Author: Lou Bloomfield

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.

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 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 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).

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.

What are watts and amps? – NS

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What are watts and amps? – NS

The watt is the standard unit of power—that is, it’s the way in which we measure how much energy is being transferred to or from sometime each second. 1 watt is equivalent to 1 joule of energy per second. A 100 watt light bulb consumes 100 joules of electric energy each second. Anytime energy moves from one place to another, you can determine how much power is flowing. For example, the food energy in a jelly donut is about 1 million joules, so if you eat 1 jelly donut in 100 seconds, you receive 10,000 watts of power. Since your body only consumes about 100 watts of power while you are resting, it will take you 10,000 seconds to use up all that food energy.

The amp (or ampere) is the standard unit of electric current—that is, its the way in which we measure how many electric charges flow past a certain point each second. 1 amp is equivalent to 1 coulomb of electric charge per second. Since 1 coulomb of electric charge is the charge on 6,240,000,000,000,000,000 protons, even a current of only 1 amp means that a great many electric charges are passing each second. The current passing through a 100-watt light bulb is roughly 1 amp on average, while the current used in starting a car is about 100 amps.

How does an internal voltage regulator type auto alternator work and are they an…

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How does an internal voltage regulator type auto alternator work and are they any better than an external regulator type? – H

An alternator is a device that uses rotary motion to generate electricity. As the car engine turns, it spins a magnet (the rotor) in the alternator and this spinning magnet induces electric currents in a set of stationary wire coils (the stator). The alternator’s ability to generate electric currents by spinning a magnet past stationary wires is an example of electromagnetic induction. Induction is a general phenomenon in which a moving or changing magnetic field creates an electric field, which in turn pushes electric charges through a conducting material. Overall, some of the engine’s mechanical energy is converted into electric energy.

The amount of energy given to each electric charge that flows through the wires in the stator depends on the speed with which the magnet turns and the strength of that magnet. Whether it’s internal or external, the voltage regulator monitors this energy per charge—also known as the voltage—to make sure that it’s correct. If not, it adjusts the strength of the alternator’s magnet. It can do this because the alternator’s magnet is actually an electromagnet and its strength depends on how much current is flowing through its wire coils. The voltage regulator carefully adjusts the current flowing through the electromagnet in order to obtain the proper output voltage from the alternator. Actually, the alternator itself produces alternating current, so a set of solid-state diodes converts this alternating current into direct current. A car’s electric system, particularly its battery, operates on direct current. Since the alternator’s operation is the same whether the voltage regulator is inside it or external to it, neither version should be better than the other.

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.

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 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.