First slide

Potential energy

Question

Two point charges +Q and $- \frac{Q}{2}$ are kept at a separation and potential energy at the mid point joining the charges is -0.8 U. Now +2Q charge is added to each of the point charge. Then potential energy at the same point has a magnitude of

Moderate

A

3.2 U
B

2.4 U
C

3.6 U
D

7.2 U

Solution

Initially, $U = K \frac{(+ Q) (- \frac{Q}{2})}{d} = - \frac{1}{2} \frac{{KQ}^{2}}{d}$
Finally $U^{1} = K \frac{(+ 3 Q) (2 Q - \frac{Q}{2})}{d} = \frac{9}{2} . \frac{{KQ}^{2}}{d}$
$\Rightarrow U^{1} = 9 \times 0.8 U = 7 .2 U$

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