ELECTRICAL MATERIAL SCIENCE FOR ENGINEERING STUDENT SLIDE.pptx

ELECTRICAL MATERIAL SCIENCE

1. ELECTRICAL MATERIAL SCIENCE RUTHERFORD-THOMSON ATOMIC MODEL E EC 315 B.A BAKURA 1

2. ATOM Atom is made up of three fundamental particles i.e. electron, proton and neutron An electron is sub atomic particles, having negative charge Proton is sub-atomic particles, having positive charge neutrons are the type of sub-atomic particles with no charge. They are neutral Protons and neutrons are bound into the atom’s nucleus as a result of strong nuclear force B.A BAKURA 2

3. VARIOUS ATOMIC MODELS PROPOSED BY SCIENTISTS OVER THE FIRST FEW DECADES Thomson’s plum pudding model Rutherford’s nuclear model Bohr’s model Sommerfeld's model Vector model Wave-mechanical model B.A BAKURA 3

4. J.J THOMSON’S PLUM PUDDING ATOMIC MODELS The plum pudding model is one of several historical scientific models of the atom. First proposed by J. J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model tried to explain two properties of atoms known then: that electrons are negatively charged particles and that atoms have no net electric charge. The plum pudding model has electrons surrounded by a volume of positive charge, like negatively charged "plums" embedded in a positively charged "pudding". J.J Thomson tried to explain the arrangement of positive charge and the electrons inside the atom. The total positive charge inside the atom is equal to the total negative charge carried the electrons, so that every atom is electrically neutral. B.A BAKURA 4

5. CONT. The picture below depict how Thomas carried out his experiment. Thomson’s model of an atom is similar to a plum pudding. The negatively charge represent plums. Embedded in a positively charge, pudding. B.A BAKURA 5

6. CONT. Limitations of Thomson’s atomic model. He failed to explain the stability of an atom because his model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom. Thus, this theory also failed to account for the position of the nucleus in an atom B.A BAKURA 6

7. Rutherford model The Rutherford model was devised by the New Zealand-born physicist Ernest Rutherford to describe an atom. Rutherford directed the Geiger–Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum pudding model of the atom was incorrect. Rutherford's new model for the atom, based on the experimental results, contained new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom. This region would be known as the "nucleus" of the atom. B.A BAKURA 7

8. CONT. Most of the Alpha particles passed through the foil without undergoing any deflection. This is the reason said the spare is empty Few Alpha particles underwent deflection through small angles. Very few were deflected back through an angle greater than 90° B.A BAKURA 8

9. CONT. Limitation of Rutherford atomic models Theory could not explain the distribution of electrons in the orbit. The theory did not explain the stability of the atom as a whole. The model assumed the electrons outside the nucleus either to be stationary or moving in a circle Thus, if the electron were stationary they would have attracted towards nucleus. Or continuously lost it energy down to the nucleus if moving in a circle. B.A BAKURA 9

10. Bohr’s model Prof. Neil Bohr in (1913) overcome the drawback of Rutherford model by applying quantum theory. He said, the electron can revolve in certain obits without losing energy. These orbits are called stationary orbits. however, when excess energy is delivered, the electron jumps from lower level to higher level called ( excitation) and from higher level to lower level or ground called (de- excitation), then excess energy is given out as one packet called photons. Energy is expressed as E = or E = hf where h is plank’s constant, c is velocity of light and ℷ is wavelength of light emitted. En2, En1 = energies of the atom before and after emissions of radiations respectively. E= hf = En2 – En1, not that En2 is always greater than En1   B.A BAKURA 10

11. Cont. B.A BAKURA 11

12. Derivation of energy level of hydrogen atom En2 = = x …………therefore Z = 1 En1 = x hf = - ] or f = - ]   B.A BAKURA 12

13. Cont. Now C = f = = - ] or = - ] Let R = = = Rydberg’s constant   B.A BAKURA 13

14. Cont. ∵ = R - ] now, wave number v = v = R - ] ……. for hydrogen   B.A BAKURA 14

15. Cont. Not that, in the hydrogen spectrum following series are observed 1 Layman series : layman series consist of all wavelength which are emitted when electron jumps from different higher orbits to the final orbits with n =1 2 Balmer series with n=2 3 Paschen series with n=3 4 Bracket series with n=4 5 Pfund series with n=5 B.A BAKURA 15

16. Challenge 1 Calculate the value of kinetic, potential and total energy of an electron revolving bohr’s first orbit in an hydrogen atom. Solution K.E = = when z = 1 = = Now, 1eV =   B.A BAKURA 16

17. Cont. i ∵ K.E = = 13.6eV ii Potential energy is twice the kinetic energy in magnitude but is negative P.E = -2 x K.E = -2x13.6 = -27.2 eV iii total energy = K.E + P.E = 13.6 – 27.2 = -13.6 eV (Ans.)   B.A BAKURA 17

18. Cont. Assignment 1 : Calculate the value of kinetic, potential and total energy of an electron revolving bohr’s first orbit in an hydrogen atom. B.A BAKURA 18

19. Challenge 2 An electron in the n=5 energy level of a hydrogen atom emits a photon of wavelength 1.281x10 -6 m. To what energy level does it move? Solution Data = 1.281x10 -6 m, n=5, Rydberg’s constant (R) = 1.097x10 7 m -1 ∵ = R - ] = = 1.097x10 7 m -1 - ] 7.806x10 = 1.097x10 7 - ]   B.A BAKURA 19

20. Cont. Divide both side by 1.097x10 7 ∵ = 0.071158 = 0.071158 + 0.04 = 0.111158 = 1 = = 9 n = = 3 (Ans.) Therefore, the electron relax at n=3 middle of photon with the wavelength of 1281nm.   B.A BAKURA 20

21. Assignment 2 An electron in the n=4 energy level of a hydrogen atom emits a photon of wavelength 1281nm. To what energy level does it move? B.A BAKURA 21

22. Einstein Relation The equation which relates the mobility µ ( of electrons or holes) and the diffusion coefficient (of electrons D n or holes) D p is known as Einstein Relation. Mobility characterizes how quickly an electron or hole can move though a semiconductor, when electric field is applied to it. The process of electrons or holes moving from the higher concentration region to the lower concentration region is known as diffusion. Diffusion coefficient is the quantity of substance that diffusing from one region to another. B.A BAKURA 22

23. Einstein's photoelectric Equation The earlier idea of Planck's that the light wave consists of tiny bundles of energy called photons or quanta, Einstein suggest an explanation of the photoelectric equation. He looked at the way in which electrons tried to leave the metal; he established that it actually takes energy for an electron to leave the surface of the metal. He named this energy, work function. Finally, he compared energy received and energy used by the electrons at the surface. Therefore, energy supplied to the electron = hf (energy from the photon) while energy used by the electron is either used as a work function to escape from the surface OR is left from the electron after it has escape, in which case it is in the form of E k Hence, energy supplied (hf) = energy used (w + E kmax ) this is Einstein equation B.A BAKURA 23

24. Understanding photoelectric effect Is the phenomenon of ejection of electrons from the surface of a metal when light of a suitable wavelength falls on it . Note that, Heinrich Hertz discovered photoelectric effect in 1887 when he came across radio wave, which remained unexplained until 1905 when Albert Einstein postulated the existence of quanta of light (photons). Photons carrying an energy can release the electrons from the surface of a metal so long as energy of the photons is greater than the work function. Work function is the minimum energy required to release an electron from the material or a metal. Threshold frequency is the minimum frequency of incident radiation which can cause the photoelectric emission to start. B.A BAKURA 24

25. Laws of photoelectric emission 1- photoelectric current is directly proportional to the incident of light 2- the energy of electron emission is independent of the intensity of incident and depends only on the frequency and nature of light 3- for every metal there is a limiting frequency of radiation which can cause emission of photoelectrons. This is called threshold frequency 4- photoelectric emission is independent of the temperature of photo cathode B.A BAKURA 25

26. Sample problem For a certain frequency of light, the maximum kinetic energy of a photoelectron from sodium cathode is 2.03 x 10 -19 calculate the wavelength of light used. E = W + E xmax or = hf + E xmax , recall E=hf or , and W = hf where h = 6.63x10 -34 , f = 5.31x10 14 ,c = 3x10 8 E xmax = 2.03 x 10 -19 Solution = hf + E xmax , = = 6.63x10 -34 x 5.31x10 14 + 2.03 x 10 -19 Therefore , ℷ = 3.58x10 -7 ans.   B.A BAKURA 26

27. Cont. A potassium metal plate is irradiated with light of wavelength 5x10 -7 m in an arrangement, as shown below, the threshold frequency of potassium is 5.55x10 14 Hz. Calculate the energy of a photon incident on the metal plate Using a suitable calculation, prove that the ammeter will show a reading B.A BAKURA 27

28. Cont. E = = 3.978x10 -19 j W = hf = 6.63x10-34 x 5.55x1014 = 3.68 j Therefore, E = W + Exmax, 3.98x10^-19= 3.68 x 10^-19 + E xmax E xmax = 3.98x10^-19 - 3.68 x10^-19= 3.004x10^-20 j The energy of the photon is greater than the work function. Hence, the ammeter will read.   B.A BAKURA 28

29. Cont. Note: When the energy of the photon is less than the work function (E < W ) no movement When the energy of the photon is equal the work function(E=W ), there is a lift but no movement When the energy of the photon is greater than the work function (E>W ) there is both a lift and movement B.A BAKURA 29

30. Cont. from the graph below, which photocell with the same frequency of light emits photoelectrons of higher kinetic energy? Choose from potassium or sodium. Name the energy level shown as A B.A BAKURA 30