i) The surface area of any sphere of radius R is Surface Area = 4pR2. ii) The en
ID: 1980844 • Letter: I
Question
i) The surface area of any sphere of radius R is Surface Area = 4pR2.ii) The energy reaching us on the Earth from the Sun is about 1 kilowatt per
square meter. (That means that if you put a square meter of cardboard on the
ground, it will soak up 1 joule of energy from the Sun every second.)
iii) The sun is about 93 million miles away from the Earth.
iv) Firewood has an energy content of about 15 megajoules per kilogram.
v) The mass of the sun, as determined from Newton’s Universal Law of Gravity,
is msun = 2 * 1030 kg.
Using the above facts, calculate the expected lifetime of the sun in years.
Explanation / Answer
lifetime = (energy) / (rate [energy/time] at which sun emits energy) E = 0.007 x M c2 where c is the speed of light and M is the amount of mass in the sun that is capable of undergoing the above nuclear reactions. where Msun is the total mass of the sun, 2 x 1030 kilograms. We therefore can calculate that the total energy the sun has to burn is around 1.3 x 1044 Joules. Dividing 3.8 x 1026 Watts (the rate at which the sun is giving off energy) into this number gives an approximate value of 10 billion years for the sun's lifetime.
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