1. Electric charges produce electric field
Electromagnetic Waves So far we have learned that: 1. Electric charges produce electric field; 2. Moving charges (electric current) produce magnetic field; 3. Changing magnetic flux induces emf in a circuit which drives electric charge to produce electric field. Maxwell noticed that for the nature of symmetry, a changing electric flux should induce a magnetic field and an electric current. Indeed, after Maxwell’s theoretical work and Hertz’s experimental discovery, generation and applications of electromagnetic waves totally changed the world. Since E&M waves can propagate through vacuum, radio and TV become a reality.
Important results from Maxwell’s predictions: Emax 1 =c= (1) = 3.0×108 m/s, is the speed of light Bmax µ 0ε 0 (2) Both fields are transverse traveling waves with E and B vectors perpendicular to each other. Both vectors are perpendicular to the propagation direction. (3) E and B form electromagnetic waves in free space and travel at the speed of light c. From this Maxwell predicted that light is just an electromagnetic waves. (4) Average power of E&M waves per unit area:
Emax Bmax c 2 1 2 I= Emax = Bmax = 2cµ 0 2µ 0 2µ 0 (5) Radiation momentum p delivered by E&M waves perpendicular incident into surface is: 2U U • for total absorption: p = • for total reflection: p = c c where U is the incident energy into the surface.
Example: Sunlight just outside Earth’s atmosphere has an intensity of 1.4 kW/m2. What is the Emax and Bmax ? Emax = 2cIµ 0 = 2 • 3 • 108 • 1.4 • 103 • 4π • 10 −7 = 1027V/m
Bmax =Emax/c=1027/(3•108)=3.42•10-6 T This numerical result shows that electric field is far more important than The magnetic field for the light wave. Wavelength of Electromagnetic Waves Since E&M waves is resulted from the oscillation of electric and magnetic waves, the relation between its speed, frequency and the wavelength should be similar to that of sound waves, i.e. speed = wavelength × frequency. ∴ c = fλ The scale of wavelengths of known E & M waves and the visible is shown on the next slide.
V B G Y
R
Question: If human beings are most sensitive to 550 nm greenyellow light, it can explain why we use them in the traffic light. But human beings are not that sensitive to the red light. Why we use red light then?
Important results from Maxwell’s predictions: Emax 1 =c= (1) = 3.0×108 m/s, is the speed of light Bmax µ 0ε 0 (2) Both fields are transverse traveling waves with E and B vectors perpendicular to each other. Both vectors are perpendicular to the propagation direction. (3) E and B form electromagnetic waves in free space and travel at the speed of light c. From this Maxwell predicted that light is just an electromagnetic waves. (4) Average power of E&M waves per unit area:
Emax Bmax c 2 1 2 I= Emax = Bmax = 2cµ 0 2µ 0 2µ 0 (5) Radiation momentum p delivered by E&M waves perpendicular incident into surface is: 2U U • for total absorption: p = • for total reflection: p = c c where U is the incident energy into the surface.
Example: Sunlight just outside Earth’s atmosphere has an intensity of 1.4 kW/m2. What is the Emax and Bmax ? Emax = 2cIµ 0 = 2 • 3 • 108 • 1.4 • 103 • 4π • 10 −7 = 1027V/m
Bmax =Emax/c=1027/(3•108)=3.42•10-6 T This numerical result shows that electric field is far more important than The magnetic field for the light wave. Wavelength of Electromagnetic Waves Since E&M waves is resulted from the oscillation of electric and magnetic waves, the relation between its speed, frequency and the wavelength should be similar to that of sound waves, i.e. speed = wavelength × frequency. ∴ c = fλ The scale of wavelengths of known E & M waves and the visible is shown on the next slide.
V B G Y
R
Question: If human beings are most sensitive to 550 nm greenyellow light, it can explain why we use them in the traffic light. But human beings are not that sensitive to the red light. Why we use red light then?
