physics 2d lecture slides · 2003. 10. 31. · saw what light does, now examine matter •...
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Physics 2D Lecture SlidesOct 31
Vivek SharmaUCSD Physics
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Quiz 4 : Good Show !
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The 18 “Commandments” of this Universe
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Saw what light does, Now examine matter
• Fundamental Characteristics of different forms of matter – Rest Mass (m) – Electric Charge (q)
• Measurable – using some combination of E & B fields interacting
with the particle
– Or E/B and some other macroscopic forcee.g. Drag Force
The “magic” is that one is measuring tiny tiny numbers using Macroscopic devices
( )F q E v B= + ×
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Thomson’s Determination of e/m of the Electron
• In E Field alone, electron lands at D• In B field alone, electron lands at E• When E and B field adjusted to canceleach other’s force electron lands at F
e/m = 1.7588 x 1011 C/Kg
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Millikan’s Measurement of Electron Charge
Find charge on oil drop is always in integral multiple of some Qqe = 1.688 x 10-19 Coulombs
me = 9.1093 x 10-31 KgFundamental properties (finger print) of electron (similarly can measure proton properties etc)
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Where are the electrons inside atoms?Early Thought: “Plum pudding” model Atom has a homogenous
distribution of Positive charge with electrons embedded in them(atom is neutral)
e-
e-
e-e-
e-
e-
e-
e- e-
e-
e-e-
e-e-
e-e-
e-
e-
e-
Positively charged matter + Core
• How to test these hypotheses? Shoot “bullets” at the atom and watch their trajectory. What Kind of bullets ? •Indestructible charged bullets Ionized He++ atom = α++ particles •Q = +2e , Mass Mα=4amu >> me , Vα= 2 x 10 7 m/s (non-relavistic) [charged to probe charge & mass distribution inside atom]
?
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Plum Pudding Model of Atom
• Non-relativistic mechanics (Vα/c = 0.1)• In Plum-pudding model, α-rays hardly scatter because
– Positive charge distributed over size of atom (10-10m)– Mα >> Me (like moving truck hits a bicycle)– predict α-rays will pass thru array of atoms with little scatter (~1o)
Need to test this hypothesis Ernest Rutherford
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Probing Within an Atom with α Particles
• Most α particles pass thru gold foil with nary a deflection• SOME (≅10-4) scatter at LARGE angles Φ• Even fewer scatter almost backwards Why
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“Rutherford Scattering” discovered by his PhD Student (Marsden)
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Rutherford Discovers Nucleus (Nobel Prize)
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Force on α-particle due to heavy Nucleus
2
particle trajectory is hyperbolicScattering angle is related to impact par.
Impact Parameter cot2
kq Qbm v
α
α α
α
θ⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠
•Outside radius r =R, F ∝ Q/r2
•Inside radius r < R, F ∝ q/r2 = Qr/R2
•Maximum force at radius r = R
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Rutherford Scattering: Prediction and Experimental Result2 2 4
22 2 414 ( / 2)
2
k Z e NnAnR m v Sinα α ϕ
∆ =⎛ ⎞⎜ ⎟⎝ ⎠
•# scattered Vs φ depends on :•n = # of incident alpha particles•N = # of nuclei/area of foil•Ze = Nuclear charge• Kα of incident alpha beam•A= detector area
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Rutherford Scattering & Size of Nucleus
2
distance of closest appoach r size of nucleus1Kinetic energy of = K = 2
particle will penetrate thru a radius r until all its kinetic energy is used up to do work AGAINST the Coulomb potent
m vα α βα
α
∝
( ) ( )
-
Al2
15
2
15
-10
2
For K =7.7.MeV, Z 13
2
ial of the
Size of Nucleus = 10Siz
Nucleus:21 K = 8
2
4.9
e of Ato
m = 1
10
0
2
kZ
Ze em v MeV k
er mK
kZerK
mm
r
α
α
α β
α
α
−
=
⇒ =
=
×
=
=⇒
=
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Spectrum of Light and Structure Within Atom
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Emission Line Spectrum of Elements
•Emission line appear dark because of photographic exposure
Absorption spectrum of NaWhile light passed thru Na vaporis absorbed at specific λ
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Kirchhoff’ Experiment : “D” Lines in Na
D lines darken noticeably when Sodium vapor introduced Between slit and prism
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Discharge Tube
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Spectrum of LightContinuous
Blackbody SpectrumDiscrete Emission Line Spectrum of elements
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Hydrogen Spectrum
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Distribution of electron Inside Atoms ?
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Distribution of electron Inside Atoms ?