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 — D, at right angles to the conducting surface and outwards from the centre of the sphere. On any charged conductor in electrostatic equilibrium the electric field just outside must be perpendicular (normal) to the surface — any tangential component would push the free charges along the surface until it disappeared. Because the sphere carries positive charge, the field points away from the positive charge, that is radially outward from the centre. So the field lines leave the surface at right angles and point outwards.
- (a)tangential to the conducting surface — A tangential field would keep driving the free surface charges sideways; in equilibrium the field must be perpendicular to the conductor's surface, not along it.
- (b)at right angles to the conducting surface and towards the centre of the sphere — The 'right angles' part is right, but field lines point away from a positive charge; inward-pointing lines would indicate a negatively charged sphere.
- (c)at any angle to the conducting surface — The field is not at an arbitrary angle — just outside a conductor in equilibrium it is always perpendicular to the surface.
Just outside any charged conductor in electrostatic equilibrium the electric field is perpendicular to the surface, because a tangential component would move the free charges. Field lines begin on positive charges and point away from them, so for a positively charged sphere they radiate outward from the surface.
Two rules settle the answer: the field of a conductor is normal (perpendicular) to its surface, and it points away from positive charge. Combining them gives lines at right angles to the surface and directed outward.
- The electric field just outside a charged conductor is always perpendicular to its surface.
- A tangential field component cannot exist in equilibrium — it would move the free charges.
- Field lines start on positive charges and point away from them (outward for a positive sphere).
- Outside it, an isolated charged sphere behaves as if all its charge were at its centre.
Normal to the surface and pointing outward — option (d).
- Choosing 'towards the centre' for a positive sphere — that direction is for a negative charge.
- Thinking field lines can run along (tangential to) a conductor's surface.
Tests the two rules for a charged conductor: the field is perpendicular to the surface and points away from positive charge.
No directly related past PYQ was found.
- practice — not a real PYQ
The electric field just outside the surface of a charged conductor is directed
- (a)parallel to the surface
- (b)perpendicular to the surface
- (c)at 45° to the surface
- (d)in a random direction
Answer(b) perpendicular to the surface — otherwise the free surface charges would move.
- practice — not a real PYQ
The electric field lines around an isolated negative point charge are directed
- (a)radially outward
- (b)radially inward
- (c)tangentially
- (d)in closed loops
Answer(b) radially inward — field lines point towards a negative charge.