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Help Desk

Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine tburbine@mtholyoke.edu www.xanga.com/astronomy100. Help Desk. There is an Astronomy Help Desk in HAS 205.  It will be open from Monday through Thursday from 7-9 pm. E = mc 2 and KE = 1/2 mv 2 HW. Purpose of this Homework

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Help Desk

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  1. Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.eduwww.xanga.com/astronomy100

  2. Help Desk • There is an Astronomy Help Desk in HAS 205.  It will be open from Monday through Thursday from 7-9 pm.

  3. E = mc2 and KE = 1/2 mv2 HW • Purpose of this Homework • To teach you that if you can get lots of energy from a small amount of material if you can turn all the matter into energy • You need a much bigger mass to produce the same amount of energy through kinetic energy (through impact with a velocity)

  4. The nuclear bomb dropped on Hiroshima released 8.4 x 1013 joules of energy. What would be the mass of nuclear material that would produce this amount of energy if you assume all the nuclear material was converted to energy? Show your work. • E = mc2 • M=E/c2 • c = 3 x 108 m/s • c2 = 9 x 1016 m2/s2 • M = 8.4 x 1013 J / 9 x 1016 m2/s2 • M = 9.3 x 10-4 kg

  5. An asteroid is on a collision course with Earth? What must the mass of the asteroid be to produce that much energy (8.4 x 1013 joules) if it hits the Earth with a velocity of 20 km/s (20,000 m/s)? Show your work. • KE = kinetic energy • KE = ½ m v2 • m = KE/(½ v2) • m = 2KE/v2 • m = 2* 8.4 x 1013 J/(20000*20000) • m = 4.2 x 105 kg

  6. Also • You need to list units for answer!!!!!! • Like kilograms for the previous questions

  7. Element Homework • So you learn that there are lots of elements with different properties • And each element has a number of isotopes • Isotope – same number of protons, different number of neutrons

  8. Latest Homework • Was not done to torture you • Was done so you realize that different planets have different accelerations due to gravity and different escape velocities • How would that information be used on a test?

  9. For example, • A body is the same size as the Earth but has twice the Earth’s mass. What would be the acceleration of gravity on this body? • A) 4.9 m/s2 • B) 9.8 m/s2 • C) 19.6 m/s2 • D) 2.45 m/s2 • E) 39.2 m/s2

  10. For example, • A body is the same size as the Earth but has twice the Earth’s mass. What would be the acceleration of gravity on this body? • A) 4.9 m/s2 • B) 9.8 m/s2 • C) 19.6 m/s2 • D) 2.45 m/s2 • E) 39.2 m/s2 Because of formula F = G M1 M2 d2

  11. Homework Assignment(Due Thursday March 3rd) • Make up a test question • Multiple Choice • A-E possible answers • 1 point for handing it in • 1 point for me using it on test • The question needs to be on material that will be on the March 10th exam

  12. Next Homework (due Tuesday, March 1st) • In Joules, calculate the typical energy of one • Gamma ray • X-ray • Ultraviolet light • Visible light • Infrared light • Radio wave • photon

  13. Light • Light is a form of energy

  14. Light • These are all forms of light • Gamma rays • X-rays • Ultraviolet light • Visible light • Infrared light • Radio waves

  15. Light • Can act as a particle • Can also act as a wave

  16. Particle aspect • Particles called photons stream from the Sun and can be blocked by your body

  17. Wave aspect

  18. Thomas Young Experiment • http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/

  19. Characteristics of waves • velocity = wavelength x frequency • Wavelength = distance • Frequency = cycles per second = hertz

  20. For light • c = wavelength x frequency • In vacuum, speed of light stays the same • So if wavelength goes up • Frequency does down • f = frequency • λ = wavelength • c = λ x f

  21. Show animation • Electromagnetic spectrum

  22. Calculations • c = λ x f • So if the wavelength is 1 x 10-12 m • 3 x 108 m/s = 1 x 10-12 m * f • f = 3 x 108 m/s/1 x 10-12 m • f = 3 x 1020 s-1 = 3 x 1020 Hz

  23. Calculations • c = λ x f • So if the frequency is 1 x 1015 Hz • 3 x 108 m/s = λ * 1 x 1015 Hz • λ = 3 x 108 m/s/1 x 1015 Hz • λ = 3 x 10-7 m

  24. Energy of light • Energy is directly proportional to the frequency • E = h * f • h = Planck’s constant = 6.626 x 10-34 J/s • since f = c/λ • Energy is inversely proportional to the wavelength • E = hc/λ

  25. Higher the frequency, Higher the energy of the photon Higher the wavelength, Lower the energy of the photon

  26. Calculations • What is the energy of a radio wave with a frequency of 1 x 107 Hz? • E = h * f • h = Planck’s constant = 6.626 x 10-34 J/s • E = 6.626 x 10-34 J/s * 1 x 107 • E = 6.626 x 10-27 J

  27. Calculations • What is the energy of a gamma ray photon with wavelength of 1 x 10-15 m • E = hc/λ • h = Planck’s constant = 6.626 x 10-34 J/s • E = 6.626 x 10-34 J/s * 3 x 108 m/s / 1 x 10-15 m • E = 1.99 x 10-10 J

  28. Next Homework (due Tuesday, March 1st) • In Joules, calculate the typical energy of one • Gamma ray • X-ray • Ultraviolet light • Visible light • Infrared light • Radio wave • photon

  29. So why are some types of radiation dangerous? • Higher the energy, the farther the photons can penetrate • So gamma and X-rays can pass much more easily into your the body • These high-energy photons can ionize atoms in cells • Ionization means removes electrons from an atom

  30. More dangerous

  31. When you measure an astronomical body • You measure intensity • Intensity – amount of radiation

  32. How do you use light to determine what is in an astronomical body?

  33. electrons

  34. Energy levels where an electron can reside To go to a higher energy level, an electron needs to gain energy To go to a lower energy level, an electron needs to lose energy

  35. Rules • An electron can not jump to a higher energy level unless it gains energy from somewhere else • Absorbs a photon • Gains kinetic energy from an impacting particle • To go to a lower energy level, the electron must lose energy • Emits a photon • Electron jumps can occur only with the particular amounts of energy representing differences between possible energy levels

  36. So which of these transitions is not possible • A • B • C • D • E

  37. So which of these transitions is not possible • A • B • C • D • E

  38. Show animation • Production of emission lines

  39. Heated hydrogen gas Emission line spectrum White light through cool hydrogen gas Absorption line spectrum

  40. Show animation • Production of absorption lines

  41. Types of spectra • Emission – radiation is emitted at characteristic wavelengths • Material is “hot” so electrons keep on bumping into each other and transferring kinetic energy toe ach other so they jump between particular energy levels • Absorption – radiation is absorbed at characteristic wavelengths • Radiation passes through the material

  42. So why is this important • Different elements have different number of electrons • Different elements have different energy levels for their electrons

  43. So • Different elements can absorb light at specific energies • Different elements can emit light at specific energies • So if you can measure the wavelength of the light from an astronomical body, you can determine whats in it

  44. Emission line spectra

  45. Show animation • Composition of a mysterious gas

  46. How can you all this to determine velocities? • Doppler Shift – The wavelength of light changes as the source moves towards or away from you • Since you know the wavelength position of emission or absorption features • If the positions of the features move in wavelength position, you know the source is moving

  47. So • Source moving towards you, wavelength decreases • Source moving away from you, wavelength increases

  48. Questions?

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