Solid angle a solid angle in steradians is the following ratio: where A is the spherical surface area and r is the radius of the considered sphere. Solid angles are often used in physics, in particular astrophysics. The solid angle of an object that is very far away is roughly proportional to the ratio of area to squared distance. Here "area" means the area of the object when projected along the viewing direction. The solid angle of a sphere measured from any point in its interior is 4 sr, and the solid angle subtended at the center of a cube by one
Surface stress same, they are very different in solid–gas or solid–solid interface, which will be discussed in details later. Since both terms represent a force per unit length, they have been referred to as “surface tension”, which contributes further to the confusion in the literature. Definition of surface free energy is seemly the amount of reversible work formula_1 performed to create new area formula_2 of surface, expressed as: Gibbs was the first to define another surface quantity, different from formula_4, that is associated with the reversible work per unit area needed to elastically stretch a pre-existing surface. Surface stress can be derived