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Black-body Radiation & the Quantum Hypothesis

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Presentation on theme: "Black-body Radiation & the Quantum Hypothesis"— Presentation transcript:

1 Black-body Radiation & the Quantum Hypothesis
Max Planck Physics 100 Chapt 20

2 Black-body Radiation l peak = 2.9 x 10-3 m T(Kelvin) UV IR
Light intensity UV IR

3 lpeak vs Temperature T l peak = 3100K 58000K 2.9 x 10-3 m T(Kelvin)
(body temp) 2.9 x 10-3 m 3100 =9x10-6m infrared light 58000K (Sun’s surface) visible light 2.9 x 10-3 m 58000 =0.5x10-6m

4 “Room temperature” radiation

5 Photo with an IR camera

6 IR Cat

7 IR house

8 Theory & experiment disagree wildly
the UV catastrophe Theory & experiment disagree wildly Pre-1900 theory

9 Planck’s solution h = 6.6 x 10-34 Js “Planck’s constant”
EM energy cannot be radiated or absorbed in any arbitrary amounts, but only in discrete “quantum” amounts. The energy of a “quantum” depends on frequency as Equantum = h f h = 6.6 x Js “Planck’s constant”

10 Other “quantum” systems

11 The quantum of the US monetary system
We don’t worry about effects of quantization Because the penny’s value is so small

12 Suppose the quantum were a $1000 bill
A quantum this large would have an enormous effect on “normal” transactions

13 The quantum of the US Income tax system

14 US Income tax with a $1 quantum
Number of taxpayers

15 US Income tax with a $1000 quantum
Number of taxpayers All these guys don’t have to pay anything Quantum effects are negligible to these taxpayers Quantum effects are huge to these guys

16 How quanta defeat the UV catastrophe
Without the quantum With the quantum Low frequency, small quantum, Negligible effects high frequency, large quantum, huge effects

17 Planck’s quantum is small for “ordinary-sized” objects but large for atoms etc
pendulum f = 1 Hz Hydrogen atom f  2x1014 Hz about the same as the electron’s KE Equant= hf =(6.6x10-34Js)x(2x1014Hz) Equant= hf =6.6x10-34Jsx1Hz =(6.6 x 2) x J very tiny =6.6x10-34J =1.3 x 10-19J

18 Typical energies in “ordinary” life
Typical energy of a tot on a swing: Etot = mghmax = 20kgx10m/s2x1m = 20kgx10m/s2x = 20kgx hmax = 200 kgm2/s2 = 200 J much, much larger than Equant=6.6x10-34J

19 Typical electron KE in an atom
Energy gained by an electron crossing a 1V voltage difference 1 “electron Volt” Energy = q V - - - 1eV = 1.6x10-19C x 1V 1V = 1.6x10-19 Joules similar Equant = 1.3 x 10-19J for f  2x1014 Hz

20 Classical vs Quantum world
At atomic & subatomic scales, quantum effects are dominant & must be considered In everyday life, quantum effects can be safely ignored Laws of nature developed without consideration of quantum effects do not work for atoms This is because Planck’s constant is so small

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