point charge

子路

2009-05-31 12:16 am

first of all i have to say the question gave inaccurate data/unit
but i can't still correct it

so i give 10pt even for a short Q

圖片參考：http://i256.photobucket.com/albums/hh182/zilu_photo/Capture8.jpg

回答 (1)

天同

2009-05-31 9:32 am

✔ 最佳答案

The given charges are 12(?) and 8J(??) are quite ambiquous !!!
Assume they are of charges Q and q, say. The work done in bringing them closer equals the gain in potential energy of the system.
Potential energy when the two charges are 10 cm (=0.1 m) apart
V(10cm) = kQq/0.1 Joules
where k = 1/(4/pi/e), e is the permitivity of free space and pi = 3.14159...
Potential energy when the two charges are 6 cm (=0.06 m) apart
V(6cm) = kQq/0.06 Joules
Hence, gain in potential energy
= (kQq/0.06 - kQq/0.1) Joules
= kQq[1/0.06 - 1/0.1] Joules
Work done = gain in potential energy = kQq[1/0.06 - 1/0.1] Joules

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