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112物理分科測驗–04


<單選題>空間中有相距 \(2d\) 的兩靜止點電荷,已知將此兩點電荷緩慢移近至相距 \(d\) 靜止。需作功 \(W\)。若再將兩點電荷緩慢移近至相距 \(d/5\) 靜止,則需再作多少功?
(A)10\(W\)
(B)8\(W\)
(C)5\(W\)
(D)4\(W\)
(E)\(W\)

答案

設兩同號電荷 +Q, +q。\( U_{2d} = \frac{kQq}{2d} \),\( U_d = \frac{kQq}{d} \),\( W = U_d - U_{2d} = \frac{kQq}{2d} \)。\( U_{d/5} = \frac{5kQq}{d} \),再作功 \( W' = U_{d/5} - U_d = \frac{4kQq}{d} = 8W \),選(B)。 報錯
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