Physics 213 General Physics

1. 0 Physics 213General Physics Lecture 14

2. Last Meeting: Electric Generators, Alternating Current Today: Electromagnetic Waves, Maxwell’s Equations

3. Electromagnetic Properties, Summary (1) Electric field Sources (2) No magnetic field sources (3) Changing magnetic field produces an electric field (Faraday’s Law) (4) Moving charge (current) produces magnetic fields.

4. Changing E field produces B field • A changing E field produces a B field. • Can be viewed as formed from a “displacement current”.

5. EM Waves by an Antenna • Two rods are connected to an ac source, charges oscillate between the rods (a) • As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = 0 moves away from the rod (b) • The charges and field reverse (c) • The oscillations continue (d)

6. EM Waves by an Antenna, B field • Because the oscillating charges in the rod produce a current, there is also a magnetic field generated • As the current changes, the magnetic field spreads out from the antenna • The magnetic field is perpendicular to the electric field

7. Electromagnetic Waves Can Propagate in Free Space • A changing magnetic field produces an electric field • A changing electric field produces a magnetic field • These fields are in phase • At any point, both fields reach their maximum value at the same time

8. Electromagnetic Waves are Transverse Waves • The and fields are perpendicular to each other • Both fields are perpendicular to the direction of motion • Therefore, em waves are transverse waves

9. Properties of EM Waves • Electromagnetic waves are transverse waves • Electromagnetic waves travel at the speed of light (3x108 m/s) • Because EM waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave

10. Properties of EM Waves, 2 • The ratio of the electric field to the magnetic field is equal to the speed of light • Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects placed in their path

11. Properties of EM Waves, 3

12. The Spectrum of EM Waves • Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths • c = ƒλ • Wavelengths for visible light range from 400 nm to 700 nm • There is no sharp division between one kind of EM wave and the next

13. The EMSpectrum • Note the overlap between types of waves • Visible light is a small portion of the spectrum • Types are distinguished by frequency or wavelength

16. Right-Hand Rule Review Sheet

17. In an RLC circuit, the maximum current is 1 amp. What is the average current? • 21/2 A • (1/21/2) A • (1/2)A • Non of the above

18. An AC voltage source, with a peak output of 200 V, is connected to a 50- resistor. What is the effective (or rms) current in the circuit? • 2.8 A • 4.0 A • 5.6A • 2.0 A

19. A 100 Hz AC voltage source, with a peak output of 170 V, is connected to an inductor. If the rms current through the inductor is 1 A, what is the value of the inductor? • 1.9 H • 190 mH • 19 mH • 1.9 mH

20. In a circuit made up of inductor L, resistance R, ammeter, battery and switch in series, the current is greatest at which of the following times, as measured after the switch is closed? • Zero • One time constant • At time t=L/R • Ten time constants