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PRACTICE / Electrostatic potential energy

Add one charge without recounting old pairs

Two fixed +3 nC charges are already in place. A third charge of −2 nC is brought slowly from infinity to a location 0.30 m from each existing charge.

What work does the external agent do during this addition?

Organize the givens & assumptions
  • Two fixed sources: +3 nC each
  • Added charge: −2 nC
  • Two source distances: 0.30 m each

Fixed point sources in vacuum; k = 9.0×10⁹ N m²/C². Ignore the probe’s effect on the sources. Zero potential at infinity. The external transfer is quasistatic with no change in kinetic energy; existing sources do not move.

  1. A3.6×10−7 J3.6\times10^{-7}\ \mathrm J
  2. B−1.8×10−7 J-1.8\times10^{-7}\ \mathrm J
  3. C0 J0\ \mathrm J
  4. D−3.6×10−7 J-3.6\times10^{-7}\ \mathrm J

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