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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

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  • Alternating currentcircuits 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
AC Generators
  • The output of an ac generator
    • ac generator symbol
    • Formula for instantaneous voltage (Dv)


resistors in an ac circuit
Resistors In An ac Circuit
  • Current and voltage arein stepwith each other.(in phase)
rms current
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
  • Rms voltage
    • 120 V rms in our homes
ohm s law in an ac circuit
Ohm’s Law In An ac Circuit
  • Ohm’s Lawis valid for resistors in ac circuits.
  • Capacitors in a dc circuit
    • Charging
      • As the voltage across the capacitor increases, the current decreases.
    • RC time constant


capacitors in an ac circuit
Capacitors In An ac Circuit
  • In a capacitive ac circuit, the current always leads the voltage by 90o.


capacitive reactance
Capacitive Reactance
  • Capacitive reactance (ohms)
    • Similar to resistance in a dc circuit
      • Decreases with frequency
    • Opposes a change in ac voltage
    • Formulas:
inductive reactance
Inductive Reactance
  • Inductive reactance (ohms)
    • Similar to resistance in a dc circuit
      • Increases with frequency
    • Opposes a change in ac current
    • Formulas:


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 90o.
    • The instantaneous voltage across the capacitor lags the instantaneous current by 90o.
the rlc series circuit
The RLC Series Circuit
  • Impedance (Z)
    • The total resistance (ohms) in an ac circuit
    • Formula:


  • Phasors
    • Rotating vectorswhich are used to represent the voltage across each component.
      • Separated by 90o
    • Phase angle (f)


phase angle
Phase Angle
  • Phase angle
schematic symbols
Schematic Symbols
  • Review of circuit symbols
    • Table 21.2 (pg. 705)
    • Electronic Symbol Handout


power in an ac circuit
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:cosfis called the power factor

  • Mechanical Resonance

91, 95

resonance in a series rlc circuit
Resonance In A Series RLC Circuit
  • Resonant frequency (fo)
    • The frequency at which XL = XC
    • Important in tuning circuits
      • Radios, televisions, cell phones
      • Airport metal detectors
    • Formula:



1 - 4, 6, 7

Pg. 726

  • What is a transformer?


transformer voltage
Transformer Voltage
  • A transformer steps up or steps down ac voltages
    • Primary and secondary coils


transformer power
Transformer Power
  • The power input to the primary equals the power output at the secondary.
transformer efficiency
Transformer Efficiency
  • Efficiencies are between 90% and 99%.
the transformer equation
The Transformer Equation

Np/Ns is the turns ratio

the transformer
The Transformer
  • Applications:
    • AC/DC electronic equipment
    • Power distribution
    • Automobile ignition systems

219, 220

maxwell s theory
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).


maxwell s hypothesis
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’s Prediction
  • Maxwell came up with the concept of electromagnetic waves and predicted that they would travel at the speed of light.


maxwell s conclusion
Maxwell’s Conclusion
  • He concluded that light waves are electromagnetic in nature.
hertz s discovery
Hertz’s Discovery
  • Hertz was the first to generate and detect electromagnetic waves in the laboratory.


em resonance
EM Resonance
  • Hertz achieved resonance and transferred electromagnetic energy between two coils which were several meters apart.
    • Resonant frequency(fo)formula
properties em radiation
Properties EM Radiation
  • Hertz showed that electromagnetic radiation exhibited wave properties.
    • Reflection
    • Refraction
    • Diffraction
    • Interference
    • Polarization

231, 232

the speed of em waves
The Speed of EM Waves
  • Hertz showed that electromagnetic radiation traveled at the speed of light.
hertz s contribution
Hertz’s Contribution
  • Hertz’s experiments provided the first proof of Maxwell’s theories.
production of em waves by an antenna
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

properties of em waves
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
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
Energy and Momentum
  • Electromagnetic waves carry both energy and momentum.
the speed of em waves48
The Speed of EM Waves
  • Important formula:
visible light
Visible Light
  • Range of wavelengths for visible light:
    • 400 nmto 700 nm


the spectrum of em waves
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


review the doppler effect
Review-The Doppler Effect
  • Sound waves exhibit the Doppler effect
    • Trains
    • Race cars
the doppler effect for em waves
The Doppler Effect For EM Waves
  • Astronomy
    • The expanding universe
      • Red shift
      • Blue shift
      • Rotation of galaxies

9 - 15

Pg. 726