Alternating
Current (AC) varies in magnitude and direction.
the
voltage from an AC voltage source can be written as
t. in
an AC circuit, the current through a resistor is directly
proportional to and in phase with the voltage across the
resistor.
for
quantities which vary with time in an AC circuit, like voltage and
current, the rms (root-mean-square) values are a useful description;
they correspond to constant values in a DC circuit which would
produce the same power.
the
capacitive reactance of a capacitor in an AC circuit is
analogous to the resistance of a resistor; its value is given by
XC = 
.
for
a given voltage, more current will flow through a capacitor's circuit
if the frequency is higher and less current if the frequency is
lower.
the
AC current through a capacitor is out of phase by 90° with the
AC voltage across the capacitor; the current "leads" the voltage.
the
inductive reactance of an inductor in an AC circuit is
analogous to the resistance of a resistor; its value is given by
XL = 2¹ f L .
for
a given voltage, more current will flow through an inductor's circuit
if the frequency is lower and less current if the frequency is
higher.
the
AC current through an inductor is out of phase by 90° with the
AC voltage across the inductor; the current "lags" the voltage.
in
determining the current through a series RCL circuit, the phase
relationships between voltage across and current in the various
elements is important and can be explained using phasor
diagrams.
the
impedance of an AC circuit is analogous to the resistance in a
DC circuit.
the
total impedance of a series RCL circuit is given by
. the
power dissipated by an AC circuit depends on the phase relation
between current and voltage.
an
electric circuit may have a resonant frequency -- a frequency at
which it dissipates far more power than at other frequencies.
the
resonant frequency of a series RCL circuit is fo
= 
.
Maxwell's
Equations show the strong, fundamental connection between Electricity
and Magnetism.
a
changing electric field E and/or a changing magnetic field B can
cause a wave composed of oscillating strengths in the electric field
E and the magnetic field B; this is called an electromagnetic (EM)
wave.
examples
of electromagnetic (EM) waves include light,
infra-red and
ultra-violet radiation, gamma
rays from a nucleus, X-rays, and radio and
television waves.
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(c) Doug Davis, 2002; all rights reserved
