Alternating Current Circuits And Electromagnetic Waves

1. Alternating Current Circuits And Electromagnetic Waves Chapter 21

2. Introduction • Alternating currentcircuits play an important part in our lives. • Electrical power production and transmission • Communication systems • Entertainment • Medicine 224, 229

3. We will study • Circuits containing a source of emf and one or more of the following in series: • Resistors • Capacitors • Inductors

4. 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

6. AC Generators • The output of an ac generator • ac generator symbol • Formula for instantaneous voltage (Dv) 21.1/21.2

7. Resistors In An ac Circuit • Current and voltage arein stepwith each other.(in phase)

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

9. 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.

10. Rms Voltage • Rms voltage • 120 V rms in our homes

11. Ohm’s Law In An ac Circuit • Ohm’s Lawis valid for resistors in ac circuits.

12. Review • Capacitors in a dc circuit • Charging • As the voltage across the capacitor increases, the current decreases. • RC time constant 21.4

13. Capacitors In An ac Circuit • In a capacitive ac circuit, the current always leads the voltage by 90o. 21.5

14. Capacitive Reactance • Capacitive reactance (ohms) • Similar to resistance in a dc circuit • Decreases with frequency • Opposes a change in ac voltage • Formulas:

15. 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

16. In an inductive ac circuit, the voltage always leads the current by 90o. ELItheICEman 21.7

17. 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.

18. The RLC Series Circuit • Impedance (Z) • The total resistance (ohms) in an ac circuit • Formula: 221

19. Phasors • Phasors • Rotating vectorswhich are used to represent the voltage across each component. • Separated by 90o • Phase angle (f) 222

20. Phase Angle • Phase angle

21. Schematic Symbols • Review of circuit symbols • Table 21.2 (pg. 705) • Electronic Symbol Handout 23

22. 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.

23. Power Formulas: NOTE:cosfis called the power factor

24. Review • Mechanical Resonance 91, 95

25. 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: 223

26. QUESTIONS 1 - 4, 6, 7 Pg. 726

27. Transformers • What is a transformer? 218

28. Transformer Voltage • A transformer steps up or steps down ac voltages • Primary and secondary coils 218

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

30. Transformer Efficiency • Efficiencies are between 90% and 99%.

31. The Transformer Equation Np/Ns is the turns ratio

32. The Transformer • Applications: • AC/DC electronic equipment • Power distribution • Automobile ignition systems 219, 220

33. 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

34. 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.

35. 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

36. Maxwell’s Conclusion • He concluded that light waves are electromagnetic in nature.

37. Hertz’s Discovery • Hertz was the first to generate and detect electromagnetic waves in the laboratory. 21.17

38. Hertz’s Apparatus

39. EM Resonance • Hertz achieved resonance and transferred electromagnetic energy between two coils which were several meters apart. • Resonant frequency(fo)formula

40. Properties EM Radiation • Hertz showed that electromagnetic radiation exhibited wave properties. • Reflection • Refraction • Diffraction • Interference • Polarization 231, 232

41. The Speed of EM Waves • Hertz showed that electromagnetic radiation traveled at the speed of light.

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

43. 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

44. EM Waves 228

45. 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.

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

47. Energy and Momentum • Electromagnetic waves carry both energy and momentum.

48. The Speed of EM Waves • Important formula:

49. Visible Light • Range of wavelengths for visible light: • 400 nmto 700 nm 69

50. 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