Chapter 21Alternating Current Circuits and Electromagnetic Waves

1. Alternating Current2. Resistor in an AC circuit3. Capacitor in an AC circuit4. Inductor in an AC circuit5. RLC series circuit6. Resonance7. Transformer8. Electromagnetic waves9. Properties of electromagnetic waves10. Doppler effect and electromagnetic waves

AC Circuit What is alternating current? What is an AC circuit?

Resistor in an AC Circuit What is it? The voltage source The concept of phase Behavior of the resistor What is the power

dissipated by the resistor?

What is rms current? Why do we need it? What is the average

power? What about ohm’s law?

Example – rms Current1. For the circuit show, calculate the maximum

current and voltage, rms current and voltage and rms power.

Capacitors in an AC Circuit Behavior of the

capacitor What is the Voltage

and Current relationship?

What is capacitive reactance?

How do I calculate it? How do I use ohm’s

law?

Inductors in an AC Circuit Behavior of the

inductor Voltage and current

relationship What is inductive

reactance? How do I calculate it? How do I apply

ohm’s law?

Example - Inductor

1. In a purely inductive ac circuit as shown, Vmax=100V.

1. If the maximum current is 7.5A at 50 Hz, calculate the inductance L.

2. At what angular frequency ω is the maximum current 2.5 A?

The RLC Series Circuit What is the

current in the circuit?

What about voltage and current relationship?

Phasor Diagrams What is a

phasor? diagram?

How do I use it?

Phasor Diagram for RLC series circuit

What is the maximum Voltage?

What is the phase? What about rms

values?

Impedance of a Circuit What is

impedance? How do I calculate

it? How do I apply

ohms’ law?

Example - RLC Circuit

1. A coil of resistance 35 Ω and inductance 20.5 H is in series with a capacitor and a 200-Vrms, 100-Hz source. The rms current in the circuit is 4.0A.

1. Calculate the capacitance in the circuit.2. What is Vrms across the coil?

Summary of Circuit Elements, Impedance and Phase Angles

Power in an AC Circuit How do I calculate the average

power? What is a power factor? Power losses in a capacitor and an

inductor

Example - RLC Circuit

1. In a certain series RLC circuit, Irms=9.0A, Vrms=180V, and the current leads the voltage by 37°.

1. What is the total resistance in the circuit?2. Calculate the magnitude of the reactance of the

circuit (XL-XC).

Resonance in an AC Circuit What is resonance in

an AC circuit? When does is occur? How do I calculate

resonance frequency? Example of devices

that use resonance

Example - Resonance in an AC Circuit1. Calculate the resonant frequency of a

circuit of negligible resistnace containing an inductance of 40mH and a capacitance of 600pF.

Transformers What is a

transformer? How does it work? How do I use it? What about

power?

Example - Transformers1. An AC adapter for telephone-answering unit

uses a transformer to reduce the line voltage of 120Vrms to a voltage of 9.0 V. The rms current delivered to the answering system is 400mA.

1. If the primary (input) coil in the transformer in the adapter has 240 turns, how many turns are there on the secondary (output) coil?

2. What is the rms power delivered to the transformar? Assume an ideal transformer.

Nikola Tesla 1865 – 1943 Inventor Key figure in

development of AC electricity High-voltage

transformers Transport of electrical

power via AC transmission lines

Beat Edison’s idea of DC transmission lines

Why do we use AC and not DC What would you want to conserve? Why do we have high voltage

transmission lines when our homes only have 120V?

James Clerk Maxwell 1831 – 1879 Electricity and

magnetism were originally thought to be unrelated

in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena

More of Maxwell’s Contributions Electromagnetic theory of light Kinetic theory of gases Nature of Saturn’s rings Color vision Electromagnetic field

interpretation Led to Maxwell’s Equations

Maxwell’s Predictions

Hertz’s Confirmation of Maxwell’s Predictions 1857 – 1894 First to generate and

detect electromagnetic waves in a laboratory setting

Showed radio waves could be reflected, refracted and diffracted

The unit Hz is named for him

Hertz’s Basic Apparatus How does it work? What about resonance

frequency? What is the source for

energy transfer? What are

electromagnetic waves?

How do I measure the speed of the waves?

Electromagnetic Waves Produced by an Antenna Acceleration of charge Antenna

Charges and Fields, Summary

Electromagnetic Waves, Summary

Electromagnetic Waves are Transverse Waves

Properties of EM Waves

The EMSpectrum

Doppler Effect and EM Waves

The light of a moving source is blue/red shifted by

/0 = vr/c

0 = actual wavelength

emitted by the source

Wavelength change due to

Doppler effect

vr = radial velocity

Blue Shift (to higher frequencies)

Red Shift (to lower frequencies)

vr

Example – EM Waves

1. What are the wavelength ranges in

1. AM radio band (540-1600 kHz) and2. The FM radio band (88-108 MHz)?

Example – EM Waves

1. A speeder tries to explain to the police that the yellow warning lights on the side of the road looked green to her because of the Doppler shift. How fast would she have been traveling if yellow light of wavelength of 580 nm had been shifted to green with a wavelength of 560 nm?