Energy Levels And Spectra

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Energy Levels And Spectra

  1. 1. How Do Electrons in Atoms absorb Energy? Quantum Behaviour
  2. 2. Energy Levels and Spectra <ul><li>We will find out: </li></ul><ul><ul><li>How electrons in atoms absorb energy from photons </li></ul></ul><ul><ul><li>How spectra tell us the type of atom present </li></ul></ul>
  3. 4. v v v v v E2 E1 The Electron Goes from Energy Level E1 to E2 Energy Absorbed = hf = E2 – E1
  4. 5. v v v v v v Energy of Emitted photon = hf = E2 – E1 E2 E1
  5. 6. Absorption Spectra These lines show where particular wavelengths (frequencies) have been absorbed. They are emitted in all direction so are not added back in.
  6. 7. Examples <ul><li>A photon is absorbed by an atom and an electron is excited from energy level -13eV to -8eV. What is the frequency of the photon? Wavelength? </li></ul><ul><li>E2 – E1 = hf </li></ul><ul><li>E2 – E1 = (-8) – (- 13) = 5eV = 5 x 1.6 x 10 -19 Joules </li></ul><ul><li>= 8 x 10 -19 Joules = hf </li></ul><ul><li>Therefore f = 8 x 10 -19 / 6.6 x 10 -34 = 1.2 x 10 15 Hz </li></ul><ul><li>As c = f x  therefore  = c / f = 3 x 10 8 / 1.2 x 10 15 </li></ul>
  7. 8. Phasors <ul><li>Phasors represent waves in terms of: </li></ul><ul><ul><li>Amplitude – Length </li></ul></ul><ul><ul><li>Frequency – Number of turns on 1 second </li></ul></ul>But a photon also has frequency but the magnitude is arbitrary in this case
  8. 9. Phasors and Interference <ul><li>When more than one phasor interact, they interfere. </li></ul><ul><li>We add them ‘tip-to-tail’ to see the outcome – constructive or destructive interference </li></ul>In Phase – Phasors Add  /2 out of phase Out of Phase – Phasors Cancel Resultant The phasor magnitude is normally 1
  9. 10. Phasors <ul><li>But how do we know the phase of a phasor at any one time..? </li></ul><ul><li>It depends on the path! </li></ul><ul><li>But how does the phasor know which path to take..? </li></ul>
  10. 11. Calculating Phase Position <ul><li>Phasors leave a source in phase – i.e. lined up </li></ul><ul><li>To calculate their relative phase at a time and path length we use: </li></ul><ul><li>Trip Time = Distance / Speed </li></ul><ul><li>Speed = f x  = c where c = speed of light = 3 x 10 8 m/s </li></ul><ul><li>Number of Rotations = Rotations per second x Number of seconds </li></ul>This is frequency
  11. 12. Question 10s <ul><li>Complete the questions using the 3 equations </li></ul><ul><li>Trip Time = Distance / Speed </li></ul><ul><li>Speed = f x  = c where c = speed of light = 3 x 10 8 m/s </li></ul><ul><li>Number of Rotations = Rotations per second x Number of seconds </li></ul>
  12. 13. Answer – 10s
  13. 14. Probability <ul><li>Because all events are possible, there must be a probability of them happening </li></ul><ul><li>Probability = Resultant Amplitude 2 </li></ul><ul><li>This is the probability of those phasors arriving at that point </li></ul><ul><li>It is interpreted as an intensity </li></ul>
  14. 15. <ul><li>Sample exam Question </li></ul>

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