### All GRE Subject Test: Physics Resources

## Example Questions

### Example Question #1 : Special Topics

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

**Possible Answers:**

It is halved.

No change.

It is quartered.

It doubles.

It quadruples.

**Correct answer:**

It is halved.

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 #1 : Astrophysics

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:**

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:**

Energy

Magnetic field strength

Number of moons

Temperature

Mass

**Correct answer:**

Mass

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.

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