CAIE A-Level Physics 18.3.1 Charged Spherical Conductors

Practise explaining why a charged spherical conductor can be treated externally as a point charge at its centre, and applying that model to field calculations.

Exam points

explain why the charge on a spherical conductor outside the sphere may be treated as a point charge at its centre

[Maximum number: 3]

An isolated metal sphere of radius r is charged so that the electric field strength at its surface is $E_{0}$ .

On Fig. 6.1, sketch the variation of the electric field strength E with distance x from the centre of the sphere. Your sketch should extend from x=0 to x=3 r.

Fig. 6.1

from x=0 to x=r: E=0

B1

from x=r to x=3 r : curve with negative gradient of decreasing magnitude passing through ( $r, E_{0}$ )

B1

line passing through ( $2 r, E_{0} / 4$ ) and ( $3 r, E_{0} / 9$ )

B1

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18.3.1—That, for a point outside a spherical conductor, the charge on the sphere question 1