PHOTOELECTRIC EFFECT

1. PHOTOELECTRIC EFFECT

2. INTRODUCTION • DEFINATION; • Is the phenomenon of electrons emmission when an electomagnetic radiation of sufficient frequency is radiated on a metal surface.

3. Light energy and quantum theory • Light energy is propagated as small particles of energy called quanta-plural or quantum-singular • In light,these discrete amount of energy are called photons. • under this theory the energy possed by a photon is given by

4. E = hf • where: • E = Energy • h = planks constant ; valued at 6.63 × 10-34js • f = Frequency of the radiation....differs from materia to material.

5. From general wave equation: • V(c) = fƛ max • So; f = C/ƛmax......(1) • Remembering...E = hf.....(2) • Substituting equation 1 in 2: • E = hc/ƛmax...........

6. Where: • h = planks constant...6.63 ×10-34 • c = velocity • ƛ = wavelength • E = energy of a photon

7. Terms used; • Work function • Minimum amount of energy needed to dislodge an electron from a metal surface. • S.I Units ...joules (j) or eV • Where 1eV = 1.6 ×10-19j

8. Threshhold frequency ( fo) • Minimum frequency needed to emit photo electrons from a metal surface . • FO = C/ƛmax • C = velocity • ƛ = Wavelength of the light wave • fo= Threshhold frequency.

9. Threshhold wavelength (ƛo) • Maximum wavelength beyond which no photoelectric emission take place. • Wo= hfo

10. Rem: • For any radiation of frequency f less than fo,hf will be lower than wo and emission will not occur. • When the frequency of the radiation is fo,hfo = wo,emission occurs

11. When the frequency of the radiation f>fo,hf > wo and the excess energy in this case appears as the kinetic energy of the emitted electron. • hf-wo=1/2mev2 • where.... • me =Mass of the electron (9.11× 10 -31kg) • V = Velocity of the electron emmitted

12. Cut – off potential /stopping potential • Is the minimum potential or maximum negative potential for which photoelectric effect is observed. • Its symbol is Vs

13. Photons • Packets or particles in fform of which light energy travels or is transfered. • The energy of a photon is given by; • Energy (E)=hf • Where h = planks constant • F = frequency

14. Electron Volt (eV) • Energy required by an electron falling through a potential difference (p.d) of 1Volt • Using E = qV, where q = charge • V = potential difference • 1eV = 1.6 ×10 -19 j

15. Factors affecting photo elecric effect • We have three main factors • Intensity of the radiation • Energy of the radiation • Type of metal

16. Intensity of the radiation • Is the rate of energy flow per unit area when the radiation is normal to the area. • Intensity = work( W)/Area(A) ×time(t) • I = W/At......... but w/t = power (p) • Therefore... • I = P/A

17. Energy of the radiation • When an experiment is conducted on investigating the above,its noted that the higher the frequency of the radiation,the more the kinetic enrgy of the photo – electrons.

18. Type of the metal • Every metal surface has its own minimum frequency of the radiation called thethreshhold frequency (fo) below whch no photo emmission takes place, no matter how intence it is.

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20. Take away home assignment • Show mathematically the relationship between wo,fo and E.

21. EINSTENS EQUATION OF PHTOELECTRIC EFFECT • Whe a photon strikes an electron,its energy is absorbed by the electron. • Some of the energy is used to dislodge the electron from the metal surface while the rest appers as the K.E of ttheemited electron.

22. Expressed as; • Energy of Photon (hf) = Energy needed to dislodge an ( hfo ) + Maximum K.E gained by • Electron from the metal surface electron (1/2meV2)

23. Therefore ; • hf = hfo+1/2meV2

24. In energy radiation experiment ,different colour codes are used to show the frequency and their correspondind stopping potential Vs;

26. Graphically represented as; • Graph of stopping potential (Vs) against frequency (f) • Y axis • Stopping • Potential • (Vs) • fo • x-axis • -Wo/e Frequency ( 10^14 hz)

27. Graph is a straight line graph. • From the Einsteins photo electric equation; • hf = hfo + 1/2meV2............(a) • By work energy theorem; • eVs = 1/2meV2.................... (b) • Substituting eqn (b) in (a),then; • hf = hfo+ eVs...............................(c) • eVs = hf – hfo(divide through by e)

28. giving us...... • Vs = h/ef +h/efo • But hfo = wo • Therefore; • Vs = hf/e – wo/e • Where...e = 1.6×10^-19c • h = 6.63 × 10^-34j • comparing with.... y = mx + c........then.... • y = Vs x = f • m = h/e c = -wo/e

29. examples...... • Sodium has work function of 2.3eV.calculate; • Its threshhold frequency • The maximum velocity of the photoelectron produced when its surface is illuminated by light of wavelength 5.0× 10^-7 • The stopping potential of this energy

30. Maxmum K.E of the Photoelectrons emmitted from a metal is 1.8 ×10^-19j when the frequency of the incident radiation is 8.5 ×10^14hz.Calculate the maximum wavelength of the radiation that can just emmit an electron from the metal.

31. Application of photoelectric effect: • Photoemmissive cells • Photo voltaic cells • Photoconductive cells or light dependent resistors (L.D.R)

32. ASSIGNMENT: • Read and make notes on the application of the above about photoellectric effect. • Do question no. 6 & 7 on page 133 of K.L.B.

33. REM.............. ....... CHICKEN WITHOUT TEETH AND MILK SRTUGGLES TO FEED ITS CHICKS ........SOO...MAKE USE OF EVEN THAT SEEMS IMPOSSIBLE....