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Physics · 2026 · 3 marks
CBSE 2026 · Region 2 · Set 2 · Q25
(i) Draw electric field lines and equipotential surfaces for a system of two equal and opposite point charges separated by some distance. (ii) Why…
(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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