DP Physics (last assessment 2024)

Question 20N.3.SL.TZ0.15

Date	November 2020	Marks available	[Maximum mark: 3]	Reference code	20N.3.SL.TZ0.15
Level	SL	Paper	3	Time zone	TZ0
Command term	Explain, Show that	Question number	15	Adapted from	N/A

15.

[Maximum mark: 3]

20N.3.SL.TZ0.15

(a)

Show that the apparent brightness $b \propto \frac{A T^{4}}{d^{2}}$ , where $d$ is the distance of the object from Earth, $T$ is the surface temperature of the object and $A$ is the surface area of the object.

[1]

Markscheme

substitution of $L = σ A T^{4}$ into $b = \frac{L}{4 {πd}^{2}}$ giving $b = \frac{σ A T^{4}}{4 {πd}^{2}}$

Removal of constants $σ$ and $4 π$ is optional

(b)

Two of the brightest objects in the night sky seen from Earth are the planet Venus and the star Sirius. Explain why the equation $b \propto \frac{A T^{4}}{d^{2}}$ is applicable to Sirius but not to Venus.

[2]

Markscheme

equation applies to Sirius/stars that are luminous/emit light «from fusion» ✓

but Venus reflects the Sun’s light/does not emit light «from fusion» ✓

OWTTE

Syllabus sections

Option D: Astrophysics

Option D: Astrophysics » Option D: Astrophysics (Core topics) » D.1 – Stellar quantities

Option D: Astrophysics » Option D: Astrophysics (Core topics)

Question 20N.3.SL.TZ0.15

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