A black body is an object that absorbs (or emits) all of the radiation incident on it, and does not reflect or transmit any radiation.

Define black body radiation .

Black body radiation is the thermal radiation emitted by all objects, in the form of electromagnetic waves, due to their temperature.

How does the peak wavelength of an object's emitted radiation change as its temperature increases?

The peak wavelength decreases , shifting to shorter wavelengths as the object's temperature increases.

What real object provides the best approximation to an ideal black body?

Stars provide the best real-world approximation to an ideal black body.

Since a good absorber of radiation is also a good .......... , a perfect black body is the best possible emitter.

Since a good absorber of radiation is also a good emitter , a perfect black body is the best possible emitter.

True or False? A black body only absorbs radiation - it does not emit any.

False. A perfect black body absorbs and emits all wavelengths of radiation; a good absorber is also a good emitter.

Define the Stefan-Boltzmann Law .

The Stefan-Boltzmann Law states that the total energy emitted by a black body per unit area per second is proportional to the fourth power of its absolute temperature.

What two factors determine a star's luminosity?

Its surface temperature Its surface area

In the Stefan-Boltzmann law, temperature must be measured in .......... , not degrees Celsius.

In the Stefan-Boltzmann law, temperature must be measured in kelvin , not degrees Celsius.

True or False? Doubling a star's surface temperature doubles its luminosity, assuming its surface area stays constant.

False. Luminosity is proportional to T 4 , so doubling the temperature increases luminosity by a factor of 16 , not 2.

Define Wien's displacement law .

Wien's displacement law states that the black body radiation curve for different temperatures peaks at a wavelength that is inversely proportional to the temperature.

Why do hotter stars appear white or blue, while cooler stars appear red or yellow?

Hotter stars have their peak intensity at shorter wavelengths (blue/white end of the spectrum); cooler stars peak at longer wavelengths (red/yellow end).

What units must temperature be measured in when using Wien's Law?

Kelvin (K) - convert from degrees Celsius if the temperature is given in °C.

Wien's Law states that λ max is inversely proportional to the .......... of the object.

Wien's Law states that λ max is inversely proportional to the absolute temperature of the object.

True or False? A star with a longer peak wavelength has a higher surface temperature than one with a shorter peak wavelength.

False. Peak wavelength is inversely proportional to temperature, so a longer peak wavelength means a lower surface temperature.

International A Level (IAL)PhysicsEdexcelFlashcards5. Thermodynamics, Radiation, Oscillations & CosmologyBlack Body Radiation

Black Body Radiation (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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Define black body.

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Cards in this collection (18)

Define black body.
A black body is an object that absorbs (or emits) all of the radiation incident on it, and does not reflect or transmit any radiation.
Define black body radiation.
Black body radiation is the thermal radiation emitted by all objects, in the form of electromagnetic waves, due to their temperature.
How does the peak wavelength of an object's emitted radiation change as its temperature increases?
The peak wavelength decreases, shifting to shorter wavelengths as the object's temperature increases.
What real object provides the best approximation to an ideal black body?
Stars provide the best real-world approximation to an ideal black body.
Since a good absorber of radiation is also a good .........., a perfect black body is the best possible emitter.
Since a good absorber of radiation is also a good emitter, a perfect black body is the best possible emitter.
True or False?
A black body only absorbs radiation - it does not emit any.
False.
A perfect black body absorbs and emits all wavelengths of radiation; a good absorber is also a good emitter.
Define the Stefan-Boltzmann Law.
The Stefan-Boltzmann Law states that the total energy emitted by a black body per unit area per second is proportional to the fourth power of its absolute temperature.
State the Stefan-Boltzmann Law equation.
$L = σ A T^{4}$
Where L is luminosity (W), A is surface area, T is absolute temperature (K), and σ is the Stefan-Boltzmann constant.
How is the surface area of a spherical star of radius r calculated?
$A = 4 π r^{2}$
What two factors determine a star's luminosity?
- Its surface temperature
- Its surface area
In the Stefan-Boltzmann law, temperature must be measured in .........., not degrees Celsius.
In the Stefan-Boltzmann law, temperature must be measured in kelvin, not degrees Celsius.
True or False?
Doubling a star's surface temperature doubles its luminosity, assuming its surface area stays constant.
False.
Luminosity is proportional to T⁴, so doubling the temperature increases luminosity by a factor of 16, not 2.
Define Wien's displacement law.
Wien's displacement law states that the black body radiation curve for different temperatures peaks at a wavelength that is inversely proportional to the temperature.
State Wien's Law equation.
$λ_{m a x} T = 2.9 \times 10^{- 3} m K$
Why do hotter stars appear white or blue, while cooler stars appear red or yellow?
Hotter stars have their peak intensity at shorter wavelengths (blue/white end of the spectrum); cooler stars peak at longer wavelengths (red/yellow end).
What units must temperature be measured in when using Wien's Law?
Kelvin (K) - convert from degrees Celsius if the temperature is given in °C.
Wien's Law states that λ_max is inversely proportional to the .......... of the object.
Wien's Law states that λ_max is inversely proportional to the absolute temperature of the object.
True or False?
A star with a longer peak wavelength has a higher surface temperature than one with a shorter peak wavelength.
False.
Peak wavelength is inversely proportional to temperature, so a longer peak wavelength means a lower surface temperature.

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