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

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Presentation on theme: "Black-body Radiation & the Quantum Hypothesis Physics 100 Chapt 20 Max Planck."— Presentation transcript:

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

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

3 peak vs Temperature peak = 2.9 x 10 -3 m T(Kelvin) T 310 0 K (body temp) 2.9 x 10 -3 m 310 0 =9x10 -6 m 5800 0 K (Suns surface) 2.9 x 10 -3 m 5800 0 =0.5x10 -6 m infrared light visible light

4 Room temperature radiation

5 Photo with an IR camera

6 IR Cat

7 IR house

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

9 Plancks solution 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 E quantum = h f h = 6.6 x 10 -34 Js Plancks constant

10 Other quantum systems

11 The quantum of the US monetary system We dont worry about effects of quantization Because the pennys 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 All these guys dont have to pay anything Number of taxpayers Quantum effects are negligible to these taxpayers Quantum effects are huge to these guys

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

17 Plancks quantum is small for ordinary- sized objects but large for atoms etc ordinary pendulum f = 1 Hz Hydrogen atom f 2x10 14 Hz E quant = hf=6.6x10 -34 Jsx1Hz =6.6x10 -34 J E quant = hf =(6.6x10 -34 Js)x(2x10 14 Hz) =(6.6 x 2) x 10 -34+14 J =1.3 x 10 -19 J very tiny about the same as the electrons KE

18 Typical energies in ordinary life Typical energy of a tot on a swing: Etot = mgh max h max = 20kgx = 200 kgm 2 /s 2 = 200 J much, much larger than E quant =6.6x10 -34 J = 20kgx10m/s 2 x= 20kgx10m/s 2 x1m

19 Typical electron KE in an atom 1 electron Volt Energy gained by an electron crossing a 1V voltage difference 1V -- - Energy = q V 1eV = 1.6x10 -19 C x 1V = 1.6x10 -19 Joules E quant = 1.3 x 10 -19 J similar for f 2x10 14 Hz

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


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