CBSE 2026 · Region 2 · Set 2 · Q25 · 3 marks
(i)Draw electric field lines and equipotential surfaces for a system of two equal and opposite point charges separated by some distance.(ii)Why electric field $\displaystyle \overrightarrow{\mathrm{E}}$ at a point on an equipotential surface must be perpendicular to the surface at that point ?
(i)
Draw electric field lines and equipotential surfaces for a system of two equal and opposite point charges separated by some distance.
(ii)
Why electric field $\displaystyle \overrightarrow{\mathrm{E}}$ at a point on an equipotential surface must be perpendicular to the surface at that point ?
Marking-scheme solution
(i)
(ii)
If the field were not normal to the equipotential surface, it would have a non-zero component along the surface. To move a unit test charge against the direction of the component of the field, work would have to be done. But this is in contradiction to the definition of an equipotential surface.
Alternatively:
$\displaystyle \Delta \mathrm{V}=-\overrightarrow{\mathrm{E}} \cdot \overrightarrow{\Delta \mathrm{l}}$
$\displaystyle 0=-\mathrm{E} \Delta l \cos \theta$
$\displaystyle \theta=90^{\circ}$
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CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.