Calculating Area and Volume of a sphere through line element

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Homework Help Overview

The discussion revolves around calculating the area and volume of a sphere using a specified line element in polar coordinates. The original poster presents their attempts and the actual answers for area and volume, which they are struggling to derive.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definitions of the variables involved in the metric tensor components and their relation to the area and volume calculations. Questions are raised about the nature of the area being calculated and the necessity of the radial component in the volume calculation.

Discussion Status

There is an ongoing exploration of the correct interpretation of the metric components and their application to the problem. Some participants offer insights into the relationships between the components and the calculations, while others seek clarification on the original problem statement and the specifics of the original poster's approach.

Contextual Notes

Participants note that the original poster may be omitting important details from the problem statement, which could be affecting their calculations. There is also a mention of the need for clarity regarding the definitions of the variables used in the metric tensor.

Tony Stark
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Homework Statement


Flat space-time in polar coordinate is considered. The line element is
ds2= -dt2+dr2+r2(dθ2+sin2θdΦ2)

The actual answers are given below, but I can't come up to them. Need urgent help.

Homework Equations


dA = √g11g22 dx1 dx2
dV = √g11g22g33 dx1 dx2dx3

The Attempt at a Solution


g1= 1
g2= r2
g3= r2sin2θ

⇒dA = √1.r2 dr.dθ
dA= r dr dθ
(ACTUAL ANSWER= dA = r2 sinθ dθ dΦ)

Cant calculate Volume
(ACTUAL ANSWER= dV= r2sinθ dθ dΦ dr)
 
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The area is surface area, not area of the projection. That is why the answer is in ##d\theta d\phi##.
What variables do your g1, g2, g3 correspond to? It seems like g1 is the dr term, g2 is the d\theta term, and g3 is the d\phi term.
Why can't you calculate the volume?
 
RUber said:
The area is surface area, not area of the projection. That is why the answer is in ##d\theta d\phi##.
What variables do your g1, g2, g3 correspond to? It seems like g1 is the dr term, g2 is the d\theta term, and g3 is the d\phi term.
Why can't you calculate the volume?
Why would the answer be dΘdΦ instead of drdΦ?
G1,G2,G3 are basis four vector, then how could they be dr,dθ and dΦ?
Explanation needed.
 
Perhaps I am unfamiliar with your application. However, these look like spherical coordinates to me, and the surface area does not require a change in r.
g1 is 1, which corresponds to the square of the factor used for a change in r dr. g2 is r^2, which corresponds to the square of the factor used for a change in theta d theta.
g3 is r^2 sin^2 theta which corresponds to the square of the factor used for a change in phi d phi.
So, from what you have shown, it seems clear that you should be using sqrt(g2 g3) d theta d phi to calculate dA.
 
What is the problem statement you were given, word for word? You seem to be omitting important details.
 
vela said:
What is the problem statement you were given, word for word? You seem to be omitting important details.
According to the question, I had to calculate the area and volume of line element described above.
 
May I have certain more specification into the mistake I am doing...:bow:
 

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