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Let a uniform surface charge density of 5 nC/m2 be present at the z = 0 plane, a uniform line charge density of 8 nC/m be located at x = 0, z = 4 and a point charge of 2 μC be present at P(2, 0, 0). If V = 0 at A(0, 0, 5), the V at B(1, 2, 3) is
  • a)
    10.46 kV
  • b)
    1.98 kV
  • c)
    0.96 kV
  • d)
    3.78 kV
Correct answer is option 'B'. Can you explain this answer?
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Let a uniform surface charge density of 5 nC/m2be present at the z = 0...
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Let a uniform surface charge density of 5 nC/m2be present at the z = 0...
Solution:

Given:
Surface charge density σ = 5 nC/m²
Line charge density λ = 8 nC/m
Point charge q = 2 C
Point A(0, 0, 5)
Point P(2, 0, 0)
Point B(1, 2, 3)

To find:
Potential difference between points A and B i.e. VAB

Concept:
We will use the principle of superposition to find the potential at point B due to each of the charges separately and then add them up to get the total potential.

Calculation:
1. Potential due to the surface charge density σ at point B:
Consider a small element of area dA on the surface charge.
The distance between the element and point B is r = [(1-0)² + (2-0)² + (3-0)²]½ = 3.74 m
The potential due to this element is dV = kdAσ/r
The total potential due to the surface charge is Vσ = ∫dV = kσ/r ∫dA
The surface is infinite in extent, but the potential contribution from any point on the surface will be the same at point B. So, we can assume a circular surface of radius R around point B and then take the limit R → ∞.
Vσ = kσ/r ∫dA = kσ/r ∫2πR²sinθdθdφ = 2πkσR/r
As R → ∞, Vσ → 0 (because σ is finite)

2. Potential due to the line charge density λ at point B:
Consider a small element of length dl on the line charge.
The distance between the element and point B is r = [(1-0)² + (2-0)² + (3-4)²]½ = 2.45 m
The potential due to this element is dV = kλdl/r
The total potential due to the line charge is Vλ = ∫dV = kλ/r ∫dl
The line is infinite in extent, but the potential contribution from any point on the line will be the same at point B. So, we can assume a cylindrical surface of radius R around the line and then take the limit R → ∞.
Vλ = kλ/r ∫dl = 2πkλln(R/r)
As R → ∞, Vλ → ∞ (because λ is infinite)

3. Potential due to the point charge q at point B:
The distance between the point charge and point B is r = [(1-2)² + (2-0)² + (3-0)²]½ = 2.45 m
The potential due to this point charge is Vq = kq/r

4. Total potential at point B:
VAB = VB - VA = Vσ + Vλ + Vq - 0 (because VA = 0)
VAB = 2πkσR/r + 2πkλln(R/r) + kq/r
To find the value of R that minimizes VAB, we differentiate VAB with respect to R and equate it to zero:
dVAB/dR = 2πkσ/r - 2πkλ/r + kq/R² =
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Let a uniform surface charge density of 5 nC/m2be present at the z = 0 plane, a uniform line charge density of 8 nC/m be located at x = 0, z = 4 and a point charge of 2 μC be present at P(2, 0, 0). If V = 0 at A(0, 0, 5), the V at B(1, 2, 3) isa)10.46 kVb)1.98 kVc)0.96 kVd)3.78 kVCorrect answer is option 'B'. Can you explain this answer?
Question Description
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