Simple AC circuit calculations
Over the course of the next few chapters,
you will learn that AC circuit measurements and calculations
can get very complicated due to the complex nature of
alternating current in circuits with inductance and
capacitance. However, with simple circuits involving nothing
more than an AC power source and resistance, the same laws
and rules of DC apply simply and directly.
Series resistances still add, parallel
resistances still diminish, and the Laws of Kirchhoff and
Ohm still hold true. Actually, as we will discover later on,
these rules and laws always hold true, it's just that
we have to express the quantities of voltage, current, and
opposition to current in more advanced mathematical forms.
With purely resistive circuits, however, these complexities
of AC are of no practical consequence, and so we can treat
the numbers as though we were dealing with simple DC
quantities.
Because all these mathematical relationships
still hold true, we can make use of our familiar "table"
method of organizing circuit values just as with DC:
One major caveat needs to be given here: all
measurements of AC voltage and current must be expressed in
the same terms (peak, peak-to-peak, average, or RMS). If the
source voltage is given in peak AC volts, then all currents
and voltages subsequently calculated are cast in terms of
peak units. If the source voltage is given in AC RMS volts,
then all calculated currents and voltages are cast in AC RMS
units as well. This holds true for any calculation
based on Ohm's Laws, Kirchhoff's Laws, etc. Unless otherwise
stated, all values of voltage and current in AC circuits are
generally assumed to be RMS rather than peak, average, or
peak-to-peak. In some areas of electronics, peak
measurements are assumed, but in most applications
(especially industrial electronics) the assumption is RMS.
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REVIEW:
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All the old rules and laws of DC (Kirchhoff's
Voltage and Current Laws, Ohm's Law) still hold true for
AC. However, with more complex circuits, we may need to
represent the AC quantities in more complex form. More on
this later, I promise!
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The "table" method of organizing circuit
values is still a valid analysis tool for AC circuits.
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