Chapter 21 Notes
College Physics by Giambattista et al.
Basic AC Circuits
INPUT
ac voltage
E(t) = Em sin ωt
OUTPUT
ac current
i(t) = Im sin (ωt – φ)
E(t)
Em
0
t
T
2T
–Em
How does the current compare with the driving voltage (emf)?
Peak is reduced by Ohm’s Law for ac circuits:
I = E/Z
Waveform is shifted by a phase angle φ:
–90° ≤ φ ≤ +90°
Root-Mean-Square Value
 a type of average related to standard deviation
rms 
peak
2

Ohm’s Law:
Vrms = IrmsZ
or
Vm = ImZ
Power Law:
Pav = I2rmsR
or
Pav = ½ I2mR
Components of Basic AC Circuits:
1) Resistors – cause heat to dissipate but do not introduce a phase shift
2) Capacitors – store charge, impede low frequencies, adds (–) phase shift
3) Inductors – store current, impede high frequencies, add (+) phase shift
Inductors are coils, rated by their inductance L, SI unit: henry (H)
IMPEDANCE Z
RESISTANCE R
REACTANCE X
CAPACITIVE XC
XC 
Ohm’s Law:
John B. Ross, Ph.D.
VR = IR
1
C
VC = IXC
IUPUI Physics Dept.
INDUCTIVE XL
X L  L
  2f
VL = IXL
Chapter 21 Notes
Phase Diagram:
College Physics by Giambattista et al.
XL
+90°
R 0°
XC
–90°
Resonance

phenomenon found in a variety of physical systems

discuss pumping legs on swing, cracking glass with sound, musical instruments,
Tacoma Narrows Bridge in Wash.
What happens when the driving frequency equals ω0?

Maximum current

Minimum total impedance

Current in phase with driving voltage

Maximum power transferred from source to circuit
XL = XC 
John B. Ross, Ph.D.
0 
1
LC
IUPUI Physics Dept.