Lecture18 2013
- 1. Lecture 18 • Maxwell’s equations and electromagnetic waves: propagation and generation •Energy transport by electromagnetic fields: the Poynting vector •Momentum transfer by absorption or reflection of light: radiation pressure
- 2. Midterm #2: Thursday, November 14 • Exam is in Galileo •8:10-9:25: Arrive early so you can begin on time •Cover sheet posted next week. •Focus on material since Midterm #1: from magnetic fields and forces to electromagnetic waves! •Review/practice in recitations 11/13
- 3. Closer look: how Faraday’s Law and Ampère’s Law govern propagation of light: (Faraday’s Law) (Ampère’s Law) http://www.amanogawa.com/archive/PlaneWave/PlaneWave.html
- 4. Wave equation is linear in EM field: superpositions of waves are waves, too Standing waves: Individual traveling waves have E, B in phase with each other… but the resulting standing waves have E, B offset from each other! http://webphysics.davidson.edu/applets/Superposition/GroupVelocity.html
- 5. Generation of electromagnetic waves: accelerating charges http://webphysics.davidson.edu/applets/retard/Retard_FEL.html
- 6. Energy transport by electromagnetic fields Energy density in any region with electric and magnetic fields: EM traveling wave? ∝ Poynting vector gives carried by wave, and direction that power is being delivered
- 7. Poynting vector isn’t just for EM waves! Example: A current i runs through a uniform cylindrical resistor R with radius a and length . Use the Poynting vector to find the electromagnetic power delivered to the resistor. a ℓ i
- 8. EM waves: time-averaged power and intensity is / / B Time-average over many cycles? “intensity”= Savg =
- 9. Example: HMC quantum optics pump laser Example: The pump laser used in the HMC quantum optics lab is a 50-mW violet diode laser (wavelength 405nm). If it is focused to a circular spot with an effective diameter of 2mm, what is the intensity of the light? What is the maximum electric field amplitude?
- 10. Radiation pressure: momentum transfer on absorption or reflection of light is / Light carries momentum = / Radiation pressure:
- 11. Example: IKAROS Interplanetary Kite-craft Accelerated by Radiation Of the Sun Example: The IKAROS mission (2010-2012) achieved the first demonstration of radiation pressure propulsion in interplanetary travel. The IKAROS spacecraft had a mass of 307kg and a trapezoidal solar sail with a 20m diagonal, as shown. What acceleration did it achieve due to radiation pressure?
- 12. IKAROS attitude control via reflectance control
- 13. Summary •Poynting vector gives power/area delivered by EM fields •EM fields of light oscillate fast: usually deal with time-averaged power and intensity of light •Radiation pressure: Force/area on absorber = Force/area on reflector =