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PRS Questions (queestions after Midterm 2) PRS question: Thermal Radiation The graph below shows the blackbody spectra of three different stars. All three stars have the same size. Which of the stars is at the highest temperature? 1. 1) Star A 2) Star B 3) Star C Brightness A Hotter objects emit more total radiation per unit surface area. Stephan-Boltzmann Law 4 E = σT B C Wavelength 1 PRS question: Thermal Radiation The graph below shows the blackbody spectra of three different stars. All three stars have the same size. Which of the stars is at the highest temperature? • 1) Star A 2) Star B 3) Star C Brightness A Hotter objects emit bluer photons (with a higher average energy.) Wien Law λmax = 2.9 x 106 / T(K) [λmax in nm] B T(K) = 2.9 x 106 / λmax [nm] C λmax λmax Wavelength PRS: The Doppler Effect Two identical stars are moving towards the Earth. Star A’s emission lines (that are normally at visible wavelengths) are observed to be at ultraviolet wavelengths. Star B’s emission lines are observed to be at X-ray wavelengths. From these observations you conclude that: 1) both stars are moving away from the Earth 2) Star A is moving towards the Earth faster than Star B 3) Star B is moving towards the Earth faster than Star A 4) Star B is actually moving away from the Earth 2 Uranus is almost 20 times farther from the Sun than the Earth. Consider one of the moons of Uranus that has no atmosphere. How much colder is the surface of that moon compared to the surface of the Earth’s Moon? 1. 2. 3. 4. 5. 20 times colder 40 times colder 200 times colder 400 times colder 1000 times colder I’d like to know the answer to this PRS Question The sunlight that we actually measure • Flux = energy per area per second • Area of a sphere = 4πR2 • Flux = Luminosity 4πR2 If we somehow know the luminosity and we measure the flux, we can solve for the distance to the object! 3 Uranus is almost 20 times farther from the Sun than the Earth. How much lower is the flux of Sunlight at Uranus compared to the flux at the Earth? 1. 2. 3. 4. 5. 20 times lower 40 times lower 200 times lower 400 times lower 1000 times lower Flux = Luminosity 4π R2 The things that a telescope must do... Our eyes work via the process of: 1) transmission 2) reflection 3) absorption 4) emission 5) none of the above 4 Consider a gas cloud in space, such as the one shown at right. Suppose that it starts to collapse suddenly. What will happen to the temperature of the cloud as PRS it collapses? 1. The cloud will cool down. 2. The cloud will warm up. 3. The cloud temperature will remain the same. 4. This cannot be answered without additional information. PRS PRS PRS. Which of the following principles of physics is NOT important for understanding the formation of the Solar System? 1. 2. 3. 4. 5. Conservation of angular momentum Conservation of linear momentum Conservation of energy Newton’s law of gravity Actually, all of these laws of physics are important factors This is an extra-credit prs, as explained in lecture. 5 PRS Question: Look carefully at this picture. What might you conclude about the way that Saturn formed? 1. Tidal forces due to encounters between Saturn and comets/asteroids are the main origin of the rings 2. Saturn is a “mini solar system”. The physics that led to the formation of the rings are the same as those that formed the Sun and its planets, etc. Note: we didn’t do the PRS question this semester, but it is a useful question to think about: PRS Question: Why are Saturn’s rings confined to a thin plane? 1. Tidal forces 2. Newton’s 1st law 3. Conservation of energy 4. Conservation of angular momentum angular momentum = m x v x r 6 What happens next? • Kinetic energy = 1/2 mv2 PRS Question: Consider a balloon full of air. Inside that balloon, which of the following elements will move the slowest? 1. 2. 3. 4. Hydrogen (atomic weight = 1) Helium (atomic weight = 4) Nitrogen (atomic weight = 14) Oxygen (atomic weight = 16) 14N is a stable isotope with atomic number = 7 and atomic mass = 14 What happens when carbon-14 radioactively decays into nitrogen-14? PRS question 1. 2. 3. 4. The carbon becomes ionized A proton turns into a neutron A neutron turns into a proton Neutrons and protons both change forms “Parent” isotope 14C radioactive (6 protons, 8 neutrons) decay “Daughter” isotope 14N 7 Radioactive Decay • When a radioactive isotope suddenly changes, the process is called radioactive decay. • Most common carbon isotope: carbon-12 (6 protons & 6 neutrons). Stable isotope. • Radioactive carbon isotope: carbon-14 (6 protons & 8 neutrons) “Parent” isotope 14C radioactive (6 protons, 8 neutrons) decay “Daughter” isotope 14N (7 protons, 7 neutrons) Radioactive decay half-life • Half-life: the amount of time required for half of the parent nuclei in a solid to decay PRS QUESTION. Suppose that one year you put 10 kilograms of a radioactive isotope in a cabinet in your laboratory. The stuff has a half-life of 10 years. If 30 years later you stumble across that stuff, you will find that you still have 1. 2. 3. 4. 5. 7.5 kilograms of the radioactive isotope 5.0 kilograms of the radioactive isotope 2.5 kilograms of the radioactive isotope 1.25 kilograms of the radioactive isotope 0.625 kilograms of the radioactive isotope 8 PRS Question: The inside of the Earth is 1. Entirely molten rock, i.e., “magma” 2. Entirely solid rock 3. Mostly magma with a small amount of solid rock 4. Mostly solid rock with a small amount of molten rock PRS Question: How were these maps of the atmospheric ozone obtained? Much of these data were collected by “TOMS”, the “Total Ozone Mapping Spectrometer” 1. 2. 3. NASA has airplanes flying around the atmosphere measuring the amount of ozone NASA has balloons floating in the ozone layer collecting samples NASA observes the spectrum of the atmosphere with a telescope 9