Physics for Poets Lecture 2

Gaurang Yodh

Development of quantum theory (and relativity) was in response to ExperimentalObservations that defied explanation in terms of Classical Physics (Newton's Lawsand Electro-Magnetic (EM) theory of Maxwell of the 19th century).

These were based on experiments which explored behavior of matter and radiation at the atomic or nuclear scales and speeds near the speed of light, not previously studied.

Experiments needed new techniques to be developed.

The three most important technical developments were:

1. Photographic film to detect “ light “ - either visible, UV or X-rays:

1839: Daguerre made “film” by coating Copper with Silver Chloride( AgCl on Cu)

1895: George Eastman Kodak – created emulsion which contained AgCl in it and needed no metallic backing.

This led to the detection of

X-rays (EM waves of very short wavelengths) and

Discovery of radioactivity.

2. Invention of Spark Coils, Which converted low voltage to high voltage which couldthen be applied across two electrodes, specifically Ruhmkorff Coils.

This made possible:

- - the technique of generating and receiving radio signals – Marconi

- - Roentgen's discovery of X-rays

Spark coil

Light from ionized air atoms

3. Invention of vacuum pumps to evacuate air from space of various kinds. 1657 von Guericke 1850 Geissler Mercury pump.

These three inventions made possible:

--- Cathode ray (modern version TV) tubes. Crookes tubes --- Method of producing X-rays.--- Dewar vessels to keep things cold.

MILESTONES OF EXPERIMENTS WHICH LEAD TO STUDY OF THE PHYSICS OF THE SMALL AND FAST !

Roentgen 1894: Discovered X-rays: He discovered EM radiationof wavelengths of atomic size – which could not be observedby our physical senses.

He used a Crookes tube (evacuated with a pump) to accelerate electrons to high speed and energy. He used spark coils to generate highvoltage to get the electrons up to high energy. And a type of photo plateto make observations.

1. DISCOVERY OF X-RAYS

These high energy electrons struck glass walls and produced a radiationwhich he accidentally discovered in watching a Barium Platinocyanide coveredplate fluoresce. This was even true if enclosed his apparatus in a light tightbox.

Wavelength of X-rays is of the order of atomic size or smaller. The quantum structure of the atom explained emission of X-ray lines.

He found this radiation was emitted from the material in the wall and it was penetrating – he took an X-ray picture of his own hand.

His apparatus quickly transformed into a commercial device to be able to see bone fractures ..

And he got the first Nobel Prize.

In 1912, Max von Laue diffracted X-raysfrom crystals - showing that (1) their wavelength was very short and(2) they were EM radiation or waves.

Bragg Measured their wavelength (also Debye and Scherrer)

X-rays are neutral penetrating radiation.

In 1954 Watson, Crick and Franklin determined the helical structureof DNA using X-rays.

Crucial X-ray picture taken by Rosalind Franklin

What was known about atoms around 1900?

1. They were small2. They were neutral as a whole3. Atom parts must have positive and negative charges4. Metals (e.g. Wires) could carry electric current.5. Negative charges could be rubbed off glass by silk !6. Sparks involved these elementary charges producing currents.7. Light could emit negative charges from certain metals:

Photo-electric effect. 8. Strong electric fields could extract negative charges from metal points.9. Electrolysis (Faraday) involved positive and negatively charged atoms or molecules.

Time was ripe to find out the fundamental properties of these charges - their charge, their mass and their size.

2. THE ELECTRON:

J.J.Thomson Established electron as a fundamentalparticle of nature. He measured its charge to massratio using a Crooke's tube.

Animation of electrons movingand being deflected by an electricor magnetic field. The green blob is a bunch of electrons producing light by exciting the phosphor atoms.

Electric current = flow of electrons

Crooke's tube:Evacuated tubeVisualization of electron beam. Size of electron is < 10^(-18) m

electric field

Qeme

Thomson's cathode ray tube

A TV tube schematic

The charge on an electron was measured by Robert Millikan (at Chicago and laterat Caltech): The oil drop experiment : Q_e

Qe or more commonly just called eQe also called just e

Fine uncharged drops of oil were allowed to fall under gravity and watched through a viewer through which he could see one oil drop fall. Then using X-rays oil drops were charged up and again watched. But now there was an electric field which could be adjusted until the drop was steady. The electric field needed depended on the charge generated by X-rays which was on the drop. Millikan found that the charge on the oil drop was always a multiple of a smallest charge; This was the charge of the electron. He found a value of e = - 1.6 x 10^(-19) Coulombs

DROP COULD NEVER GET A FRACTIONAL CHARGE, E.G. 5/3 Q

Negative sign says that the charge was attracted to the positive plate.

Here m is mass of oil drop, g is acc. Dueto gravity. If the drop is at rest

Q = mg/E

3. Discovery of Radioactivity: Spontaneous transformations of elements.

Henri Becquerel studied production of light by means other than heat.He accidentally discovered Radioactivity.He found that materials which contained high Z elements like Uranium caused photographic plates to become exposed even if they were completely wrapped in light tight covers. Some radiation was beingemitted from U : Radiation was emitted from his sample.

Marie and Pierre Curie isolated elements which possessed this activity by chemical processes, discovered and isolated Radium and Polonium So we had Uranium, Radium and Polonium – elements which emittedradiation. The rate of emission was not influenced by pressure, temperaturebut was an intrinsic property of these elements called Radioactivity.Emission occurred at random times but a well defined average rate .

Earnest Rutherford and Fredrick Soddy quantitatively studied emissions fromradioactive substances – found three types : alpha, beta and gamma emissions.They derived the rates of decay of parent nuclei and growth of daughter nuclei.

They found that alpha particles were emitted with well defined energies (nuclei of Helium atoms) and were positively charged.Beta particles were electrons in the materials they studied, which had a rangeof energies in contrast to alphasGamma particles were neutral and were EM radiation.

Example:

U(238) Th(234) + alpha(4)

Z = 92 90 2

Radioactivity involved transmutation of elements

When would a radioactive atom transmute or emit radiation was not predictable, but probability of decay was random, althoughone could predict the rate of decay. Half life.

It provided the first atomic “ bullets “ with sufficient energy to explorestructure of other atomic objects. Alpha Scattering off gold foils.

It was a nuclear property and could not be influenced by external forces or conditions.

4. Determining the Structure of the Atom:

Rutherford used energetic atomic projectiles – the alpha particles to probe the structure of atoms: He studied how the alpha particles were scattered by the gold atoms.

http://www.waowen.screaming.net/revision/nuclear/rsanim.htmlhttp://www.waowen.screaming.net/revision/nuclear/rsanim.htm

Experimental Set up

1. Very thin gold foil – few atomic layers thick2. Source of high energy alpha particle (Radium source) 3. Collimated beam4. After “ scattering “ detect scintillations on the detecting screen (by Eye !!)

Scattering of Alpha Particles by a Gold foil: Rutherford Experiment

Alpha particles is a helium atom devoid of its two electrons, hence it is positively charged with a charge = + 2e , where e is the charge on an electron.

The beam used by Rutherford was alpha particles from radioactive decay of elementlike Radium or a gas like Radon. These particles are emitted from the nucleus withabout 2 MeV (Million electron volts) of energy.

In unit of joules: 1 eV of energy is = 1.6×10−19 Joules Hence 1 MeV = 106eV=1.6×10−13 Joules

This is the kinetic energy of the alpha particle = 12

mv

2

Mass of −particle : M~4×Mproton=6.68×10−27 kg

The speed of this alpha particle is v=1.4×107m/s or 0.046c where c is the speed of light = 3×108m /s.

Physical size of alpha particle is small enough to probe into the gold atom.

As alpha particle has a +2e charge it can interact with charges in the goldatom:

negatively charged electrons which are much lighter that alpha particlesor positive charge associated with the protons inside the gold atom.

Alpha particles will be barely deflected by electrons as the electrons are muchlighter than alpha particles. Alpha particles will be deflected by positively charged protons in the gold nucleus.

To probe into the gold atom, alpha particle has to have sufficient energy to overcomeany electric repulsion due to the positive charges.

Plum pudding model of atom is expected to produce only small deflections of thealpha particles.

But if the positive charges were concentrated in a small massive object – the nucleusthen when alpha particle has a head on collision with the much more massive gold nucleus it can bounce backwards. This was what was observed by Rutherford and colleagues in Manchester. They were able to explain what theyobserved in terms of collisions of + charged alpha particles with a + charged nucleus of the gold atom.

From his experiment Rutherford could establish that:

1. Atoms have a positively charged nucleus

2. Charge on the nucleus was equal in magnitude to the total number of electrons in the atom, and was positive

3. Size of the nuclear diameter was 10,000 times smaller than that of the atom.

Thus the atom was planetary in design, the electrons whirling around the nucleus and the atom was mostly empty space !!

In such a planetary atom, classical physics predicted that the electronswould radiate away all their energy and spiral into the nucleus in 100 millionthof a second !! The atom would be unstable......

http://www.waowen.screaming.net/revision/nuclear/rsanim.htm

http://www2.biglobe.ne.jp/~norimari/science/JavaApp/e-Scatter.html

In a nuclear atom, the electrons would spiral down into the nucleusin less than a microsecond and atom would collapse if classical physics applied !

5. The spectra of light emitted by atoms:

If you heat atoms to high temperature they emit light – note the yellowlight if you burn paper – or the bluish light of gas burning in your kitchen.

Using a prism you can decompose light into the colors it contains.

These colors extend over a range of wavelengths (or equivalently frequencies)And the intensity distribution as a function of wavelength is called the spectrum.

If you study the spectrum of EM radiation emitted by a particular atom, you should learn something about its structure – as EM radiation is emitted byaccelerated electrons.

In the 19th century study of spectra of substances became a very important field of study and they were measured in exquisite detail.

Each element emits its own characteristic spectrum – distinct from that of allothers.

A spectrum of hydrogen lines : Emission by H in a tube excited byhigh frequency electrical discharge....

To make nuclear particle tracks become visible, thick emulsions loaded withsilver chloride were developed (in the states by Eastman).

I show some tracks and interactions of atomic particles in the slides on the next page.

You do not “ see “ the particle, you see only the developed grains of silveralong the track – these dark spots have an intrinsic size of a micron (or a millionthof a meter).

The pictures show tracks are of high energy cosmic rays colliding with an emulsion nucleus and spraying out other nuclear particles.This is from about 1950 (C.F. Powell and colleagues in Bristol)

NUCLEAR EMULSIONS AND PARTICLE DETECTION

Tracks of charged particles (nuclear size or smaller)

Cosmic rayNucleus ofa high Znucleus

Energetic nucleus colliding withan emulsionnucleus