Parametric Equation of a sphere

In summary, the student attempted to solve a problem involving a sphere that lies between two planes, but was not sure how to parametrically solve for theta and phi. They found that z=3cos(phi) and y=3*cos(phi), and that theta was 0<\theta<2\pi.
  • #1
anubis01
149
1

Homework Statement


Find parametric equations for the part of sphere x2+y2+z2=9 that lies between the planes y=1 and y=2.


Homework Equations





The Attempt at a Solution



Okay knowing that the p=3 I wrote the parametric equations for a sphere as
x=3sin[tex]\phi[/tex]cos[tex]\theta[/tex] y=3sin[tex]\phi[/tex]sin[tex]\theta[/tex]
z=3cos[tex]\phi[/tex] now the phi bound is 0<[tex]\phi[/tex]<[tex]\pi[/tex]
but I'm not sure what to write for the [tex]\theta[/tex] bound.
 
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  • #2
It would be easier if they said between z=2 and z=1, right? Then you would just have to restrict phi. There's no reason why you can't interchange say, y and z in your parametrization.
 
  • #3
Oh okay so I can say z=3cos[tex]\phi[/tex]
and from there plug in the values to find the [tex]\phi[/tex] bound to be cos(1/3)-1<[tex]\phi[/tex]<cos(2/3)-1 and the [tex]\theta[/tex] bound should just be 0< [tex]\theta[/tex]<2[tex]\pi[/tex]
 
  • #4
anubis01 said:
Oh okay so I can say z=3cos[tex]\phi[/tex]
and from there plug in the values to find the [tex]\phi[/tex] bound to be cos(1/3)-1<[tex]\phi[/tex]<cos(2/3)-1 and the [tex]\theta[/tex] bound should just be 0< [tex]\theta[/tex]<2[tex]\pi[/tex]

Right. But they did say y=1 and y=2. You'll also have to tweak your parametrization so y=3*cos(theta) instead of z=3*cos(theta).
 
  • #5
oh so then phi bound should be 0<[tex]\phi[/tex]<[tex]\pi[/tex] and since y=3cos[tex]\theta[/tex] the bounds for [tex]\theta[/tex] are
cos(1/3)-1<[tex]\theta[/tex]<cos(2/3)-1
 
  • #6
anubis01 said:
oh so then phi bound should be 0<[tex]\phi[/tex]<[tex]\pi[/tex] and since y=3cos[tex]\theta[/tex] the bounds for [tex]\theta[/tex] are
cos(1/3)-1<[tex]\theta[/tex]<cos(2/3)-1

Sorry, sorry! I meant to say your parametrization has z=3*cos(phi) and you want to change it so that y=3*cos(phi). Leave theta as it is.
 
  • #7
alright i think i got it now. So bound for theta 0<[tex]\theta[/tex]<2[tex]\pi[/tex]
y=3cos[tex]\phi[/tex] the bounds for phi are then
cos(1/3)-1<[tex]\phi[/tex]<cos(2/3)-1

and the parametric equation for x=3sin[tex]\phi[/tex]cos[tex]\theta[/tex]
and z=3sin[tex]\phi[/tex]sin[tex]\theta[/tex]
 
  • #8
anubis01 said:
alright i think i got it now. So bound for theta 0<[tex]\theta[/tex]<2[tex]\pi[/tex]
y=3cos[tex]\phi[/tex] the bounds for phi are then
cos(1/3)-1<[tex]\phi[/tex]<cos(2/3)-1

and the parametric equation for x=3sin[tex]\phi[/tex]cos[tex]\theta[/tex]
and z=3sin[tex]\phi[/tex]sin[tex]\theta[/tex]

That looks good to me.
 
  • #9
alright thank you for all the help, its much appreciated.
 

Related to Parametric Equation of a sphere

1. What is the parametric equation for a sphere?

The parametric equation for a sphere is given by x = r sinθ cosφ, y = r sinθ sinφ, and z = r cosθ, where r is the radius of the sphere, θ is the polar angle, and φ is the azimuthal angle.

2. How is the parametric equation of a sphere derived?

The parametric equation of a sphere is derived by using the spherical coordinate system, where the radius r, polar angle θ, and azimuthal angle φ are used to locate points on the surface of a sphere.

3. What are the advantages of using the parametric equation of a sphere?

The parametric equation of a sphere allows for a more efficient way of representing points on the surface of a sphere, as it only requires two variables (θ and φ) instead of three (x, y, and z). It also allows for easier calculations and visualizations in three-dimensional space.

4. Can the parametric equation of a sphere be used in other contexts?

Yes, the parametric equation of a sphere can be used in various fields such as computer graphics, physics, and engineering, as it provides a convenient way to represent points on a spherical surface.

5. How is the parametric equation of a sphere different from the standard equation of a sphere?

The standard equation of a sphere, x² + y² + z² = r², represents a sphere in Cartesian coordinates, while the parametric equation x = r sinθ cosφ, y = r sinθ sinφ, and z = r cosθ represents it in spherical coordinates. The parametric equation allows for a more flexible and versatile representation of a sphere, as it can easily be transformed and manipulated in 3D space.

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