ChargedparticlesandQuantumphysics.ppt

(1)

Quantum

Physics

(2)

Quantization of charge

• JJ Thomson



measured specific charge = q/m

• R Millikan

(3)

(4)

(5)

J.J. Thomson: Finding e/m

1. F_B = F_C

2. Bqv = mv2/r

 v2 = B2r2q2/m2

3. qV_A = ½mv2

 v2 = 2qV

A/m

B = Magnetic Field (T)

r = radius of path of particles with B-field only (m) V_A = accelerating potential

for particles (V)

v = velocity of particles (ms-1)

Combining these equation to eliminate v:

(6)

Specific Charge, q/m

Alpha particle

= 2.41 x 107 Ckg-1

Electron

= 1.76 x 1011 Ckg-1

Proton

= 9.58 x 107 Ckg-1

These distinct

‘particles’ were known to be different because of their ‘specific

charge’.

Millikan’s experiment enabled the charge

(7)

(8)

Millikan’s Oil-Drop

Experiment

http://omniknow.com/scripts/wiki.php?term=Oil-drop_experiment

(9)

F_G is balanced by F_E

m

g

=

e

V/d

q

= m

g

d/V

Millikan’s Oil-Drop Experiment

Weight F_G = mg

(10)

Millikan’s Oil-Drop Experiment

Robert Millikan’s

(11)

Conclusions

• Charge exists in

quanta

(small discrete units)

• The smallest

quanta

= 1.6 x 10

-19

C

(12)

Measuring

e

Blue Box

(

Page 62,

(13)

• Max Planck

 Planck’s constant, h

• Maxwell

 speed of light , c

• Einstein

 E = mc2

• De Broglie (matter waves)

  = h/mv = h/p

(14)

(15)

The Photo-electric Effect

• Known to Hertz

• Explained by Einstein …

• …who was disbelieved by Millikan

• Further evidence then found by Millikan • Light comes in small discrete packets of

energy.

(16)

The Photo-electric Effect

(17)

(18)

photoelectric effect.swf

(19)

(20)

Photocurrent v Intensity

(21)

The Classical (Wave) Theory

Could not explain why:

• KE of electrons is independent of light intensity

• KE_max depends on light frequency

• The threshold frequency is different for different materials

(22)

Einstein’s Explanation

• The photon • E = h

as suggested by Max Planck

(h = Planck’s constant = 6.6 x 10-34Js)

  = c = 3.0 x 108 ms-1

as suggested by Maxwell

h

 =



+ (½ mv

2 )

(23)

Einstein’s Explanation

h



=



+

(½ mv

2

)

Potential well

(24)

Einstein’s Explanation

h



=



+ (½ mv

2 )

max

• _h



_{= Incoming energy of photon}



= ‘

work function

’ of material

• (½ mv

2

)

max

= maximum KE of

emitted electron

(25)

Worked Example (Duncan,

p392

)

Threshold wavelength for a material =

0.65m (650nm). Calculate the:

(i) Threshold frequency,  =

(ii) Work function,  =

(iii) Maximum speed of electrons emitted by

violet light ( = 400nm) =

₀ = c/₀ = 3.0 x 108 / 650 x 10-9 = 4.6 x 1014Hz

(26)

The

electron-volt

1eV

=

the work done (in joules) on an

electron when accelerated across a

potential difference of 1 Volt

= 1.6 x 10

-19

J

e.g. 3.0eV = 4.8 x 10

-19

J

(27)

(28)

Questions

SAQs 16.3 – 16.9

(Physics 1, pages 137-8) SAQs 16.10 – 16.12

(Physics 1, page 141)

Duncan, Advanced Physics:

(29)

Evidence – Atomic Line Spectra

(30)

Emission Spectra

Argon Iodine

Mercury

(31)

Atomic Line Spectra

Line spectra.swf

(32)

Atomic

Line

Spectra

(33)

(34)

(35)

(36)

Hydrogen Emission Spectrum

Lyman series 

Ground state (n = 1)

Balmer  n = 2

(37)

(38)

(39)

(40)

Electron Diffraction

(41)

(42)

The De Broglie Equation



=

h/mv

= h/p

Matter waves

What is your personal

(43)

De Broglie Equation

• SAQs 16.13 (Page 142, Physics 1)

• Questions (Page 143, Physics 1)

• Questions 23 – 29 (Pages 410 – 411,

(44)

(45)