Transcript AC Theory powerpoint File

Inductor
Arising from
Faraday's law, the
inductance L may
be defined in terms
of the emf
generated to
oppose a given
change in current:
Resistor A.C. Response
Capacitor A.C. Response
Inductor A.C. Response
ELI the ICE man
Impedance
While Ohm's Law applies directly to resistors in DC or in
AC circuits, the form of the current-voltage relationship in
AC circuits in general is modified to the form:
•
•
• where I and V are the rms or "effective" values. The
quantity Z is called impedance. For a pure resistor, Z =
R. Because the phase affects the impedance and
because the contributions of capacitors and inductors
differ in phase from resistive components by 90 degrees,
a process like vector addition (phasors) is used to
develop expressions for impedance.
Impedance, Z
• RC circuit
LC circuit
Series LCR Circuit
•
The resonance of a series RLC circuit occurs when the inductive and
capacitive reactances are equal in magnitude but cancel each other
because they are 180 degrees apart in phase. The sharp minimum in
impedance which occurs is useful in tuning applications.
Tuning in
• LC circuits have a resonance
property like mechanical
systems such as a pendulum
or a mass on a spring: there is
a special frequency that it likes
to oscillate at, and therefore
responds strongly to.
• They can be used to tune in to
a specific frequency, for
example in the station selector
of a radio or television.
• In an LC circuit, electric charge
oscillates back and forth just
like the position of a mass on a
spring oscillates.
mechanical spring
position:
x
LC circuit
charge:
Q
velocity:
current:
kinetic energy:
inductor's energy:
potential energy:
capacitor's energy:
frequency:
frequency:
Audio Crossover network
Speaker systems
Impedance Matching
As a general rule, the maximum power transfer from an active device like an
amplifier to an external device like a speaker occurs when the impedance of
the external device matches that of the source. That optimum power is 50%
of the total power when the impedance of the amplifier is matched to that of
the speaker. Improper impedance matching can lead to excessive power use,
distortion, and noise problems.