GRE Subject Test: Physics : Special Topics

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

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All GRE Subject Test: Physics Resources

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Example Questions

Example Question #1 : Astrophysics

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

Possible Answers:

It quadruples. 

It doubles. 

It is halved. 

No change.

It is quartered. 

Correct answer:

It is halved. 

Explanation:

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

, where  is the wavelength of maximum emission of the object (measured in ), and  is the temperature in . 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.   

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:

Correct answer:

Explanation:

Stefan-Boltzmann's equation for a blackbody states:

, where  is the energy emitted each second,  is the Stefan-Boltzmann constant, and  is the temperature of the blackbody. Therefore, if we triple the temperature, the energy will increase by a factor of 

Example Question #2 : Special Topics

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:

Mass

Number of moons

Temperature

Magnetic field strength

Energy

Correct answer:

Mass

Explanation:

The kinetic theory of gas states:

. Whenever the velocity of the individual gas particles is larger than the escape speed of the planet, 

, the gas particles will leak into space and deplete the atmosphere. The physical property that determines this is the mass of the planet. 

All GRE Subject Test: Physics Resources

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