# probability of striking the circular ring by gas molecules

In kinetic theory we use probabilistic case to derive pressure, no. Of molecules having speed c to c+dc or in such cases.and to derive such equations we introduce a term called "SOLID ANGLE"

I come to know that this is a kind of 3d angle or a closed angle like the angle surrounded by a cone towards its apex And equation for solid angle is dw=dAcosx/r^2 Although I don't know where the formula come from! But this is a very useful indeed. In kinetic theory it is used to determine the no. Of particles striking a unit area element dA

But I am a little bit confused about the solid angle cause it tells about the he angle of the cone but can I consider the angle of a pyramid towards its apex as it's base is a square not circle????

Always when we tell about solid angle we take the reference of the angle of a cone which has a circular base but why cone why not about a pyramid

1)in kinetic theory we consider the probability of molecule striking the elementary area dA is = dw/4π But what will be the probability of molecule strike a circular ring not area, Consider the circular perimeter of a cone???

2)in 2D what will be the probability of molecule strike a unit length of boundary as I can not consider solid angle there cause solid angle can only be considered in 3D So what will be the 2D form of solid angle???

• Please anyone solve my problem – user101134 Apr 4 '16 at 6:16