April 26th, 2006 AC Circuits

PHYS 102Where

T is the period of oscillation.

April 26th, 2006 AC Circuits

PHYS 102Transformers:

One symbol used to denote a transformer is:

April 26th, 2006 AC Circuits

PHYS 102Transformers:

Looks like two solenoids in close proximity to one another.

April 26th, 2006 AC Circuits

PHYS 102Transformers:

VP= NP(Induced EMF); VS= NS(Induced EMF); same induced EMF

S PN NCalled step-up transformer

P SN NCalled step-down transformer

If primary current is alternating in time, then the magnetic flux in transformer core is changing!

Induced EMF

April 26th, 2006 AC Circuits

PHYS 102Transformers:

Remember: Power input must equal power output (Always)

April 26th, 2006 AC Circuits

PHYS 102Transformers:

Have we all seen a transformer?

April 26th, 2006 AC Circuits

PHYS 102How 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.

April 26th, 2006 AC Circuits

PHYS 102Resistors:

We say that the current is “in phase” with the driving voltage.

April 26th, 2006 AC Circuits

PHYS 102Capacitors:

We say that the current is “out of phase” with the driving voltage. Current in a capacitor leads the voltage by 90o.

April 26th, 2006 AC Circuits

PHYS 102Capacitors:

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.

Resembles current through resistor.

Since the sine function has a maximum value of 1, we could define a peak current.

April 26th, 2006 AC Circuits

PHYS 102Capacitors:

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.

April 26th, 2006 AC Circuits

PHYS 102Inductors:

We say that the inductor current is “out of phase” with the driving voltage. Current in an inductor lags behind the voltage by 90o.

April 26th, 2006 AC Circuits

PHYS 102Inductors:

I tV

Ltp( ) sin( )

2

IV

L

V

XPp p

L

X LL

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.