Chapter 38. The End of Classical Physics€¦ · · 2009-04-08Chapter 38. The End of Classical ... form that travel in straight lines but are easily blocked by solid objects

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.

Chapter 38. The End of Classical Physics Studies of the light emitted by gas discharge tubes helped bring classical physics to an end. Chapter Goal: To understand how scientists discovered the properties of atoms and how these discoveries led to the need for a new theory of light and matter.


Topics: •  Physics in the 1800s •  Faraday •  Cathode Rays •  J. J. Thomson and the Discovery of the

Electron •  Millikan and the Fundamental Unit of Charge •  Rutherford and the Discovery of the Nucleus •  Into the Nucleus •  The Emission and Absorption of Light •  Classical Physics at the Limit

Chapter 38. The End of Classical Physics


Physics in the 1800s

In 1800 Volta invented the battery, and then immediately discovered that an electric current through water decomposes the water into hydrogen and oxygen, a process called electrolysis.



Electrical Conduction in Gases

In the 1820s, Faraday showed that 1. Current flows through a low-pressure gas, creating an

electric discharge. 2. The color of the discharge depends on the type of gas in

the tube. 3. Regardless of the type of gas, there is a separate, constant

glow around the cathode.


Cathode Rays

In the 1850s it was found that a solid object sealed inside a Faraday tube casts a shadow on the glass wall. This discovery suggested that the cathode emits rays of some form that travel in straight lines but are easily blocked by solid objects. These rays were dubbed cathode rays. We now know that cathode rays are high speed electrons.


Thomson’s Crossed-Field Experiment

•  In 1895 Thomson measured the deflection of cathode-ray particles by both a magnetic and electric field.

•  Parallel-plate electrodes and the poles of a magnet were placed so that the electric and magnetic fields

were perpendicular to each other, thus creating what came to be known as a crossed-field experiment.

•  Thomson was the first to measure the charge-to-mass ratio q/m of cathode rays (electrons).

•  q/m = 1.76 × 1011 C/kg


F=0 gives v if E and B known.

Only e/m determined from deflections.


Millikan and the Fundamental Unit of Charge • Millikan observed oil droplets in an electric field. • He found that some of his droplets were positively

charged and some negatively charged, but all had charges that were integer multiples of a certain minimum charge value.

•  That value, the fundamental unit of charge that we now call e, is measured to be

• We can then combine the measured e with the measured charge-to-mass ratio to find that the mass of the

electron is



EXAMPLE 38.2 Suspending an oil drop

QUESTION:


EXAMPLE 38.2 Suspending an oil drop



Rutherford and the Discovery of the Nucleus •  In 1896 Rutherford’s

experiment was set up to see if any alpha particles were deflected from gold foil at large angles.

• Not only were alpha particles deflected at large angles, but a very few were reflected almost straight backward toward the source!



The Electron Volt

• Consider an electron accelerating (in a vacuum) from rest across a parallel plate capacitor with a 1.0 V potential difference.

•  The electron’s kinetic energy when it reaches the positive plate is 1.60 × 10−19 J.

•  Let us define a new unit of energy, called the electron volt, as 1 eV = 1.60 × 10−19 J.


EXAMPLE 38.5 Energy of an electron

QUESTION:





This is the negative of the energy to remove an electron.


Into the Nucleus •  The atomic number Z of an element describes the

number of protons in the nucleus. Elements are listed in the periodic table by their atomic number.

•  There are a range of neutron numbers N that happily form a nucleus with Z protons, creating a series of nuclei having the same Z-value but different masses. Such a series of nuclei are called isotopes.

• An atom’s mass number A is defined to be A = Z + N. It is the total number of protons and neutrons in a nucleus.

•  The notation used to label isotopes is AZ, where the mass number A is given as a leading superscript. The proton number Z is not specified by an actual number but, equivalently, by the chemical symbol for that element.



The Emission and Absorption of Light

Hot, self-luminous objects, such as the sun or an incandescent lightbulb, form a rainbow-like continuous spectrum in which light is emitted at every possible wavelength. The figure shows a continuous spectrum.


The Emission and Absorption of Light

The light emitted by one of Faraday’s gas discharge tubes contains only certain discrete, individual wavelengths. Such a spectrum is called a discrete spectrum. Each wavelength in a discrete spectrum is called a spectral line because of its appearance in photographs such as the one shown.


Blackbody Radiation The heat energy Q radiated in a time interval Δt by an object with surface area A and absolute temperature T is given by

where σ = 5.67 ×10−8 W/m2K4 is the Stefan-Boltzmann constant. The parameter e is the emissivity of the surface, a measure of how effectively it radiates. The value of e ranges from 0 to 1. A perfectly absorbing—and thus perfectly emitting—object with e = 1 is called a blackbody, and the thermal radiation emitted by a blackbody is called blackbody radiation.


The wavelength of the peak in the intensity graph is given by Wien’s law (T must be in kelvin):

Blackbody Radiation


EXAMPLE 38.7 Finding peak wavelengths

QUESTIONS:








Discrete Spectra

• Not only does low-density gas emit discrete wavelengths, but it also may absorb discrete wavelengths.

• Every wavelength absorbed by the gas is also emitted, but not every emitted wavelength is absorbed.

•  The wavelengths in the hydrogen spectrum can be represented by the Balmer formula


Classical Physics at the Limit

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Chapter 38. The End of Classical Physics€¦ · · 2009-04-08Chapter 38. The End of Classical ... form that travel in straight lines but are easily blocked by solid objects