Q#26 (Past Exam Paper – June 2012 Paper 41 & 43 Q6)
A solid metal sphere of radius R is isolated in space. The sphere is positively charged so that the electric potential at its surface is VS. The electric field strength at the surface is ES.
(a) On the axes of Fig. 6.1, show the variation of the electric potential with distance x from the centre of the sphere for values of x from x = 0 to x = 3R.
Fig. 6.1
[3]
(b) On the axes of Fig. 6.2, show the variation of the electric field strength with distance x from the centre of the sphere for values of x from x = 0 to x = 3R.
Fig. 6.2
[3]
[Total: 6]
Solution:
(a)
sketch: from x = 0 to x = R, potential is constant at VS
smooth curve through (R, VS) and (2R, 0.5VS)
smooth curve continues to (3R, 0.33VS)
{Electric potential: V = Q / 4πϵ0x where x is the distance from the centre
For a conductor, the potential is constant inside the sphere. So, from x = 0 to x = R (that is, inside the sphere), potential = VS and is constant. This is a horizontal line in the graph.
Hence, we also have the point (R, VS).
VS = Q / 4πϵ0R
From the equation for potential above, the potential V is inversely proportional to x. The greater the value of x, the smaller the potential.
V = Q / 4πϵ0x
When x = R, V = Q / 4πϵ0R = VS
When x = 2R, V = Q / 4πϵ0(2R) = ½ VS (2R, 0.5 VS)
When x = 3R, V = Q / 4πϵ0(3R) = 1/3 VS (2R, 0.33 VS)
}
(b)
sketch: from x = 0 to x = R, field strength is zero
smooth curve through (R, E) and (2R, 0.25E)
smooth curve continues to (3R, 0.11E)
{Electric field strength: E = Q / 4πϵ0x2 where x is the distance from the centre
For a conductor, the field strength is zero inside the sphere. So, from x = 0 to x = R (that is, inside the sphere), field strength = zero. This is a horizontal line in the graph.
At the surface of the sphere, the field strength is maximum. This is the point (R, 1.0 ES).
From the equation for field strength above, the field strength E is inversely proportional to x2. The greater the value of x, the smaller the potential.
E = Q / 4πϵ0x2
When x = R, E = Q / 4πϵ0R2 = ES (R, ES)
When x = 2R, E = Q / 4πϵ0(2R)2 = ¼ ES (R, 0.25ES)
When x = 3R, E = Q / 4πϵ0(3R)2 = 1/9 ES (R, 0.11ES)
}
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