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GRE Subject Test: Physics : Astrophysics

Study concepts, example questions & explanations for GRE Subject Test: Physics

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33 Practice Tests Question of the Day Flashcards Learn by Concept

Example Questions

Example Question #1 : Astrophysics

If the temperature of a blackbody doubles, what occurs to the wavelength of maximum emission?

Possible Answers:

It is quartered.

It is halved.

No change.

It quadruples.

It doubles.

Correct answer:

It is halved.

Explanation:

This is an application of Wein's Law that states the following:

$\lambda_{max}=\frac{0.0029Km}{T}$ $\displaystyle \lambda_{max}=\frac{0.0029Km}{T}$, where $\lambda_{max}$ $\displaystyle \lambda_{max}$ is the wavelength of maximum emission of the object (measured in meters $\displaystyle meters$), and $\displaystyle T$ is the temperature in Kelvin $\displaystyle Kelvin$. Since the wavelength and the temperature are inversely proportional, if we double the temperature, we must cut the wavelength by the same proportion. Therefore, the temperature must be halved.

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Example Question #2 : Special Topics

By what factor will the energy emitted each second off the surface of a blackbody change if the the temperature of the object is tripled?

Possible Answers:

$\displaystyle 3$

$\displaystyle 27$

$\frac{1}{3}$ $\displaystyle \frac{1}{3}$

$\displaystyle 81$

$\displaystyle 9$

Correct answer:

$\displaystyle 81$

Explanation:

Stefan-Boltzmann's equation for a blackbody states:

$F=\sigma T^4$ $\displaystyle F=\sigma T^4$, where $\displaystyle F$ is the energy emitted each second, $\sigma$ $\displaystyle \sigma$ is the Stefan-Boltzmann constant, and $\displaystyle T$ is the temperature of the blackbody. Therefore, if we triple the temperature, the energy will increase by a factor of 3^4=81. $\displaystyle 3^4=81.$

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Example Question #2 : Astrophysics

Using the kinetic theory of gas, what is the fundamental property of a planet that dictates if it is able to retain an atmosphere?

Possible Answers:

Number of moons

Mass

Temperature

Energy

Magnetic field strength

Correct answer:

Mass

Explanation:

The kinetic theory of gas states:

$v_{rms}=\sqrt{\frac{3kT}{m}}$ $\displaystyle v_{rms}=\sqrt{\frac{3kT}{m}}$. Whenever the velocity of the individual gas particles is larger than the escape speed of the planet,

$v_e=\sqrt{\frac{2GM}{r}}$ $\displaystyle v_e=\sqrt{\frac{2GM}{r}}$, the gas particles will leak into space and deplete the atmosphere. The physical property that determines this is the mass of the planet.

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All GRE Subject Test: Physics Resources

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GRE Subject Test: Physics : Astrophysics

Study concepts, example questions & explanations for GRE Subject Test: Physics

All GRE Subject Test: Physics Resources

Example Questions

Example Question #1 : Astrophysics

Example Question #2 : Special Topics

Example Question #2 : Astrophysics

All GRE Subject Test: Physics Resources

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