motor and a three phase motor? Does that make one of them more efficient, better, or longer lasting than the other? — EJ, Houston, TX
To keep the center component or "rotor" of an electric motor spinning, the magnetic poles of the electromagnets surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic poles. With single-phase electric power, producing that rotating magnetic environment isn't easy. Many single-phase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of the other electromagnets. The result is magnetic poles that seem to rotate around the rotor and that start it turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the starting process is clearly rather complicated in a single phase motor.
In a three phase motor, the complicated time structure of the currents flowing through the three power wires makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general, three phase motors start more easily and are somewhat more energy efficient during operation than single phase motors.
. How do remote garage door openers work? — JD, Greenville, SC
The communication from the remote to the opener is done with radio waves. When you push the button on the remote, it produces a brief burst of radio waves at a specific frequency and with a selected pattern of pulses. A radio receiver in the opener is continuously looking for a transmission at that same frequency and with that same pattern of pulses. While other garage door openers may use radio waves of the same frequency, it's extremely unlikely that they will make use of the same pattern of pulses. This pattern of pulses is the security code that prevents unauthorized opening of your garage door. These security codes have grown longer and more sophisticated over the years. Early garage door openers had no security code at all and could be opened by almost any radio transmission at the right frequency. You could drive around neighborhoods with a remote and open garage doors right and left. But now the security codes are complicated enough that opening someone else's garage door is almost impossible.