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# Alternating Current Circuits And Electromagnetic Waves - PowerPoint PPT Presentation

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

• The output of an ac generator

• ac generator symbol

• Formula for instantaneous voltage (Dv)

21.1/21.2

• Current and voltage arein stepwith each other.(in phase)

• 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

• 120 V rms in our homes

• 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

21.4

• In a capacitive ac circuit, the current always leads the voltage by 90o.

21.5

• Capacitive reactance (ohms)

• Similar to resistance in a dc circuit

• Decreases with frequency

• Opposes a change in ac voltage

• Formulas:

• Inductive reactance (ohms)

• Similar to resistance in a dc circuit

• Increases with frequency

• Opposes a change in ac current

• Formulas:

21.6/21.7

• In an inductive ac circuit, the voltage always leads the current by 90o.

ELItheICEman

21.7

ELItheICEman

• 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.

• Impedance (Z)

• The total resistance (ohms) in an ac circuit

• Formula:

221

• Phasors

• Rotating vectorswhich are used to represent the voltage across each component.

• Separated by 90o

• Phase angle (f)

222

• Phase angle

• Review of circuit symbols

• Table 21.2 (pg. 705)

• Electronic Symbol Handout

23

• 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.

• Mechanical Resonance

91, 95

• Resonant frequency (fo)

• The frequency at which XL = XC

• Important in tuning circuits

• Radios, televisions, cell phones

• Airport metal detectors

• Formula:

223

1 - 4, 6, 7

Pg. 726

• What is a transformer?

218

• A transformer steps up or steps down ac voltages

• Primary and secondary coils

218

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

• Efficiencies are between 90% and 99%.

Np/Ns is the turns ratio

• Applications:

• AC/DC electronic equipment

• Power distribution

• Automobile ignition systems

219, 220

• 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

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

21.20

• He concluded that light waves are electromagnetic in nature.

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

21.17

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

• Resonant frequency(fo)formula

• Hertz showed that electromagnetic radiation exhibited wave properties.

• Reflection

• Refraction

• Diffraction

• Interference

• Polarization

231, 232

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

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

• 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

228

• Electromagnetic waves are transverse waves.

• They travel at the speed of light.

• Light is an electromagnetic wave

• They can travel through a vacuum.

• The ratio of the electric field strength (E) to the magnetic field strength (B) equals the speed of light.

• Electromagnetic waves carry both energy and momentum.

• Important formula:

• Range of wavelengths for visible light:

• 400 nmto 700 nm

69

• 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

• Astronomy

• The expanding universe

• Red shift

• Blue shift

• Rotation of galaxies

9 - 15

Pg. 726