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Alternating Current Circuits And Electromagnetic Waves

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Chapter 21

- 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

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

- 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

- Composed of fluctuating electrical and magnetic waves

- Electromagnetic waves

- 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)

- The average valueof the current over one cycle is zero.
- Power formula:

- 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

- Charging

- 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:

- Similar to resistance in a dc circuit

- Inductive reactance (ohms)
- Similar to resistance in a dc circuit
- Increases with frequency

- Opposes a change in ac current
- Formulas:
21.6/21.7

- Similar to resistance in a dc circuit

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

21.7

- 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

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

- 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.

- Power Formulas:
NOTE:cosfis called the power factor

- 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

- Primary and secondary coils

- 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

- Antennas are used to produce electromagnetic waves.

- Electromagnetic waves are generated if this happens rapidly.

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

- 400 nmto 700 nm

- 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

- Sound waves exhibit the Doppler effect
- Trains
- Race cars

- Astronomy
- The expanding universe
- Red shift
- Blue shift
- Rotation of galaxies

- The expanding universe

9 - 15

Pg. 726