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PRS Questions(queestions after Midterm 2)

The graph below shows the blackbody spectra of three different stars.All three stars have the same size. Which of the stars is at thehighest temperature?

1) Star A2) Star B3) Star C A

B

C

Wavelength

PRS question: Thermal Radiation

1. Hotter objects emit more total radiation per unitsurface area. Stephan-Boltzmann Law E = σT4

Brightness

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The graph below shows the blackbody spectra of three different stars.All three stars have the same size. Which of the stars is at thehighest temperature?

1) Star A2) Star B3) Star C A

B

C

Brightness

Wavelength

• Hotter objects emit bluer photons (with a higheraverage energy.) Wien Law λmax = 2.9 x 106 / T(K) [λmax in nm]

T(K) = 2.9 x 106 / λmax [nm]

λmaxλmax

PRS question: Thermal Radiation

PRS: The Doppler EffectTwo identical stars are moving towards the Earth. Star A’s emission

lines (that are normally at visible wavelengths) are observed to beat ultraviolet wavelengths. Star B’s emission lines are observed tobe at X-ray wavelengths. From these observations you concludethat:

1) both stars are moving away from the Earth2) Star A is moving towards the Earth faster than Star B3) Star B is moving towards the Earth faster than Star A4) Star B is actually moving away from the Earth

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Uranus is almost 20 times farther fromthe Sun than the Earth. Consider one ofthe moons of Uranus that has noatmosphere. How much colder is thesurface of that moon compared to thesurface of the Earth’s Moon?

1. 20 times colder2. 40 times colder3. 200 times colder4. 400 times colder5. 1000 times colder

I’d like to knowthe answer to

this PRS Question

The sunlight that weactually measure

• Flux = energy per areaper second

• Area of a sphere = 4πR2

• Flux = Luminosity4πR2

If we somehow know the luminosityand we measure the flux, we can solvefor the distance to the object!

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Uranus is almost 20 times farther fromthe Sun than the Earth. How much lower

is the flux of Sunlight at Uranuscompared to the flux at the Earth?

1. 20 times lower2. 40 times lower3. 200 times lower4. 400 times lower5. 1000 times lower

Flux = Luminosity4π R2

Our eyes work via the process of:1) transmission2) reflection3) absorption4) emission5) none of the above

The things that a telescope must do...

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What will happen to thetemperature of the cloud as

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.

Consider a gas cloud in space, suchas the one shown at right. Supposethat it starts to collapse suddenly.

PRSPRS

PRS

PRS. Which of the followingprinciples of physics is NOT

important for understanding theformation of the Solar System?

1. Conservation of angular momentum2. Conservation of linear momentum3. Conservation of energy4. Newton’s law of gravity5. Actually, all of these laws of physics

are important factorsThis is an extra-credit prs, as explained in lecture.

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PRS Question:Look carefully at thispicture. What might youconclude about the waythat Saturn formed?

1. Tidal forces due to encounters between Saturn and comets/asteroidsare the main origin of the rings2. Saturn is a “mini solar system”. Thephysics that led to the formation of therings are the same as those that formedthe Sun and its planets, etc.

PRS Question:Why are Saturn’s rings

confined to a thin plane?1. Tidal forces2. Newton’s 1st law3. Conservation of energy4. Conservation of angular momentum

angular momentum = m x v x r

Note: we didn’t do the PRS question this semester, but it is a usefulquestion to think about:

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What happens next?

1. Hydrogen (atomic weight = 1)2. Helium (atomic weight = 4)3. Nitrogen (atomic weight = 14)4. Oxygen (atomic weight = 16)

PRS Question:

Consider a balloon full of air. Inside that balloon, which of the following elements will move the slowest?

• Kinetic energy = 1/2 mv2

14N is a stable isotope with atomicnumber = 7 and atomic mass = 14

14C 14N radioactive

decay

“Parent” isotope “Daughter” isotope

1. The carbon becomes ionized2. A proton turns into a neutron3. A neutron turns into a proton4. Neutrons and protons both change forms

What happens when carbon-14 radioactively decaysinto nitrogen-14?

(6 protons, 8 neutrons)

PRS question

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Radioactive Decay• When a radioactive isotope suddenly

changes, the process is called radioactivedecay.

• Most common carbon isotope: carbon-12(6 protons & 6 neutrons). Stable isotope.

• Radioactive carbon isotope: carbon-14(6 protons & 8 neutrons)

14C 14N radioactive

decay

“Parent” isotope

(6 protons, 8 neutrons)

“Daughter” isotope

(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 aradioactive isotope in a cabinet in your laboratory. The stuff has ahalf-life of 10 years. If 30 years later you stumble across that stuff, you will find that you still have

1. 7.5 kilograms of the radioactive isotope2. 5.0 kilograms of the radioactive isotope3. 2.5 kilograms of the radioactive isotope4. 1.25 kilograms of the radioactive isotope5. 0.625 kilograms of the radioactive isotope

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PRS Question:The inside of the Earth is

1. Entirely molten rock, i.e., “magma”2. Entirely solid rock3. Mostly magma with a small amount of

solid rock4. Mostly solid rock with a small amount of

molten rock

PRS Question: How were these maps ofthe atmospheric ozone obtained?

Much of these data were collected by“TOMS”, the “Total Ozone Mapping

Spectrometer”1. NASA has airplanes flying

around the atmospheremeasuring the amount of ozone

2. NASA has balloons floating inthe ozone layer collectingsamples

3. NASA observes the spectrumof the atmosphere with atelescope