# Solid angle or steradian for measuring

1. Aug 25, 2009

### monty37

where do we use solid angle or steradian for measuring,in what way
is theuseful over normal angle?

2. Aug 25, 2009

### Gear300

They are useful for proving the generality of Gauss's law for closed surfaces.

3. Aug 25, 2009

### Integral

Staff Emeritus

Consider a sphere and a cone emanating from the center, now how do you apply "normal" angles to describe the cone?

4. Aug 26, 2009

### penguin007

They are also useful in the measure of radiant intensity.

5. Aug 26, 2009

### Born2bwire

Geometric phase problems sometimes has them, like with Foucalt's pendulum.

6. Aug 26, 2009

### TheDestroyer

They are very useful, of course, if you review the definition of Radian, you'll find the it's the angle where the length of the arc is equal to the radius, in different angles, you'll be taking the ratio between the arc and the radius.
In 3D case, you'll find yourself talking about an area and a cone, this cone doesn't necessarily have to be based on a circle, maybe on some elliptical shape, this gives some kind of a generalisation to the definition of an angle in higher dimensions, that it's a ratio, more than being a physical quantity.

If I'm not right, I'd love someone to correct for me :)

Good luck :)

7. Aug 26, 2009

### Bob S

Prove that the surface area of a sphere of radius R is 4 pi R2. The 4 pi steradians is the solid angle of the entire surface.

8. Aug 26, 2009