The electric field lines from an isolated positively charged conducting sphere are
- A.tangential to the conducting surface
- B.at right angles to the conducting surface and towards the centre of the sphere
- C.at any angle to the conducting surface
- D.at right angles to the conducting surface and outwards from the centre of the sphere
Correct answer
Explanation
The correct answer is D, at right angles to the conducting surface and outwards from the centre of the sphere. Field lines always leave a conductor normal to its surface, and for a positive charge they point away from it.
In electrostatic equilibrium the charges on a conductor are at rest, so the field just outside cannot have any component along the surface; otherwise it would push charges and set up a current. The surface is therefore an equipotential, and field lines meet it at 90°. Electric field lines start on positive charges and end on negative ones, so from a positively charged sphere they run radially outward, as if from a point charge at the centre. Inside the conductor the field is zero.
A is wrong: a tangential field would move charges along the surface, which cannot happen in equilibrium.
B is wrong: inward lines would mean a negative charge on the sphere.
C is wrong: the angle is not free; it is always 90° at a conductor's surface.
Exam tip: field lines are normal to a conductor's surface, start on positive and end on negative charge, and the field inside a conductor is zero.
Practice Questions
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