(7)   Electrical Safety     

   

Heat from Electricity

If you connect a wire with resistance R to a voltage V, a current I = V/R will flow, and from what was stated above, electric energy will be consumed at the rate

W = I V = I2R watts             (=joules/sec)

Engineering students memorize the formula by the rhyme:

            "Twinkle, twinkle little star
              Power equals I-squared R"

    However, energy in nature is conserved--remove it in one form and it appears in another. In this case, the energy turns into heat. In an incandescent lightbulb, most of the resistance is in a thin coiled wire ("filament") made of the metal tungsten, which gets so hot that it radiates visible light

        (Tungsten is used because it can stand high temperature without melting. Even so it gradually evaporates, and the evaporated metal is deposited on the glass bulb, causing it to blacken near the filament. That evaporation makes the wire thinner and weaker, until it breaks, which sets a limit to the lifetime of the bulb. "Halogen lamps" have quartz glass envelopes and contain a chemically active gas, which combines with the deposited tungsten and ultimately returns most of it to the filament, which then lasts longer.)

    The heating wires in an electric toaster, a room heater or an electric oven operate in a similar way, but only get red-hot.

    Fluorescent lights [Note spelling!] are filled with dilute gas, whose atoms get accelerated by electricity, collide and emit light. That light emission comes from individual atoms and is more efficient than the emission caused by heat. (The cited web page describes old-style fluorescent lights; newer compact kinds contain more complex circuitry.) Light emitting diodes use semi-conductors and are even more efficient. Microwave ovens first convert the electric energy to short-wave radio waves, which are then absorbed by the food inside, Never run such an oven without something inside to absorb the energy, even if it is just a cup of water: otherwise the energy is absorbed by the oven itself, which may damage it. Electric Motors generate magnetic forces which deliver the energy to a rotating shaft

    To remember:


    In electric filament ("incandescent") lights, toasters and heaters, the energy of the current is converted to heat, which is then converted to light--mostly infra-red light which heats but cannot be seen, though lightbulbs are hot enough to also provide visible lighting. Fluorescent lights, light emitting diodes and microwave ovens do not rely on simple heating, and deliver their energy by a more efficient process.

Electric Safety

    The heat which electricity produces can be the source of danger. Two wires usually connect a house to the power station: in one electricity may be said to flow in, in the other out. Usually, the connection between them in the house also includes some load--e.g. a resistance or motor--which limits the current they draw.

    If however the two happen to touch directly, in parts not covered by insulation, a current will flow directly between them. Because the resistance of the direct path (a "short circuit" or simply "a short") is very small, that current may in principle be huge. Without extra protection (discussed in the next section), its limits would be set mainly by the small resistance of the wires themselves! That can be dangerous: the wires heat up and may melt (somewhere inside the wall, where they are hard to repair), or worse, their heat may start a fire. As newspaper stories confirm, many home fires are started by electricity.

    Two barriers prevent such mishaps: Fuses or circuit breakers, and electrical codes.

    To remember:


    Heat generated by electricity can be a fire hazard.

Electric Codes and the Grounding of Wires

Electrical codes exist in all US states (and in countries all over the world), specifying standards which electric wiring must satisfy. In new homes, new plants and renovated construction, an official inspector examines the wiring (for a fee) to make sure it conforms to all rules of the code. If so, a license is issued, without which no insurance company will insure the construction against fire. The regulations are detailed, and professional electricians must be familiar with them. In many localities homeowners may work on the wiring of their own homes, but they too must follow the code.

    All wires that carry current must have an insulation sufficient to hold back their voltage--you may not use telephone wire to carry 110 volts house current. In addition, a third bare "ground wire" (un-insulated) must follow all electrical lines, connected to all outlets and also to water pipes, and in one place, to the white "return wire." "Grounding the system" in this fashion protects users from leakages due to faulty insulation, for the following reason.

    Strictly speaking, the flow in a water pipe is not driven by pressure, but the pressure difference between its ends. The air we breathe has a pressure of one atmosphere, but that pressure alone doesn't make it move anywhere: to move air requires a difference in air pressure between two points. Similarly, a voltage V itself will not move electric current through a wire: you need a high voltage at one end, and a low voltage at the other. Only voltage differences can drive a current.

    The "value of the voltage" at any point in the circuit only becomes meaningful after we decide that some reference point in the circuit has "voltage zero" (and for purpose of calculation, any point may be chosen). Compare this with elevations on a map: only after choosing a reference point where the elevation is zero (usually, at sea level) can the values of other elevations be assigned values. If a different reference point were chosen (say, the top of the US Capitol dome in Washington), all those numbers would change.

    The ground is a weak conductor, and is usually at a single voltage--and we, who walk on it, also have that voltage. The electric circuit has two wires--a black "hot" wire in which (by common convention, with alternating currents) the current is said to flow into the house, and a white "return" wire by which it is said to flow out again.

    (Actually, AC sloshes back and forth, and each wire is positive half the time, negative half the time. But only the black line is assumed to have a direct connection to the power station.)

    If neither of the wires is connected to the ground, then the voltage of either wire, relatively to the ground, is not defined. If our finger touches one of the wires, nothing will happen: that point will be at the same voltage of the ground, but in the absence of any voltage difference, no current flows through the finger.

    However, suppose current leaks somewhere--because of imperfect insulation or because of water that leaked in. The leak has already established which part of the circuit is at the ground voltage--and if the wire we touch does not belong to that part, a current will flow from it through the finger to the ground. In some cases the leak is small and its current is just annoying--you touch a metal appliance and your fingers tingle. But with a big leak it can be downright dangerous.

    To prevent this the circuit is grounded--the white wire is connected (at one selected spot) to the bare copper wire, and that bare wire is connected to the "ground"--to water pipes or to a metal stake driven deep into the ground. In the image here, the grounding wire is covered with blue sheath--in one section separated (by weathering) from the wire itself--leading to a buried stake in a clump of poison ivy. The grounding wire is connected to all exposed parts of the circuit--to the metal boxes enclosing switches and all wire connections (a usual code requirement), as well as to metal housings of machinery and appliances. We then know they are safe to touch: only the black "hot" wire can give shock.

    The third prong on electric plugs of appliances connects its grounding to that of the outlet. Note also that power prongs usually differ from each other ("are polarized"): the one which is slightly wider connects to the "hot" side. Only appliances completely encased in an insulator may dispense with these rules.

    To remember:


    All wiring in the home must meet the standards of the national electrical code, protecting users from fire and electrical shock. For instance, the code prescribes insulation standards and requires (usually) that all connections occur inside metal boxes. One important precaution is the "grounding" of all circuits and their metal enclosures, connecting them to the ground (e.g. water pipes) and also (at one place in the house) to the white ("return") wiring. That assures that people handling appliances are safe from shock as long as they do not touch the "hot" wire.


 
Next Stop: E8.   Electric Power

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: 21 September 2009