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Class 12 › Physics › Electric Charge and Field
CBSE2024Class Class 12 · Physics2 Marks · Short✅ Verified
Figure shows the field lines of a positive charge. Is the work done by the field in moving a small positive charge from Q to P positive or negative? Give reason.
✅ Answer & Solution
<b>Answer: Work done is NEGATIVE</b><br><br><b>Setup (as per standard CBSE figure):</b><br>• Q is farther from the source positive charge<br>• P is closer to the source positive charge<br><br><b>Step 1: Compare potentials</b><br>Since potential due to a positive charge V = kq/r decreases with distance:<br>• V<sub>P</sub> > V<sub>Q</sub> (P is closer, so higher potential)<br><br><b>Step 2: Apply work-energy formula</b><br>Work done by field: W = q(V<sub>initial</sub> - V<sub>final</sub>)<br>W = q(V<sub>Q</sub> - V<sub>P</sub>)<br><br><b>Step 3: Determine sign</b><br>Since V<sub>Q</sub> = V<sub>P</sub>, we get (V<sub>Q</sub> - V<sub>P</sub>) < 0<br>Hence <b>W = 0 (negative)</b><br><br><b>Reason:</b><br>A small positive charge moving from Q (farther) to P (closer) is moving <b>against</b> the direction of the electric field. Hence the field does <b>negative work</b> on it (external agent must do positive work).
