4. The discovery of X-rays and electrons 4.1 Gas discharges

Size: px

Start display at page:

Download "4. The discovery of X-rays and electrons 4.1 Gas discharges"

Megan Atkinson
6 years ago
Views:

1 4. The discovery of X-rays and electrons 4.1 Gas discharges 19 th century: knowledge of charged atoms/molecules electrolysis discharges of rarefied gases (vacuum). near cathode: glow charge, cathode rays charged molecules? ether waves? UV light?

2 various tubes used for producing cathode rays

3 Röntgen,1895: as cathode rays impact on glass walls of tube, other rays are produced, penetrating glass walls and air. detection with fluorescent paper. no deflection by magnet. Penetration of thick bodies, absorption dependent on density of matter. Frau Roentgen s hand (1895), improved image (Albert von Kollicker, 1896).

4 X-rays: ultraviolet light, produced when cathode rays are stopped JJ Thomson and Wiechert: cathode rays: have charge to mass ratio 2000 times that of Hydrogen ion. corpuscular (particle) nature of the cathode rays: electrons (Stoney).

5 4.2 X-ray spectra X-rays: ultraviolet light, produced when cathode rays are stopped. minimum wavelength of X-rays (maximum energy) given by: ev accelerating hf max hc min Continuous x-ray spectrum produced by electrons impacting on anode known as bremsstrahlung (see YF Chapter 38.7) Impacting electrons can also knock out tightly bound inner electrons of the anode material. Refilling holes with less tightly bound electrons leads to emission of x-ray photons, and characteristic sharp spikes in spectra.

6 (YF Figures & 41.20) hc min ev accelerating Continuous x-ray spectrum from tungsten anode, as accelerating voltage increased X-ray spectrum from Molybdenum anode showing continuous spectrum and characteristic x- rays superimposed on spectrum. Spikes do not change with voltage

4.3 Thomson s experiment http://www2.kutl.kyushu-u.ac.

7 4.3 Thomson s experiment (YF 27.21)

8 accelerating potential V: thus v 2qV / m ; electrons strike screen at end of tube; path of electrons controlled by applied electric and magnetic fields; condition for a straight line: qvb = qe; F qe v E / B 2qV / m Experiments: single value for q/m, independent of material of cathode particles detected (electrons) are a common constituent of all matter. E kin qv 1 2 B 2 mv qv q m 2 E 2VB 2

9 Result: e/m = (71) C/kg value of e determined by Millikan 2007: e= (46) C electron mass m e = kg hydrogen atom: = kg mh Adaptations of Thomson s apparatus: mass-spectrometers

10 JJ Thomson: "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? -- which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science. (Recording made in From the soundtrack of the film, Atomic Physics copyright J. Arthur Rank Organization, Ltd., 1948.)

11 4.4 Millikan s oil drop experiment ( ) (Figure: drop diameter around 10-4 mm appropriate voltage makes them remain at rest in the field: mg = q E = q V/d (V, potential difference, E, electric field) Result: charges were small multiples of a basic charge e.

12 Important is precise determination of radii r of droplets. Thus: Actual experiment: Initially no applied electric field. Force balance between buoyancy and viscous drag (Stokes law) mg 4 3 r 3 g 6 rv 0 density difference Δρ=ρ oil -ρ air ρ oil η, viscosity of air, v 0 terminal velocity (see comment below). Measurement (microscope) of terminal velocity radius r.

13 Force balance: Now apply electric field E: 4 r 3 q unknown charge of droplet, determined by measuring new terminal velocity v 1 capacitor: E = V/d (d: separation of plates) 3 g qe 6rv 1 Millikan: measured charges of thousands of drops were integer multiples of elementary charge e (experimental deviation 1%).

14 Comment on terminal speed: Consider fall of object through a viscous medium Equation of motion (Newton s second law): dv( t) m dt k 6r k, drag constant (Stokes drag for sphere ) mg kv( t) perform integration (using separation of variables) to compute velocity v(t) dv dt; k 0 g v 0 m k v ln( g v) 0 m k g v m exp g mg k thus v( t) 1 exp[ t] ; k m terminal speed is then given by (limit of t ) v t k m k m t; t; v t mg k ;

Chapter 1 The discovery of the electron 1.1 Thermionic emission of electrons

Chapter 1 The discovery of the electron 1.1 Thermionic emission of electrons Learning objectives: What are cathode rays and how were they discovered? Why does the gas in a discharge tube emit light of