Question 5
The luminosity L of
a star is given by
L = 4πr2σT4
where
r is
the radius of the star,
T is
the temperature of the star,
σ is
a constant with units W m-2 K-4.
What are the SI base
units of L?
A kg
m2 s-1 B kg
m2 s-2 C kg
m2 s-3 D kg
m2 s-4
Reference: Past Exam Paper – June 2016 Paper 12 Q2
Solution:
Answer: C.
L = 4πr2σT4
[L] = [r2] [σ] [T4]
4π is a constant that has
no unit. {Note that some constants may have units.}
[L] = (m2) (W m-2
K-4) (K4) = W
The unit W (watt) is not
an SI base unit.
Watt is the unit of power.
Power = Work done / time =
(Force × distance) / t = (ma × s) / t
Power = (ma × s) / t
[L] = W = (kg) (m s-2)
(m) / s = kg m2 s-3
Can you please tell where it is told that it is Power as it can be work also?
ReplyDeleteThank you
the unit of work is Joule, not Watt.
DeleteI thought it would be Weight instead of power. Why is it not weight?
ReplyDeleteW is a known unit, which is watt
DeleteAh ok, so if it were weight, they would've used newtons? thanks!
Deleteyes
Delete