Spherical coordinates: volume bound by z=r andz^2+y^2+x^2=4

In summary, using spherical coordinates, the smaller volume bounded by the cone z=r and the sphere z^2+y^2+x^2=4 can be found by integrating (rho^2 sin phi) drho d-phi d-theta with the limits rho from 0 to 2, phi from 0 to pi/4, and theta from 0 to 2pi. This results in a volume of 2/3 pi squared. However, there may be some errors in the formula and function used, and a visual representation may be helpful in determining the correct limits.
  • #1
Unemployed
27
0

Homework Statement



Using spherical coordinares, find the smaller volume bounded by the cone z=r and the sphere z^2+y^2+x^2=4

Homework Equations



x^2+y^2+z^2=4 ; rho=2, z=rhocosphi

The Attempt at a Solution



Shot in the dark:

Tried function integrating (rho squared - rhocosphi) d-rho d-phi d-theta with these limits

rho=-2 to 2, phi=0 to p/4, theta=0 to pi/2

Got 2/3 pi squared

Have trouble visualizing, the function is probably wrong and possibly the formula..

help.

 
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  • #2
Some things wrong. For one thing, by convention you don't want to consider negative values of ρ. Another, what is the spherical element dV?

Have you drawn a picture? That will help with the limits.
 
  • #3
I think you have a typo because z=r does not define a cone. In general, to do a regular three-dimensional volume calculation, the integrand is just one, and you integrate over all volume elements in the limits. The volume element in spherical coordinates is:

r^2 sin(theta) dr d-theta d-phi
 
  • #4


Thanks. So the general formula is (rho^2 sin phi drho d-phi d-theta)

z=rho-cos-phi
r=rho-sin-phi
rho cos phi=rho-sin-phi

tan phi=1

So now with the limits rho from 0-2
phi from 0-1 or is it 0-pi/4?
and theta from 0-pi/2

I got 8/3 *(cos 0-cos 1)*pi/2

1.296
 
  • #5
chrisbaird said:
I think you have a typo because z=r does not define a cone.

You might want to reconsider that.
 
  • #6


Unemployed said:
Thanks. So the general formula is (rho^2 sin phi drho d-phi d-theta)

z=rho-cos-phi
r=rho-sin-phi
rho cos phi=rho-sin-phi

tan phi=1

Yes, good.

So now with the limits rho from 0-2
phi from 0-1 or is it 0-pi/4?

But you just showed tan(φ) = 1 so what is φ?
and theta from 0-pi/2

No. Have you drawn a picture yet?
 
  • #7
So phi is 0-pi/4
and theta is also pi/4?
 
  • #8
Unemployed said:
So phi is 0-pi/4
and theta is also pi/4?

Yes for phi. You are just guessing for theta until you draw a picture.
 
  • #9
0 to pi?
 
  • #10
Unemployed said:
0 to pi?

Sorry. I won't play the guessing game. Draw the dang picture.
 
  • #11
i meant 2pi
 
  • #12
LCKurtz said:
Sorry. I won't play the guessing game. Draw the dang picture.

I did draw a picture. It is still very hard to visualize. I am guessing 2pi because the cone is centered on the origin and it is the positive x-z plane. Please help me.
 
  • #13
If you have java running on your computer, take a look at this link. It will help you visualize what varying θ and φ do. The only thing is, since it apparently a physics application, they reverse θ and φ from the standard mathematics notation. So click on the slider to vary φ to see what the results would be varying θ in your problem.

http://www.pha.jhu.edu/~javalab/spherical/spherical.html
 
Last edited by a moderator:

1. What are spherical coordinates?

Spherical coordinates are a system of representing points in three-dimensional space using a radius, an inclination angle, and an azimuth angle. The radius is the distance from the origin to the point, the inclination angle is the angle between the radius and the z-axis, and the azimuth angle is the angle between the projection of the radius onto the xy-plane and the x-axis.

2. How are spherical coordinates related to Cartesian coordinates?

Spherical coordinates are related to Cartesian coordinates through the following equations:
x = r * sin(θ) * cos(φ)
y = r * sin(θ) * sin(φ)
z = r * cos(θ)
where r is the radius, θ is the inclination angle, and φ is the azimuth angle.

3. What is the volume bound by z=r and z^2+y^2+x^2=4 in spherical coordinates?

The volume bound by z=r and z^2+y^2+x^2=4 in spherical coordinates is given by the formula V = (2/3) * π * r^3. This can be derived by using the spherical coordinate equations to transform the Cartesian equation into spherical coordinates, and then integrating over the appropriate ranges of the inclination and azimuth angles.

4. How do you find the intersection between z=r and z^2+y^2+x^2=4 in spherical coordinates?

To find the intersection between z=r and z^2+y^2+x^2=4 in spherical coordinates, we can substitute the value of z=r into the Cartesian equation and solve for the corresponding values of θ and φ. This will give us the coordinates of the point(s) where the two surfaces intersect.

5. Can spherical coordinates be used to represent any point in three-dimensional space?

Yes, spherical coordinates can be used to represent any point in three-dimensional space. However, they are most commonly used for points that are located at a significant distance from the origin, such as in astronomy or physics applications. For points that are closer to the origin, Cartesian coordinates may be more convenient to use.

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