Potential due to a uniformly charged sphere

In summary, we are trying to find the electrical potential at r<R by subtracting the potential at infinity from the potential at r, which is the sum of the change in potential from infinity to R and the change in potential from R to r. This is because the work done against the electric field in going from infinity to r is equal to the sum of the work done from infinity to R and the work done from R to r. The potential at infinity is zero and as we move in the direction of the electric field, the potential increases. Therefore, moving from infinity to R gives us the potential at R, and moving from R to r gives us the change in potential while going from R to r.
  • #1
gracy
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http://www.phys.uri.edu/gerhard/PHY204/tsl93.pdf
To find electrical potential at r<R
I want to know why should we subtract
##V##=-##\int_∞^R\frac{kQ}{r^2}\,dr##-##\int_R^r(0)\,dr##=##\frac{kQ}{R}##
I don't know why are we subtracting these two?
 
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  • #2
gracy said:
I don't know why are we subtracting these two?
You are actually adding two pieces: (1) change in potential from ∞ to R, (2) change in potential from R to r.
 
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  • #3
Doc Al said:
You are actually adding two pieces:
Just because
##E##=##-\frac{∂V}{∂r}##

Negative sign!
 
  • #4
gracy said:
Negative sign!
Yep!
 
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  • #5
Doc Al said:
You are actually adding two pieces
And why adding them gives potential at r<R?
 
  • #6
gracy said:
And why adding them gives potential at r<R?
The work done against the electric field (which is the potential) in going from "A" to "C" equals the work done from A to B plus the work done from B to C.
 
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  • #7
potential at infinity is zero,as we move in the direction of electric field the potential increases .Moving from infinity to R gives potential at R (integration with upper and lower limits i.e final and initial positions being infinity and R),right?
 
  • #8
gracy said:
potential at infinity is zero,as we move in the direction of electric field the potential increases .Moving from infinity to R gives potential at R (integration with upper and lower limits i.e final and initial positions being infinity and R),right?
Right.
 
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  • #9
Then Moving from R to r (initial and final points being R and r respectively)gives
1)potential at r
OR
2)Change in potential while going from R to r
which one?
 
  • #10
gracy said:
Then Moving from R to r (initial and final points being R and r respectively)gives
1)potential at r
OR
2)Change in potential while going from R to r
which one?
Number 2: The change in potential. (The potential itself is defined as work from infinity to r.)
 
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1. What is the mathematical expression for the potential due to a uniformly charged sphere?

The potential due to a uniformly charged sphere can be expressed as V = kQ/R, where k is the Coulomb's constant, Q is the charge of the sphere, and R is the distance from the center of the sphere.

2. How is the potential due to a uniformly charged sphere related to the electric field?

The potential due to a uniformly charged sphere is directly proportional to the electric field. This means that as the potential increases, the electric field also increases.

3. Can the potential due to a uniformly charged sphere be negative?

Yes, the potential due to a uniformly charged sphere can be negative. This occurs when the charge of the sphere is negative. A negative potential means that work needs to be done to move a positive test charge from infinity to a point near the sphere.

4. How does the potential due to a uniformly charged sphere change with distance?

The potential due to a uniformly charged sphere decreases with distance from the center of the sphere. This can be seen in the expression V = kQ/R, where R is in the denominator. As R increases, V decreases.

5. Is the potential due to a uniformly charged sphere affected by the shape of the sphere?

No, the potential due to a uniformly charged sphere is not affected by the shape of the sphere. As long as the charge is uniformly distributed, the potential will be the same regardless of the shape of the sphere.

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