Alternating Current Circuits And

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16 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Alternating Current Circuits And
Electromagnetic Waves


Chapter 21


Introduction


Alternating current

circuits play an
important part in our lives.


Electrical power production and
transmission


Communication systems


Entertainment


Medicine


224, 229



We will study


Circuits containing a source of emf
and one or more of the following in
series:


Resistors


Capacitors


Inductors



We will study


Electromagnetic waves


Composed of fluctuating electrical and magnetic waves


Radio frequencies (RF)


Radar


Microwaves


Infrared


Visible light


Ultraviolet


X
-
rays


Gamma Rays




AC Generators


The output of an ac generator


ac generator symbol


Formula for instantaneous voltage
(
D
v)





21.1/21.2

Resistors In An ac Circuit


Current and voltage are

in step

with each other.

(in phase)






The average value

of the current
over one cycle is zero.


Power formula:



rms Current


rms current

(root mean square)


It is the direct current that would
dissipate the same amount of energy
in a resistor as an ac current of that
value.


Rms Voltage


Rms voltage



120 V rms in our homes




Ohm’s Law In An ac Circuit


Ohm’s Law

is valid for resistors in
ac circuits.









Review


Capacitors in a dc circuit


Charging


As the voltage across the capacitor
increases, the current decreases.


RC time constant



21.4

Capacitors In An ac Circuit


In a
capacitive

ac circuit, the
current

always
leads

the
voltage by 90
o.





21.5

Capacitive Reactance


Capacitive reactance (ohms)


Similar to resistance in a dc circuit



Decreases with frequency


Opposes a change in ac voltage


Formulas:



fC

2
1
X
C

Inductive Reactance


Inductive reactance (ohms)


Similar to resistance in a dc circuit



Increases with frequency


Opposes a change in ac current


Formulas:





21.6/21.7

fL

2
X
L


In an
inductive

ac circuit, the
voltage

always
leads

the current by
90
o.



E
L
I


the

I
C
E

man


21.7

E
L
I


the

I
C
E

man


Three

important relationships:


The instantaneous voltage across the
resistor is in phase with the instantaneous
current.


The instantaneous voltage across the
inductor leads the instantaneous current by
90
o.


The instantaneous voltage across the
capacitor lags the instantaneous current by
90
o.

The RLC Series Circuit


Impedance (Z)


The total resistance (ohms) in an ac circuit


Formula:









221

2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


x
2
2
)
(
C
L
X
X
R


x
2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
2
)
(
C
L
X
X
R


2
C
L
2
)
X
(X
R


Phasors


Phasors


Rotating vectors

which are used to represent the
voltage across each component.


Separated by 90
o


Phase angle
(f)






222

Phase Angle


Phase angle



Schematic Symbols


Review of circuit symbols


Table 21.2 (pg. 705)


Electronic Symbol Handout




23

Power In An ac Circuit


No power is lost in capacitors or
in
pure

inductors.


A capacitor in an ac circuit does not
dissipate energy.


A pure inductor in a ac circuit does
not dissipate energy.



Power Formulas:







NOTE:

cos
f

is called the power factor

Review


Mechanical Resonance






91, 95


Resonance In A Series RLC Circuit


Resonant frequency (f
o
)


The frequency at which
X
L

= X
C


Important in tuning circuits


Radios, televisions, cell phones


Airport metal detectors


Formula:



223

QUESTIONS


1
-

4, 6, 7

Pg. 726

Transformers


What is a transformer?





218

Transformer Voltage


A transformer
steps up

or
steps down

ac voltages


Primary and secondary coils






218

Transformer Power


The power input to the primary
equals the power output at the
secondary.



Transformer Efficiency


Efficiencies are between 90% and
99%.



The Transformer Equation







N
p
/N
s
is the turns ratio

The Transformer


Applications:


AC/DC electronic equipment


Power distribution


Automobile ignition systems



219, 220

Maxwell’s Theory


Electric field lines originate on positive charges
and terminate on negative charges.


Magnetic field lines always form closed loops.


A varying magnetic field induces an emf and
hence an electric field.


Magnetic fields are generated by moving
charges (or currents).



21.18

Maxwell’s Hypothesis


A changing electric field should
produce a magnetic field.


This could not be proven
experimentally because the electric
fields were too weak to detect at the
time.


Maxwell’s Prediction


Maxwell came up with the concept
of electromagnetic waves and
predicted that they would travel at
the speed of light.



21.20

Maxwell’s Conclusion


He concluded that light waves are
electromagnetic in nature.


Hertz’s Discovery


Hertz was the first to generate and
detect electromagnetic waves in
the laboratory.




21.17


Hertz’s Apparatus

EM Resonance


Hertz achieved resonance and
transferred electromagnetic energy
between two coils which were
several meters apart.


Resonant frequency

(f
o
)

formula


Properties EM Radiation


Hertz showed that electromagnetic radiation
exhibited wave properties.


Reflection


Refraction


Diffraction


Interference


Polarization


231, 232


The Speed of EM Waves


Hertz showed that electromagnetic
radiation traveled at the speed of
light.


Hertz’s Contribution


Hertz’s experiments provided the
first proof of Maxwell’s theories.


Production Of EM Waves By An
Antenna


Energy stored in an LC circuit is
constantly transferred between the
electric and magnetic fields.


Electromagnetic waves are generated if
this happens rapidly.


Antennas are used to produce electromagnetic
waves.



177, 178


EM Waves






228

Properties Of EM Waves


Electromagnetic waves are
transverse waves.



They travel at the speed of light.


Light is an electromagnetic wave


They can travel through a vacuum.

A Special Relationship


The ratio of the electric field
strength
(E)

to the magnetic field
strength
(B)

equals the speed of
light.



Energy and Momentum


Electromagnetic waves carry both
energy and momentum.


The Speed of EM Waves


Important formula:




Visible Light


Range of wavelengths for visible light:


400 nm

to

700 nm








69

The Spectrum Of EM Waves


Types of waves

(increasing in frequency and energy)


Radio waves


Microwaves


Infrared waves
(heat waves)


Visible light


Ultraviolet light
(UV)


X
-
rays


Gamma rays


21.23


Review

-

The Doppler Effect


Sound waves exhibit the
Doppler effect


Trains


Race cars


The Doppler Effect For EM Waves


Astronomy


The expanding universe


Red shift


Blue shift


Rotation of galaxies


The Red Shift



QUESTIONS


9
-

15

Pg. 726