Phasors and AC (sec. 31.1) Resistance and reactance (sec. 31.2) RLC series circuit (sec. 31.3) Power in AC circuits (sec. 31.4) Resonance in AC circuits (sec. 31.5)Transformers (sec. 31.6)

Alternating Current Ch. 31

C 2012 J. F. Becker

Learning Goals - we will learn: ch 31

• How phasors make it easy to describe sinusoidally varying quantities.• How to analyze RLC series circuits driven by a sinusoidal emf.• What determines the amount of power flowing into or out of an AC circuit. • How an RLC circuit responds to emfs of

different frequencies.

Phasor diagram -- projection of rotating vector (phasor) onto the horizontal axis represents the

instantaneous current.

Graphs (and phasors) of instantaneous voltage and current for a resistor.

i(t) = I cos t (source)

vR(t) = i(t) R vR(t) = IR cos t

where VR = IR is the voltage amplitude.

VR = IR

Notation: -lower case letters

are time dependent and

-upper case letters are constant.

For example, i(t) is the time

dependent current and

I is current amplitude;

VR is the voltage amplitude (= IR ).

Graphs of instantaneous voltages for RLC series circuit. (The phasor diagram is much simpler.)

Graphs (and phasors) of instantaneous voltage and current for an inductor.

i(t) = I cos t (source)

vL(t) = L di / dt vL(t) = L d(I cos t )/dt

vL(t) = -IL sin t

vL(t) = +IL cos (t + 900)

where VL = IL (= IXL)is the voltage amplitude

and = +900 is the PHASE ANGLE

(angle between voltage across and current

through the inductor).

XL = L

E L I

VL L I

Graphs (and phasors) of instantaneous voltage and current showing phase relation between current

(red) and voltage (blue).Remember: “ELI the ICE man”

Crossover network in a speaker system. Capacitive reactance: XC =1/C

Inductive reactance: XL = L

Phasor diagrams for series RLC circuit (b) XL > XC and (c) XL < XC.

Graphs of instantaneous voltages for RLC series circuit. (The phasor diagram is much simpler.)

Graphs of instantaneous voltage, current, and power for an R, L, C, and an RLC circuit. Average power for an

arbitrary AC circuit is 0.5 VI cos = V rms I rms cos

The average power is half the product of I and the component of V in phase with it.

Instantaneous current and voltage:

Average power depends on current and

voltage amplitudes AND

the phase angle :

Graph of current amplitude I vs source frequency for a series RLC circuit

with various values of circuit resistance.

The resonance frequency is at

= 1000 rad / sec(where the

current is at its maximum)

AMPLITUDE MODULATION (AM) of CARRIER WAVEresonance frequency (fo)

Electric field amplitude

AM modulatedElectric field amplitude

FREQUENCY MODULATION (AM) of CARRIER WAVEresonance frequency (fo)

Electric field amplitude

FM modulatedElectric field amplitude

A radio tuning circuit at resonance. The circles denote rms current and voltages.

Transformer: AC source is V1 and secondary provides a voltage V2 to a device with resistance R.

TRANSFORMERS

can step-up AC voltages or step-

down AC voltages.

2 /1 = N2/N1

V1I1 = V2I1 =

=d/ dt

(a) Primary P and secondary S windings in a transformer. (b) Eddy currents in the iron core

shown in the cross- section AA. (c) Using a laminated core reduces the eddy currents.

Figure 32.2b

Large step-down transformers at power stations are immersed in tanks of oil for insulation and

cooling.

Figure 31.22

Figure 31.23

A full-wave diode rectifier circuit. (LAB)

A mathematical model of Earth's

magnetic field near the core.(Courtesy: Gary Glatzmaier)

See www.physics.sjsu.edu/becker/physics51

Review

C 2012 J. F. Becker

PREPARATION FOR FINAL EXAMAt a minimum the following should be reviewed:

Gauss's Law - calculation of the magnitude of the electric field caused by continuous distributions of charge starting with Gauss's Law and completing all the steps including evaluation of the integrals.

Ampere's Law - calculation of the magnitude of the magnetic field caused by electric currents using Ampere's Law (all steps including evaluation of the integrals).

Faraday's Law and Lenz's Law - calculation of induced voltage and current, including the direction of the induced current.

Calculation of integrals to obtain values of electric field, electric potential, and magnetic field caused by continuous distributions of electric charge and current configurations (includes the Law of Biot and Savart for magnetic fields).

Maxwell's equations - Maxwell's contribution and significance.

DC circuits - Ohm's Law, Kirchhoff's Rules, series-parallel combinations, power.

Series RLC circuits - phasors, phase angle, current, power factor, average power.

Vectors - as used throughout the entire course.