I was wondering if a pitot tube is a very efficient way to measure airflow and, …

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I was wondering if a pitot tube is a very efficient way to measure airflow and, if so, what would be the conversion formula to cfm? – BN

A pitot tube determines airspeed by measuring the pressure rise that occurs when the airstream is slowed to a stop. Any time moving air encounters a closed chamber head-on, the air stops and it exchanges its kinetic energy—its energy of motion—for pressure potential energy—energy stored in the form of an elevated pressure. By measuring this elevated pressure, you can determine what the air’s kinetic energy was while it was moving and thus how fast it was moving.

Pitot tubes are used to measure airspeed in airplanes. They’re the cigar-shaped objects that project forward from the undersurfaces of airplanes near their noses. I suppose that you could use a pitot tube to measure the speed of air flowing through an air duct, but to determine the volume of air flowing through that duct, you’d need to know the dimensions of the duct. The relationship between pressure in the pitot tube and the airspeed is complicated and so is the relationship between airspeed in a real duct and the volume of air it’s carrying. Overall, this doesn’t look like an easy job.

Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at …

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Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?

The frequency of the microwaves used in most microwave ovens, 2.45 gigahertz or 2,450,000,000 cycles per second, isn’t related to any resonance of the water molecules themselves. While the isolated water molecules in steam or moist air have clear resonances associated with various vibrational and rotational modes of oscillation, these resonances are smeared out in liquid water. The water molecules in liquid water touch one another and their resonances are disturbed in much the same way that the resonances of a bell are disturbed when you touch it.

Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn’t important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don’t interfere with communication systems using nearby frequencies. I believe that there are 2 other frequencies allocated to microwave ovens, but only a few ovens make use of those frequencies.

What are the most important energy-efficient household appliances? How do their …

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What are the most important energy-efficient household appliances? How do their efficiencies compare with those of standard appliances? — LM, Klong Luang, Pathumthani, Thailand

I can think of three important energy-efficient household electric devices: (1) heat pumps, (2) electric discharge lamps (including fluorescent lamps), and (3) microwave ovens.

A heat pump is a device that transfers heat against its natural direction of flow. If you use one to heat your home, the heat pump uses electricity to transfer heat from the colder outside air to the hotter inside air, so that the inside air becomes even hotter and the outside air becomes even colder. The electricity that the heat pump uses also becomes thermal energy inside your home. Since both the electric energy and the thermal energy pumped from the air outside end up inside your home, a heat pump provides more heat than a simple space heater can provide with the same electricity. The energy efficiency of a heat pump decreases as the temperature difference between inside and outside becomes greater, but it typically provides 4 or more times as much heat to your home as a normal electric space heater would provide with the same amount of electricity. Incidentally, when the heat pump is reversed, so that it pumps heat out of your home, it is then an air conditioner.

Electric discharge lamps are between 2 and 5 times as energy efficient as normal incandescent light bulbs. The hot filament of an incandescent lamp delivers only about 10% of its electric power as visible light. In contrast, a fluorescent lamp delivers about 25% of its electric power as visible light and some gas discharge lamps (particularly low-pressure sodium vapor) deliver as much as 50% of their electric powers as visible light.

A microwave oven transfers about 50% of its electric power directly into the water molecules of the food that you are cooking. Cooking occurs quickly and because the cooking chamber doesn’t get hot, there is no power wasted in heating the oven itself or the room surrounding the oven. Depending on how large an object you are cooking, a microwave oven probably uses between 5 and 20 percent of the electricity it would take you to cook the same food in a standard oven.

How do motion detectors work?

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How do motion detectors work? — MK, Port St. Joe, FL

According to Gabriel Lombardi of Torrance, CA, most home motion detectors use infrared light to sense motion. Moving objects change the amount of infrared light striking a detector at the focus of an array of fresnel lenses. He points out that you can see this array on the front of many motion sensors. Such devices are known as passive infrared or PIR detectors. The motion detectors used in automatic door openers, such as those at the supermarket, usually use radio frequency electromagnetic waves to detect motion.

How does the power/frequency of the earth’s magnetic field compare to the magnet…

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How does the power/frequency of the earth’s magnetic field compare to the magnetic fields of electrical appliances? — MC, Independence, KA

Although I haven’t been able to find detailed lists of the magnetic fields near common appliances (such lists do exist), those fields are unlikely to be stronger than the earth’s own magnetic field. That’s because the magnetic fields in most appliances are created by electric currents and you must be quite near a relatively large current before the magnetic field of that current exceeds 0.5 gauss, the strength of the earth’s magnetic field. But while an appliance’s magnetic field is likely to be no greater than that of the earth, the appliance’s magnetic field does change with time. It reverses each time that the alternating current from the power line reverses. In the United States, that’s 120 reversals per second (60 full cycles of reversal, over and back, each second).

What does the heat anticipator do on a furnace thermostat? Does it have anything…

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What does the heat anticipator do on a furnace thermostat? Does it have anything to do with the dwell (temperature rise) of the unit? — BV, Burton, MI

A simple thermostat turns the furnace on when the temperature it senses falls below a certain value and turns the furnace off when the temperature it senses rises above that value. Because it takes time for the furnace to respond to signals from the thermostat, for the heat from the furnace to travel to the thermostat, and for the thermostat to respond to changes in the temperature around it, the furnace tends to stay on for too long after the thermostat turns it on and then to stay off for too long after the thermostat turns it off. The result is an oscillation in temperature: the home or building alternately overheats and then overcools. To reduce this oscillation, a thermostat with a heat anticipator limits the amount of time that the furnace stays on. Since the furnace turns off earlier, the temperature doesn’t overshoot as much on the high side and the furnace turns back on again more quickly once the home or building drifts below the set temperature of the thermostat. Overall, the temperature still oscillates above and below the set temperature, but those oscillations are smaller and faster.

How does a sewing machine work?

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How does a sewing machine work? — RD, APO

A sewing machine uses a spinning shaft to push a needle up and down through fabric. The rod that controls the needle’s height is attached to the spinning shaft away from the shaft’s axis of rotation so that as the shaft spins, the rod and needle move up and down. This motion resembles that of a child on a tricycle: as the front wheel turns, the child’s legs move up and down.

Thread from a spool held above the fabric passes through an eye in the needle’s tip, so that as the needle pierces the fabric, it carries the thread with it. A device beneath the fabric catches hold of this thread and pulls it rapidly around a smaller spool of thread (the bobbin). The thread from above the fabric thus fully encircles the thread from this bobbin and the two threads become permanently locked together. When the needle withdraws from the fabric, some of the thread that it carries remains behind, locked around the thread from the bobbin below. With each stroke of the needle, a new joint is created between the thread from above the fabric and the thread from below the fabric. If there are several pieces of fabric lying on top of one another, these pieces become locked together by the intertwined threads.

How much life is consumed each time you turn on a fluorescent lamp?

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How much life is consumed each time you turn on a fluorescent lamp? — BL, San Jose, CA

The starting process erodes the electrodes of a fluorescent tube through a phenomenon called sputtering. A typical fluorescent tube will last about 50,000 hours if left on continuously but only 20,000 hours if it’s turn on for just 3 hours at a time. From that tidbit, I think its fair to say that a fluorescent tube can only start about 10,000 times. If the tube costs $5, you are spending about 0.005 cents per start. If the electricity to operate that tube costs about 0.2 cents per hour, then turning the tube off for about 1.5 minutes saves the same amount of money in electricity as it costs in tube life when you turn the tube back on. In short, if you turn the lamp off for less than about 1 minute, you’re wasting money. But if you turn it off for more than 10 minutes, you’re saving money. In between, it’s not so clear. There is a myth that turning on a fluorescent lamp consumes a huge amount of electricity so that you shouldn’t turn the lamp off and on. There is simply no basis to that myth.

How does a prism work?

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How does a prism work? — RH, Louisville, KY

When light enters a material such as glass, the light slows down. That’s because the electric charges in the material delay a light wave by interacting with the wave’s electric and magnetic fields. The higher the frequency of the light wave, the more it interacts with the charges in most materials and the more that light wave slows down. Thus high-frequency violet light slows more than low-frequency red light as the two enter a piece of glass.

Because of this slowing effect, light bends when it encounters a glass surface at an angle. The wave has a width and as it encounters the glass surface, one side of the wave reaches the glass before the other side of the wave. Since the side that arrives first also slows first, the whole wave bends so that it travels more directly into the glass. Since violet light slows more than red light, the violet light also bends more than the red light. The two colors thus follow different paths through the glass.

The same bending occurs in reverse when the light leaves the glass. Light speeds up as it leaves glass and again the violet light bends more than the red light. In a prism (or any carefully cut glass, crystal, or plastic), the colors of light bend differently at each surface and follow slightly different paths both in and out of the prism. The light rays then appear separately when they strike a surface outside the prism or when you look at those light rays with your eyes.

How does waterpower work? – MA

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How does waterpower work? – MA

By “waterpower” I assume that you mean hydroelectric power. In that case, water from an elevated source enters a pipe and travels downhill to a generating plant. As the water descends, its gravitational potential energy (the stored energy associated with height and the earth’s gravity) becomes pressure potential energy (the stored energy associated with pressure) and kinetic energy (the energy of motion). By the time the water reaches the generating plant, it has enormous pressure and a modest speed.

This moving, high-pressure water is then sent through a fan-like turbine. As the water moves toward the low pressure beyond the turbine, it does work on the turbine’s rotating blades and its energy is transferred to those blades. The water gives up its energy and the turbine takes away this energy in its rotary motion. The turbine is attached to an electric generator, which uses moving magnets and wire coils to turn the turbine’s rotary energy into electric energy. The electric energy is carried away on wire to be used elsewhere. Overall, the water’s gravitational potential energy has become electric energy.