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An isolated metal sphere of radius R, carrying an electric charge Q, is situated in...

Physics
WAEC 2009

An isolated metal sphere of radius R, carrying an electric charge Q, is situated in the medium of relative permitivity, Er. A test charge is placed at a point p, distance r from the surface of the sphere. Let Eo represent the permitivity of free space. The electric potential at p is given by the expression

  • A. \(\frac{Q}{4 \pi E_o E_r}\)
  • B. \(\frac{Q}{4 \pi E_o E_r (R +r)}\)
  • C. \(\frac{Q}{4 \pi E_o E_r(R - r)}\)
  • D. \(\frac{Q}{4 \pi E_o E_rR}\)
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Correct Answer: Option B
Explanation

The electrical potential V at a point P located at a distance r from the surface of an isolated charged metal sphere is given by 

V = \(\frac{\text{Q}}{4\pi\varepsilon}\).\(\frac{1}{\text{d}}\)

Where d = R + r is the distance from the center of the sphere to point P

\(\epsilon\) = \(\varepsilon_r\)\(\varepsilon_o\) is the permittivity of the medium, with \(\epsilon_r\) being the relative permittivity and \(\epsilon_o\) the permittivity of free space

\(\therefore\) V = \(\frac{\text{Q}}{4\pi \varepsilon_r \varepsilon_o}.\frac{1}{R + r}\)


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