Study Guide

Electricity and Magnetism In the Real World

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  • Environment

    Circuit Laws and Solar Panels

    Every square meter of the earth's surface receives about 1,000 W of power while the sun shines.12 This is an incredible amount of power and energy—as much energy in a single hour, in fact, as all of humanity uses in almost a full year. Harnessing this free and ubiquitous power, however, is a little more tricky.

    Solar cells are the byproduct of decades of semiconductor technology advancements. Semiconductors are neither conductors nor insulators, but can switch back and forth when triggered. In a solar cell, sunlight is the trigger, causing the cell to spontaneously generate a current when exposed to light. We can treat solar cells as "current sources" the same way a battery in a circuit is a voltage source: while batteries add a potential V to the circuit, solar cells create a current I. Ohm's Law, of course, still holds, and every solar cell has an operating voltage that appears across it.

    But each cell has a very low voltage and doesn't create much current—and so no one cell can power a building or a home. Cells must be connected into solar panels: arrays of cells that build up the total voltage and current to useful levels. The trick is to connect them in such a way as to make both big voltages and big currents.

    Kirchhoff's Current Law states that the current around a loop of wire is constant. So connecting solar cells in series, where each cell is in the same loop as all the other cells, can't possibly change the overall current generated. But the voltage drop across each cell adds to the next one, which adds to the next one, which adds to the next one, until a large overall panel voltage has been built up in little cell-sized steps. Each of these strings of high-voltage, low-current cells can then be connected in parallel to increase the overall current of the panel.

    And just like that, you've turned your little solar cells into a device that creates high currents at a high voltage. The equation P = IV means you now have power overwhelming—and just imagine the amount of vespene gas emissions you can mitigate with solar panels.

  • Engineering

    Maglev Trains

    If you've ever tried to push two refrigerator magnets together north-to-north or south-to-south (or faced off against Magneto), you know that magnetic fields can create considerable amounts of force. When mankind isn't harnessing that force to do very scientifically important things like levitating frogs, it can be harnessed to help overcome one of nature's most annoying forces: friction.

    The two major energy losses in large trains come from friction—some from the train pushing through air (which isn't going anywhere anytime soon), and some from the steel wheels rolling on the steel track. If we were able to make a floating train, all of that friction from the wheels would be eliminated.

    Well, we could. And we did.

    (Sorry, should have mentioned you may have wanted to turn down the volume on your headphones there. Moving at almost half the speed of sound tends to be noisy.)

    These maglev trains—short for magnetic levitation—use the same forces that tack your little sister's art to the refrigerator to hurl passengers along special tracks, reaching speeds that the Little Engine That Could just can't. No matter how hard he believes in himself.

    A typical maglev system uses two sets of magnets: one to levitate the train, and another to propel the floating train down the track.13 Rather than permanent magnets (think fridge), the train's magnets are electromagnets, coils of wire that create strong magnetic fields when current flows through them—Ampère's Law in action. This field is attracted to iron guide rails, and the electromagnets are strong enough to create a force that completely offsets the force of gravity, levitating the train anywhere from a few millimeters to several centimeters above the track. Two points science.

    But a levitating locomotive, while cool, isn't particularly useful if it isn't, you know, locomotory. In order to move the train, a second set of electromagnets are embedded in the side walls of the track. These alternate orientation, so that the fields seen by the train follow a north-south-north-south pattern down the track. The train itself has two electromagnets in its own side walls, and the north end of the train is pulled towards the south end of the wall magnets at the same time the south end of the train is pushed away by the previous south wall magnet.

    This push-pull action occurs all the way along the track, each small bit of magnet accelerating the train until it reaches the high speeds maglev trains are famous for.

  • History

    The War of the Currents

    In this chapter, we focused mainly on what is called direct current, or DC, electricity. This is the kind of electricity that comes out of a battery—when you turn a 5 V battery on, you get 5 V all the time until the battery runs out of juice. This is not, however, what comes out of the wall. The electrical grid uses alternating current (or AC) electricity. Additionaly, the voltage that comes out of the wall is a sinusoid that varies (in the U.S., at least) from +120 V to -120 V at a frequency of sixty times per second.

    Why it's AC (and not DC) power that comes out of the wall was a huge debate when the electrical grid was being created. The groundwork for the U.S.'s electrical infrastructure was laid in the late nineteenth century, the "Age of Great Inventions," when railroad tycoons and oil barons and steel magnates were transforming post-Civil War America into the modern country that exists today. An integral part of that transformation was the delivery of electricity to every home in the nation.

    The AC and DC camps fought long and hard. In one corner, weighing in at one-hundred-eighty-three pounds, we had the Perspiring Power Pioneer, the Man of a Thousand Patents, the Wizard of Menlo Park—Mr. Thomas. Alva. EDISOOOOOOON!

    And in the opposite corner, the challenger, weighing in at one-hundred-forty-two pounds: the Westinghouse Powerhouse, the Scintillating Serbian Scientist, the Eclectic Electric Eccentric—Mr. Nikola. TESLAAAAAAAA!

    The power pugilists had very different visions for how electricity should flow throughout the country. Edison championed DC power, which he had popularized after inventing the light bulb. DC electricity is difficult to transmit over long distances, however, and so Edison hired a young engineer named Nikola Tesla to come up with a better solution. Tesla (who was a cool enough guy that, not only did he have a car company named after him, but he was portrayed by the one and only David Bowie in a major Hollywood film) came back to Edison with the idea of AC electricity, but Edison believed the idea to be impossible to implement.14 An industrialist named George Westinghouse came to Tesla's rescue, licensing his AC power patents and diving into the utilities market. And so, with a shock heard round the world, the opening salvos of the War of the Currents were fired.

    While Edison was a great inventor, the AC power turning on all the lights in your house right now should tip you off that Tesla was the one who was right. AC electricity is much easier to "step up" to higher voltages using a device called a transformer. A transformer generally consists of two coils of wire, one with far more coils than the other. A constantly changing current (like AC electricity) is sent through the small coil, which creates a magnetic field according to Faraday's Law. This magnetic field creates a voltage in the larger coil, which is proportional to the number of loops in the coil—creating a much larger voltage than originally present. Because of conservation of energy (pesky, we know), the amount of power must remain constant, so a stepped-up voltage will be accompanied by much lower current. P = IV is always the same.

    Long electricity lines are plagued by the build up of a large resistance over the length of the wire, which drains off power like a mynock. The power loss across a wire due to its resistance is P = I2R—the more current, the more power is lost. Because of this, high-voltage, low-current AC power is much more efficient than low-voltage, high-current DC power when sent along long wires, and that means it's much cheaper.

    AC power can then be stepped-down using a transformer with the coils reversed and sent into homes to power lights and toasters and washing machines and whatever else you plug into the wall. Transformers won't work for DC power since the crux of a transformer is Faraday's Law. Faraday's Law relies on changing magnetic fields to create voltages. The big benefit of DC power is that it never, ever changes, making electromagnetic induction impossible.

    Edison may have realized this, but resolutely refused to give up. He launched a massive smear campaign against Westinghouse and Tesla, electrocuting dogs and cats (and, in at least one instance, an elephant) with AC power to prove how dangerous it was. He went as far as ensuring the first electric chair used for capital punishment was furnished with a Westinghouse AC dynamo.

    Yet it was Westinghouse who got the contract to power the 1893 Chicago World's Fair, arguably ushering in the era of electricity.15 There are advantages to DC power—DC electricity needs less wiring to transmit, for example, and those boxes on the power cords of your computer or cell phone chargers are AC-to-DC converters, since those devices all run on DC power. However, the low cost of transmitting AC power from point-of-generation to point-of-use eventually won the war.

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