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ELB27
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Homework Statement
A metal sphere of radius ##R## carries a total charge ##Q##. What is the force of repulsion between the "northern" hemisphere and the "southern" hemisphere?
Homework Equations
The force per unit area on the surface of a conductor:
[tex]\vec{f} = \frac{1}{2\epsilon_0}\sigma^2\hat{n}[/tex]
where ##\hat{n}## is the unit vector perpendicular to the surface.
Electrostatic pressure (if I understand it right - just the magnitude of the force per unit area expressed in terms of ##E## just outside the surface):
[tex]P = \frac{\epsilon_0}{2}E^2[/tex]
The Attempt at a Solution
Since it's a sphere the charge will be evenly distributed, thus the surface charge density is: [tex]\sigma = \frac{Q}{4\pi R^2}[/tex] Substituting into the equation for the force per unit area:
[tex]\vec{f} = \frac{Q^2}{32\epsilon_0\pi^2R^4}\hat{r}[/tex] Now to find the force only on one hemisphere I multiply by the surface area of it: [tex]\vec{F} = \vec{f}2\pi R^2 = \frac{Q^2}{16\epsilon_0\pi R^2}\hat{r}[/tex] This approach sounds logical to me, but after searching through google to check my answer (including this forum - it was asked 3 times) I find 2 different approaches. One is the same as mine but another one asserts that the above result for ##\vec{F}## needs to be integrated over the hemisphere and take the ##z## component only. (see this and this ; My approach is suggested here)
While the integration approach seems to dominate (2 out of 3 results in this forum) I don't see why the need for it. My only thought is that if I take the hemisphere as a whole, the x and y components cancel, but I don't understand why my approach doesn't account for that.
I would appreciate if someone can elaborate on this.
Thanks in advance!
Edit: I think I know what's the problem. The question is asking for the force acting on the hemisphere as a whole. In this case the net force will be in the z direction and I need to integrate the z component of the result above. However, I found the repulsion force at every point on the hemisphere. Any thoughts on this?
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