Physics 22: 1-7 (All) Electromagnetic Radiation
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 22: 1-7 (All)
- Section 1 Maxwell's equations sum up the relationship between charge and magnetism.
- Section 2 Because changing electrical fields produce changing magnetic fields, and changing magnetic fields produce changing electrical fields, a wire with changing current will produce electromagnetic waves that propagate indefinitely. The electromagnetic wave derives from the E and B changing fields, acting at right angles to each other.
- Section 3 Theoretically, electromagnetic waves range from 0 to infinity in length, or from 0 to infinite frequency. In practice, we can detect EM waves in the range from less than 60 Hz (60 cycles/second, wavelength around 5 million meters) to greater than 1020 Hz (less than 3 * 10-12 m). Visible light lies in the range 4-7.5*1014 Hz, or 7.5-4*10-7 m. The velocity of EM waves depends on the of the electrical field in a medium and the of the magnetic field to that medium.
- Section 4 Maxwell realized that the speed of EM predicted by the permittivity/permeability values matched Roemer's speed of light. Michelson's measurements for the speed of light were more accurate than Roemer's; the value is now given as 2.99792458 * 108m or 3 * 108m/s for ease of use.
- Section 5 A light wave carries energy, so the space through which light is propagated has a potential energy state u = 1/2 ε0E2 + 1/2 B2/μ0. Because B = E/c, we can write the B field contribution in terms of E (u = ε0E2) or the E field contribution in terms of B (u = B2/μ0), or as a single term: u = √[BE*(ε0/μ0)].
We can also evaluate the intensity of the energy: I = power/area = (energy/time) / area = (ΔU/Δt) /A
- Section 6 Since EM waves carry energy (U = E = 1/2mv2), they carry linear momentum (p = mv), and therefore have a related force (F = Δp/Δt) which can be analyzed in terms of pressure (P = F/A). This radiation pressure is the amount of energy transferred when a beam of light is completely absorbed by an object: Δp = ΔU/c.
- Section 7 A key fundamental law is that waves add up. We can put one EM signal (information content from a program) on top of another signal (a signal of constant frequency called the carrier signal). Instruments can detect and isolate wave combinations identified carrier signals, which makes radio and TV possible.
- Displacement current:
- Ampère's law using Electric Flux:
- Speed of light, permeability and permissivity:
- Energy carried by EM Wave:
- Radiation Pressure
Read the following weblecture before chat: Maxwell's Elegant Laws
Use the simulation for Gauss's Law to visualize electric flux through a sphereical boundary.
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