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I am working through some questions and I am stuck on the working of this one:

enter image description here

Where the working to the answer is here:

enter image description here

Can someone explain the highlighted section as I can't see where it comes from?

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  • $\begingroup$ The area of a sphere is $4\pi r^2$. $\endgroup$ – joseph h Dec 18 '20 at 23:33
  • $\begingroup$ @Drjh where does the $\frac{1}{R^2}$ factor come from? $\endgroup$ – Σ baryon Dec 18 '20 at 23:35
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/488220/2451 and links therein. $\endgroup$ – Qmechanic Dec 18 '20 at 23:36
  • $\begingroup$ @Qmechanic interesting. However, I am having trouble understanding the more explicit working to the solution as I cant see how the transition was made from the surface integral to the $4\pi R^2 \cdot \frac{1}{R^2}$ $\endgroup$ – Σ baryon Dec 18 '20 at 23:42
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    $\begingroup$ The site standard for math expressions is Mathjax which is very similar to LaTex. We strongly discourage posting images of text or equations as they are not searchable by the site engine and in the case of photos of written script often hard to read. $\endgroup$ – StephenG Dec 19 '20 at 0:40
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Here is the more explicit working I found for this question which is what I was interested in: enter image description here

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