. When you make a telephone call, you send an analog signal from your phone to a central station. Is this direct current or alternating current? How do you and your neighbors share the line?
When you are talking to a friend over the telephone, the telephone company uses a special power supply to send a constant (direct current) through your telephones. Your telephone and your friend's telephone share this current so that if your telephone draws more, your friend's telephone receives less. When you talk into the microphone of your telephone, the current your telephone draws fluctuates up and down with the air pressure fluctuations at the microphone. As a result, the current through your friend's telephone fluctuates down and up, the reverse of the current fluctuations in your telephone. A speaker in your friend's telephone uses these current fluctuations to recreate the sound of your voice. When there are other extensions active in your home, they are all sharing this current so that talking into one telephone causes sound to be reproduced in all of the other telephones, both in your home and in your friend's home. While modern electronics have changed the telephone system extensively, so that this direct current sharing isn't quite the reality it was 30 years ago, all of the complicated electronic circuitry works to simulate this same relationship.
. What is the difference between a tube amplifier and a solid-state amplifier? Does the human ear prefer one over the other?
The only difference between a well-designed tube amplifier and a well-designed solid-state amplifier is the device doing the amplification. In fact, a vacuum tube and an metal-oxide-semiconductor field-effect-transistor or MOSFET are extremely similar in behavior, so that amplifiers built with the two devices can be extremely similar. If these amplifying devices are used properly in a good amplifier, that amplifier should only boost the power of its input signal and shouldn't add anything that wasn't present in the input signal. As a result, you shouldn't be able to tell whether the audio amplifier you are listening to is based on tubes or on solid-state components.
. Please explain pectin and why sugar and acid are needed when making jelly.
The molecules of pectin contain enormous chains of atoms, often hundreds or even thousands of atoms long.. Such chains are also found in cellulose and starch, and are used by plants to give them strength and structure. These chain-like molecules are naturally occurring polymers or plastics. The giant molecules in pectin are based on small molecular units of D-galacturonic acid that have joined together like strings of paper dolls. The presence of acid groups on the pectin molecules help to make pectins very water soluble and also sensitive to the acid-base balance of their environment. I am not an expert in the exact structure and chemistry of pectin, or in the proper pH needed for jellymaking, so I can't give you an exact explanation for how to control the jelling process with acids. But the jell forms because these giant molecules spread out in the viscous solution of sugar and fruit juice, and form a tangled network of filaments that span the entire container. At high temperatures, there is enough mobility in the molecular chains to allow the mixture to flow, but at room temperature, the tangle of molecular filaments prevents flow. In the language of polymer or plastic science, the mixture goes from a liquid flow regime at high temperature to an elastic plateau regime at low temperature. When you deform cold jelly, you are pulling the filaments tight but they can't disentangle themselves enough to allow the jelly to actually flow. When you deform the cold jelly too far, the filaments begin to break and the jelly tears into fragments. However, when you warm the jelly, thermal energy allows the filaments to move past one another and the jelly begins to flow like a thick (or viscous) liquid.
. What effects, if any, does storage temperature have on the height of a tennis ball's bounce?
I suspect that cool storage will prolong the life of a tennis ball in an opened can. That's because the ball's bounciness depends on its retaining air inside its rubber shell. As the ball loses air by diffusion through the rubber, it loses its ability to bounce high. Diffusion is a thermally activated process in which the individual air molecules move between the rubber molecules and migrate through the material. At lower temperatures, the air molecules will move much more slowly through the rubber and the pressure inside the ball will stay high for a longer time.
. What is the Reaumur Scale for temperature and how does it compare to degrees F, C, and K?
The Reaumur Scale was created in 1730 by French scientist Rene-Antoine Ferchault de Reaumur, who set 0 R as the freezing point of water and 80 R as the boiling point of water. Though in common use for a time, the Reaumur Scale had more or less disappeared by the end of the eighteenth century. Each degree R is equal to 5/4 of a degree C, so T(C)=T(R)*5/4. Similarly, T(F)=T(R)*9/4+32 and T(K)=T(R)*5/4+273.15.
. What is the efficiency of a 60-watt bulb to convert electricity to light?
Since only about 80% of the heat a 60-watt bulb releases is thermal radiation and only about 12% of that thermal radiation is visible light, the bulb emits about 6 watts of visible light. A halogen bulb is a little more efficient than this and a long-life bulb is a little less efficient than this.
. How do a diode and a transistor work?
A diode is normal built by touching two different pieces of semiconductor together to form what is called a "p-n junction." Semiconductors are materials that are in between good conductors and good insulators. A pure semiconductor is a very poor conductor of electricity. With careful chemical processing, a semiconductor can be made into n-type semiconductor—a semiconductor that contains a small number of mobile electrons that permit it to carry electric current. With different processing, a semiconductor can also be made into p-type semiconductor—a semiconductor that contains a small number of mobile holes for electrons that permit it to carry electric current. It may seem strange that a hole for an electron can allow electricity to flow, but imagine a highway packed with cars (electrons) bumper to bumper. If there are a couple of empty places (holes) in the bumper-to-bumper traffic, then cars (electrons) can rearrange enough that the traffic can flow. Both mobile electrons and mobile holes allow these two chemically treated semiconductors to carry current.
When an n-type semiconductor touches a p-type semiconductor, a diode is formed. The mobile electrons at the edge of the n-type semiconductor flow over the boundary (a p-n junction) and fill the mobile holes at the edge of the p-type semiconductor. This rearrangement creates a depletion region—a region near the p-n junction in which there are neither mobile electrons nor mobile holes. This depletion region normally won't carry electricity at all. But if you push electrons onto the n-type semiconductor, they will flow toward the p-n junction and replenish the missing mobile electrons. As these mobile electrons approach the p-n junction, they will repel the electrons that are filling the mobile holes on the p-type side of the junction and reopen the mobile holes. Electrons will begin to cross the p-n junction and current will flow through the diode. However, if you push electrons onto the p-type semiconductor, they will fill even more of the mobile holes there and the depletion region near the p-n junction will grow larger and more uncrossable. No current will flow through the diode. Thus a diode (a p-n junction) only carries current in one direction—electrons can only flow from the n-type semiconductor side to the p-type semiconductor side.
There are many types of transistors, so I will only describe an n-channel Metal-Oxide-Semiconductor Field Effect Transistor, or n-channel MOSFET. In this device, three layers of semiconductors are sandwiched together: an n-type piece (the source), a long, thin p-type piece (the channel), and another n-type piece (the drain). Two p-n junctions form between these three components and, since the junctions are arranged in opposite directions, they completely block current flow from the source through the channel to the drain. But a metal surface (the gate) that's separated from the channel by an extremely thin layer of oxide insulator can control the number of electrons on the channel material. If you put even a tiny bit of positive charge on the gate, it will attract electrons onto the channel and turn it from p-type semiconductor to n-type semiconductor. When that happens, both p-n junctions vanish and current can flow from the source to the drain. The MOSFET goes from being an insulating device when there is no charge on the gate to a conductor when there is charge on the gate! This property allows MOSFETs to amplify signals and control the movements of electric charge, which is why MOSFETs are so useful in electronic devices such as stereos, televisions, and computers.
. How is powder coating done?
Powder coating is done by combining the components of the coating (the binder—a polymer having giant chain-like molecules, the pigments, and the additives) to form a uniform solid, which is then pulverized to a dry powder and sprayed onto the surface to be coated. This coating is then baked to form a continuous film. There are two main classes of powder coatings: thermosetting and thermoplastic coatings. In a thermosetting film, crosslinking occurs between the molecules in the powder during baking. This crosslinking turns the baked film into a single giant molecule that can't melt or flow. In a thermoplastic film, thermal energy makes the binder molecules mobile enough to become entangled so that a continuous film forms and this film hardens upon cooling. While a thermoplastic film can still melt or flow, it can do that only at elevated temperatures. The powders are often given electric charges during spraying so that electrostatic forces will hold them in place until they're baked on.
. How are tessellations used in roofing, tiles, and quilts?
Tessellation is the covering of a surface without gaps or overlaps using one or a small number of basic shapes. It's a natural activity for roofers, tilers, and quilters, since those activities involve forming complete surfaces with a limited number of shapes. Since there are an infinite number of possible tessellations, people are always trying to create interesting new ones. You can find these in a tile catalog or a quilting guide. Tessellations appear in physics in the context of crystal structure, where surfaces and volumes must be filled completely with a few basic molecular arrangements. Quasicrystalline materials—materials with orientational order but no longer-range order—are a particularly interesting example of tessellation in physics.
. How does an electric eel produce an electric charge? I know that it can produce up to 600 volts, but what does 600 volts mean without knowing the amount of current?
The eel produces this voltage by rearranging ions in specialized muscle cells called electroplaques. While I'm not an expert in this, I suppose that they use energy derived from food to pump ions through the cell membranes of these electroplaques in order to create charge imbalances between the two surfaces of those cells. By stacking hundreds or thousands of electroplaques in series, they succeed in separating positive and negative charges to great distances on their bodies and thus produce voltage drops in excess of 600 V.
You're correct that current is an important issue here, since even household static electricity can separate enough positive charge from negative charge to reach thousands of volts. However, static electricity can reach very high voltages because there is no current flow to deplete the separated charge. In the case of an electric eel in water, the water conducts current well enough that the eel must continue to separate charge to maintain the 600-volt potential difference between its ends. I'm not sure how much current flows through the fresh water in this situation, but I would guess that it's at least 1 ampere and possibly more. That means that the eel is moving a considerable amount of charge each second and using in excess of 600 watts of power. If the eel were a salt-water fish, it wouldn't be able to reach a 600-volt potential difference at all because salt water conducts current far to well and an enormous current would flow in that case.