. How does an operational amplifier work? — BR
An operational amplifier is an extremely high gain differential voltage amplifier—a device that compares the voltages of two inputs and produces an output voltage that's many times the difference between their voltages. How the operational amplifier performs this subtraction and multiplication process depends on the type of operational amplifier, but in most cases two input voltages control how current is shared between two paths of a parallel circuit. Even a tiny difference between the input voltages produces a large current difference in the two paths—the path that's controlled by the higher voltage input carries a much larger current than the other path. The imbalance in currents between the two paths produces significant voltage differences in their components and these voltage differences are again compared in a second stage of differential voltage amplification. Eventually the differences in currents and voltage become quite large and a final amplifier stage is used to produce either a large positive output voltage or a large negative output voltage, depending on which input has the higher voltage. In a typical application, feedback is used to keep the two input voltages very close to one another, so that the output voltage actually falls in between its two extremes. At that operating point, the operational amplifier is exquisitely sensitive to even the tiniest changes in its input voltages and makes a wonderful amplifier for small electric signals.
. How does a siren work? — MM, Waterloo, Iowa
A siren uses a perforated disk or drum to alternately block and unblock a stream of air. The classic siren has a spinning disk with a pattern of holes around its periphery. This disk is spun in front of a jet of air, producing pressure pulses that we hear as sound. A more modern siren has a spinning centrifugal fan that propels air radially outward through a pattern of holes in a drum around the fan. This centrifugal siren is much louder than the disc siren because the centrifugal system pushes large pulses of air through many openings at once, whereas the disc siren only has one pulsed source of air.
. What is torque? — JPT, Calgary, Alberta
A torque is a physicist's word for a twist or a spin. When you twist the top off a jar, you are exerting a torque on the jar and causing it to undergo an angular acceleration—it begins to rotate faster and faster in the direction of your torque. Similarly, when you spin a toy top, you do this by exerting a torque on the top and it again undergoes an angular acceleration.
. How do long range metal detectors work? — AS
In general, metal detectors find metal objects by looking for their electromagnetic responses. For example, you can tell when an iron or steel object is nearby by waving a magnet around. If you feel something attracting the magnet, you can be pretty sure that there is a piece of iron or steel nearby. Similarly, if you wave a strong magnet rapidly across an aluminum or copper surface, you'll feel a drag effect as the moving magnet causes electric currents to flow in the metal surface—electric currents are themselves magnetic.
Of course, a real metal detector is much more sensitive than your hands are, but it's using similar principles to detect nearby metal. Most often, a metal detector uses a coil of wire with an alternating current in it to create a rapidly changing magnetic field around the coil. If that changing magnetic field enters a piece of nearby metal, the metal responds. If the metal is ferromagnetic—meaning that it has intrinsic magnetic order like iron or steel—it will respond strongly with its own magnetic field. If the metal is non-ferromagnetic—meaning that it doesn't have the appropriate intrinsic magnetic order—it will respond more weakly with magnetic fields that are caused by electric currents that begin to flow through it.
In a short range metal detector, the detector looks for the direct interaction of its magnetic field and a nearby piece of metal. That nearby metal changes the characteristics of the detector's wire coil in a way that's relatively easy to detect. But in a longer-range metal detector, the electromagnetic coil must actually radiate an electromagnetic wave and then look for the reflection of this electromagnetic wave from a more distant piece of metal. That's because the magnetic field of the coil doesn't extend outward forever—it dies away a few diameters of the coil away from the coil itself. For the metal detector to look for metal farther away, it needs help carrying the magnetic field through space. By combining an electric field with the magnetic field, the long-range metal detector creates an electromagnetic wave—a radio wave—that travels independently through space. Electromagnetic waves reflect from many things, particularly objects that conduct electricity. So the long-range metal detector launches an electromagnetic wave and then looks for the reflection of that wave. This wave reflection technique is the basis for sonar (sound waves) and radar (radio waves), and it can be used to find metals deep in the ground. Unfortunately, the ground itself conducts electricity to some extent, so it becomes harder and harder to distinguish the reflections from metal from the reflections from other things in the ground.
. Why is it that when you put two electric lamps into a circuit in parallel with one another, the current through the circuit increases, while when you put those two lamps in series with one another, the current through the circuit decreases?
When the two lamps are in parallel with one another, they share the current passing through the rest of the circuit. Current arriving at the two lamps can pass through either lamp before continuing its trip around the circuit. The two lamps operate independently and each one draws the current that it normally does when it experiences the voltage drop provided by the rest of the circuit. With both lamps providing a path for current, the current through the rest of the circuit is the sum of the currents through the two lamps.
But when the two lamps are in series with one another, each lamp carries the entire current passing through the circuit. Current arriving at the two lamps must pass first through one lamp and then through the other lamp before continuing its trip around the circuit. There is no need to add the currents passing through the lamps because it is the same current in each lamp. Moreover, the voltage drop provided by the rest of the circuit is being shared by the two lamps so that each lamp experiences roughly half the overall voltage drop. Since lamps draw less current as the voltage drop they experience decreases, these lamps draw less current when they must share the voltage drop. Thus the current passing through the circuit is much less when the two lamps are inserted into the circuit in series than in parallel.
. What are some general uses of X-rays other than medical? — SD, Raleigh, NC
There are so many non-medical uses for X-rays that I'll limit myself to two: industrial imaging and X-ray crystallography. Industrial X-ray imaging is used frequently in manufacturing to inspect finished materials. An important example of this imaging is in weld inspection. After a sheet of steel has been rolled into a pipe and the seam of that pipe has been welded closed, it's often important to inspect the weld to be sure that it's solid and leak free. Sometimes a weld that looks perfect to the eye has hollow spots or other flaws that can only be seen by looking through
the material of the weld. This inspection is done with high energy X-rays—X-rays that are able to penetrate a thick steel plate to look for bubbles or unwanted inclusions.
X-ray crystallography is an important tool for materials science and molecular biology. Just as the colored interference patterns that appear on a soap bubble when sunlight reflects from that bubble tell you something about the structure of that soap bubble, so the X-rays that reflect from a crystal tell you something about the structure of that crystal. X-rays experience interference after they reflect from a crystal and the interference patterns can tell you where individual atoms are located within a crystal or within the molecules from which the crystal is made. Materials scientists use this information to understand the crystals they have produced while molecular biologists use it to understand the molecular structures of complicated biological molecules.
. What is the general theory of operation of a hydraulic turbine? — GS, Fort Worth, Texas
A hydraulic turbine is essentially a fan run backward—while a fan adds energy to a passing fluid, a turbine extracts energy from a passing fluid. You can think of the fluid's effects on the turbine blades in two different but equivalent ways. In one view, the fluid is deflected by its encounter with the canted turbine blades and as the blades push the fluid in one direction, the fluid pushes the blades in the opposite direction. This reaction force that the fluid exerts on the blades causes those blades to spin and does work on them—energy is transferred from the fluid to the blades.
In the other view, the blades "fly" through the fluid like the wings of an airplane. The fluid flow around each blade is such that the pressure is higher on one side of the blade than the other and the blade experiences a net force toward the lower pressure side. The blades move in the direction of this force, so the passing fluid does work on them—energy is transferred from the fluid to the blades.
These two views are completely equivalent. The fluid leaves the turbine blades traveling more slowly or at lower pressure, and it acquires a rotation in the direction opposite the turbine's rotation.
. How might an ion engine work? — DAA, San Diego, CA
One possible ion engine uses mercury as a propellant. The mercury starts as a liquid in a small tank, but its atoms slowly evaporate to form a low-density gas. An electric discharge through this gas, such as occurs inside a fluorescent lamp, knocks electrons off some of the mercury atoms. When a mercury atom loses an electron, it becomes a positively charged mercury ion and can be accelerated from the discharge by electric fields. In the ion propulsion engine, an electric field extracts and accelerates the mercury ions toward a hole in the side of a spaceship. The mercury ions are ejected into space at enormous speeds. As they accelerate, the mercury ions exert reaction forces on the engine and these forces are what propel the spaceship forward. Overall, the mercury ions accelerate in one direction while the spaceship accelerates in the other direction. To keep the spaceship electrically neutral, the engine also ejects electrons into space. However, mercury ions provide most of the engine's thrust.
. I understand that for a steam engine to produce useful work, you need a difference in temperatures. My question is whether the difference in temperatures between cold glacier ice and the warmer air could be used to drive a steam engine and generate electricity. — LNH & AJH, Juneau, Alaska
As you clearly recognize, any heat engine—a machine that converts thermal energy into work—can only do its job while heat is flowing from a hotter object to a colder object. That limitation is imposed by the second law of thermodynamics—a statistical law that observes that the disorder of an isolated system can never decrease. A heat engine's theoretical efficiency at turning thermal energy into work improves as the temperature difference between its hotter and colder objects increases. Since the air temperature is hotter than the glacier temperature, there is the possibility to convert some of the air's thermal energy into work as heat flows from the air to the glacier. In short, what you suggest could be done.
Unfortunately, most practical heat engines work best when the hotter object is really hot. For example, a steam engine works best when the hotter object is hot enough to produce very high temperature, high pressure steam. To operate a steam engine with outside air as the hotter object and cold ice as the colder object, the steam engine would have to operate at very low pressure. In fact, it would operate well below atmospheric pressure in a carefully sealed environment. Steam might not even be the best choice for a working fluid—you might do better with a refrigerant such as the various Freon replacements. In effect, your heat engine would be an air conditioner run backward—providing electric power rather than consuming it. Although this could be done, it would probably not be cost effective. The heat exchangers needed to obtain heat from the air and to deliver most of that heat to the glacier, as well as all the machinery of the heat engine itself, would probably make the electricity you generated too expensive. Just because something can be done doesn't mean that it's worth doing. Until other sources of energy become more expensive, this one won't pay for itself.
. Why and how does water conduct electricity? — SM, Murrysville, PA
Water molecules are electrically neutral and do not accelerate in response to electric fields. For that reason, a liquid consisting only of water molecules wouldn't conduct electricity. However, real water contains things other than water molecules. Even in completely pure water, about 1 in every 10,000,000 water molecules is found to have dissociated into a hydrogen ion (H+) and a hydroxide ion (OH-). These electrically charged ions do accelerate in response to electric fields and they make it possible for even the purest water to conduct electricity weakly. Adding impurities, particularly ionic impurities such as salts, makes water an even better conductor of electricity.