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.
Chat Preparation Activities
- Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
If you want lab credit for this course, you must complete at least 12 labs (honors course) or 18 labs (AP students). One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!
© 2005 - 2018 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.