A/C
When a conductor is moved back and forth in a magnetic field, the flow of
current in the conductor will change direction as often as the physical
motion of the conductor changes direction. Several devices generating
electricity operate on this principle, producing an oscillating form of
current called alternating current. Alternating current has several valuable
characteristics, as compared to direct current, and is generally used as a
source of electric power, both for industrial installations and in the home.
The most important practical characteristic of alternating current is that
the voltage or the current may be changed to almost any value desired by
means of a simple electromagnetic device called a transformer. When an
alternating current passes through a coil of wire, the magnetic field about
the coil expands and collapses and then expands in a field of opposite
POLARITY and again collapses. If another conductor or coil of wire is placed
in the magnetic field of the first coil, but not in direct electric
connection with it, the movement of the magnetic field induces an
alternating current in the second coil. If the second coil has a larger
number of turns than the first, the voltage induced in the second coil will
be larger than the voltage in the first, because the field is acting on a
greater number of individual conductors. Conversely, if the number of turns
in the second coil is smaller, the secondary, or induced, voltage will be
smaller than the primary voltage.
The action of a transformer makes possible the economical transmission of
electric power over long distances. If 200,000 watts of power is supplied to
a power line, it may be equally well supplied by a potential of 200,000 V
and a current of 1 amp or by a potential of 2000 V and a current of 100 amp,
because power is equal to the product of voltage and current. The power lost
in the line through heating is equal to the square of the current times the
resistance. Thus, if the resistance of the line is 10 ohms, the loss on the
200,000 V line will be 10 watts, whereas the loss on the 2000 V line will be
100,000 watts, or half the available power. See Electric Power Systems.
The magnetic field about a coil in an AC circuit is constantly changing, and
the coil constantly impedes the flow of current in the circuit, because of
the quality of the inductance mentioned above. The relationship between the
voltage impressed on an ideal coil (that is, a coil having no resistance)
and the current flowing in the ideal coil is such that the current is at a
zero value when the voltage is at a maximum, and the current is at a maximum
when the voltage is at zero. Furthermore, the changing magnetic field
induces a potential difference in the coil that is equal in magnitude and
opposite in direction to the impressed potential difference. In practice,
coils always exhibit resistance and capacitance as well as inductance.
If a capacitor, also called a condenser, is placed in an AC circuit, the
current is proportional to the size of the capacitor and to the time rate of
the change of the voltage across the capacitor. Therefore, twice as much
current will flow through a capacitor that has a capacity or size of 2
farads as in a capacitor of 1 farad capacity. In an ideal capacitor the
voltage is exactly out of phase with the current. No current will flow when
the voltage is maximum because then the rate of change of voltage equals
zero. The current will be maximum when the voltage equals zero because then
the rate of change of voltage will be maximum. Current flows through a
capacitor even if there is no direct electrical connection between its
plates because the voltage on one plate induces an opposite charge on the
other plate.
It follows from the above effects that if an alternating voltage is applied
to an ideal inductance or capacitance, no power is expended. In all
practical cases, however, AC circuits contain resistance as well as
inductance and capacitance, and power is actually expended. The amount of
power depends on the relative amounts of the three quantities present in the
circuits.
TOM
>
>I'm sorry, but I have to ask the question. When I was going to electronics
>school they taught us that A/C stands for "Alternating Current". This
>means
>that the current starts out at zero (theoretically) goes positive to 120V,
>back down to zero, and the negative to 120V. So, regardless of which tine
>plugs into which side of the socket, it will be positive on the up cycle,
>and negative on the down cycle, in direct opposite proportion to the other
>tine.
>
>My question is this: Why does polarity matter, and why is such a big deal
>made out of plugs that supposedly force a particular polarity by allowing
>it
>to be plugged in only one way? Polarity IS a consideration with D/C power
>("Direct Current") because there is a definite positive side and a definite
>negative side (whether power flows from positive to negative, or negative
>to
>positive, is a debate between the physicist and the electricians that will
>continue for a long time). The transformer doesn't care which way the plug
>is put in because the diode bridge that converts the A/C to D/C is made
>specifically to clip all the negative power out regardless of which side
>it's flowing from at the time.
>
>Devices that use A/C power only shouldn't care either. Again, the negative
>side is only negative half the time, and same with the positive. For those
>of you who wonder why they don't send D/C over regular power lines, the
>answer is simple. Latency. It's hard enough to get 120V to your house as
>it is (transformer stations are used to reboost the power for areas that
>are
>a long ways from the power plant). Getting D/C to your door would be
>nearly
>impossible without a large number of repeater stations to maintain power
>levels concurrently through the lines.
>
>So, anybody know what I'm missing here to explain the "polarity" thing with
>appliance plugs? This thread has got me real curious.
>
>Kyle Elmblade
>
>
>From: "Kenneth J. Kovler" <[log in to unmask]>
>Sent: Wednesday, August 23, 2000 3:46 AM
>Subject: Re: [PCBUILD] A question for our resident EEs.
>
>
> > Once upon a time on Tue, 22 Aug 2000,at 6:19pm Mark Rode wrote about:...
> >
> > > 120 >> 12 volt transformers for PC devices such as hubs, speakers and
> > > modems never have polarized plugs.
> > > However I have always made sure that the transformers are plugged in
>at
>the
> > > correct direction relative to the printing on the device so as to put
>the
> > > hot lead on the right. This is not always easy to arrange without
>loosing
> > > some outlets on a surge protector or UPS.
> > >
> > > Because the transformers never have polarized plugs I am not sure that
>this
> > > is in fact necessary ?
> > > Is this a good idea or does it matter ?
> > >
> > > Thanks
> > > Mark Rode
> >
> > It will not make any difference since you are connecting a transformer
> > which will provide isolation between the device being powered and the
>120
> > volt circuit. Otherwise there would be a polarized plug and ground on
> > these transformers.
> >
> > ken
> >
> > Kenneth J. Kovler
>
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