Electricity and Magnetism02 MIT公开课 电磁学要点.pdf

Electricity and Magnetism • Review for Quiz #3 – Exp EB – Force in B-Field • on free charge • on current – Sources of B-Field • Biot-Savart • Ampere’s Law – Exp MF – Magnetic Induction • Faradays Law • Lenz’ Rule Apr 5 2002 Electrical Breakdown • Need lot’s of free charges • But electrons stuck in potential well of nucleus U • Need energy ∆U to jump out of well • How to provide this energy? e- ∆U Apr 5 2002 Impact Ionization e- Ukin ∆U E • Define Vion = ∆U/q Ionization potential - - • One e in, two e out • Avalanche? E Apr 5 2002 Impact Ionization (2) (1) λmfp e- λmfp : Mean Free Path E • To get avalanche we need: ∆Ukin between collisions (1) and (2) Vion * e • Acceleration in uniform Field ∆U = V – V = e E d kin 2 1 12 • Avalanche condition then E Vion /λmfp Apr 5 2002 Impact Ionization How big is Mean Free Path? (i) If Density n is big - λmfp small (ii) If size σ of molecules is big - λmfp small λmfp = 1/(n σ ) Avalan