The flux through the hemispherical surface

[ehild]

What is surface area of disk?

A disk can be a three-dimensional body like a floppy disk http://www.merriam-webster.com/dictionary/disk. In that case, it is a flat right cylinder, having a side and two bases. The surface are is twice the area of the base plus the area of the side.
In your problem, the disk is a circle. You need to calculate the flux through the circle .

 

[ehild]

So,how surface area of sphere comes out to be 4 pi r^2?Does it include 4 circles?I don't think so

Gracy, I can not believe that you never learned the area of the sphere. It does not mean that it consists of 4 circles.

 

[haruspex]

So,how surface area of sphere comes out to be 4 pi r^2?Does it include 4 circles?I don't think so

It happens to have the same area as four circles of the same radius. But it's a curved surface, so you would have to cut it into infinitely many pieces to compose it of plane areas. Anyway, I was not suggesting the closed surface necessarily be cut into plane areas. You could cut it into eight octants, say, and each would turn out to have an area $\pi r^2/2$. I was just trying to show you that there is no fundamental difference between surface area of a closed 3D region and area of a surface (which might or might not be a plane surface).

 

[gracy]

I can not believe that you never learned the area of the sphere.

Yes! I never heard ,I only knew surface area of sphere.

 

[haruspex]

Yes! I never heard ,I only knew surface area of sphere.

ehild is referring to the surface area of a sphere. There is an ambiguity here. The term sphere is used equally to mean a solid sphere or a spherical surface. The area of a spherical surface means the same as the surface area of a solid sphere. The qualifier "surface" area is only needed/appropriate when "sphere" means a solid sphere.

 

[gracy]

it should be πR^2E

I got why!Thanks haru!

 

[BvU]

It was always there!

This does not help me at all. Sorry.

 

[gracy]

This does not help me at all

What do you mean?

 

[gracy]

This does not help me at all

Oh!understood.Even when you say/write something straight forward I tend to take it logically or sarcastically to be more precise.