# Solar energy question

#### asdf1

In this question,
"Solar energy reached the earth at the rate of about 1.4 kW per square meter of surface perpendictular to the direction of the sun. By how much odes the mass of the sun decrease per second owing to this energy law?"

What is your first reaction to this question? How would you make the first move?

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#### russ_watters

Mentor
First find the surface area of a sphere with the radius of the earth's orbit. Multiply by that 1.4kW flux and you'll get the total power output of the sun. Then, apply E=mc^2...

#### Nereid

Staff Emeritus
Gold Member
What is your first reaction to this question?
That it's a homework question, and has been posted in the wrong part of PF?

#### asdf1

it's not homework~
i'm trying to self-study modern physics...

#### asdf1

russ_watters said:
First find the surface area of a sphere with the radius of the earth's orbit. Multiply by that 1.4kW flux and you'll get the total power output of the sun. Then, apply E=mc^2...
@@a
how did you think of why to "analyze it" that way?

#### Astronuc

Staff Emeritus
asdf1 said:
@@a
how did you think of why to "analyze it" that way?
The question posed - a power flux - 1.4 kW per square meter of surface.

So to get the total power (in kW), one simply multiplies the flux (energy/unit area) by the total area, as Russ indicated. This application assumes that the flux is constant in all directions.

Conversely, to get the flux at some distance, simply divide the total power (again assuming the energy is isotropically distributed) by the area of a sphere at that distance (radius of the sphere).

Remember power = energy per unit time.

#### asdf1

thank you very much!!! :)
i think physics is interesting, but i often have trouble making the first step...
thanks so much for explaining!!!

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