Download PHYS 102

AC Circuits
April 26th, 2006
PHYS 102
Where
T is the period of oscillation.
AC Circuits
April 26th, 2006
PHYS 102
Transformers:
One symbol used to denote a transformer is:
AC Circuits
April 26th, 2006
PHYS 102
Transformers:
Looks like two solenoids in close proximity to one
another.
AC Circuits
April 26th, 2006
PHYS 102
Transformers:
If primary current is alternating
in time, then the magnetic flux in
transformer core is changing!
Induced EMF
VP= NP(Induced EMF); VS= NS(Induced EMF); same induced EMF
NS  N P
Called step-up transformer
NP  NS
Called step-down transformer
AC Circuits
April 26th, 2006
PHYS 102
Transformers:
Remember: Power input must equal power output (Always)
AC Circuits
April 26th, 2006
PHYS 102
Transformers:
Have we all seen a transformer?
AC Circuits
April 26th, 2006
PHYS 102
How do the other circuit elements behave
under the influence of an AC source?
Remember, everything in the past has
been analyzed with a DC source.
AC Circuits
April 26th, 2006
PHYS 102
Resistors:
We say that the current is “in phase” with the driving voltage.
AC Circuits
April 26th, 2006
PHYS 102
Capacitors:
We say that the current is “out of phase” with the driving
voltage. Current in a capacitor leads the voltage by 90o.
AC Circuits
April 26th, 2006
PHYS 102
Capacitors:
Since the sine function has a maximum
value of 1, we could define a peak
current.
Resembles current
through resistor.
XC is called the “reactance” of a capacitor. Units = Ohms.
The slower the frequency the greater the capacitive reactance.
Low frequencies→ capacitor acts like open circuit.
High frequencies→ capacitor acts like short circuit.
AC Circuits
April 26th, 2006
PHYS 102
Capacitors:
XC is not the same as Resistance because no power is
dissipated through a capacitor.
Current leads driving voltage: it takes current to move charge
on the capacitor plates→current must flow before the voltage
can change significantly.
AC Circuits
April 26th, 2006
PHYS 102
Inductors:
We say that the inductor current is “out of phase” with the
driving voltage. Current in an inductor lags behind the voltage
by 90o.
AC Circuits
April 26th, 2006
PHYS 102
Inductors:
I (t ) 
IP 

Vp
sin(t  )
L
2
Vp
L

Vp
XL
X L  L
XL is called the “reactance” of an inductor. Units = Ohms.
The greater the frequency the greater the inductive reactance.
Low frequencies→ inductor acts like short circuit.
High frequencies→ inductor acts like open circuit.