Two point charges of -7uc and 4uc are a distance of 20
cmapart. how much work does it take to move these charges outto a
separation of 90 cm apart @ a constant speed?

Approximately 0.979 J.

Explanation:

Assume that the two charges are in vacuum. Apply the coulomb's law to find their initial and final electrical potential energy .

,

where

Note that there's no negative sign before the fraction.

Make sure that all values are in SI units:

Apply Coulomb's law:

Initial potential energy:

.

Final potential energy:

.

The final potential energy is less negative than the initial one. In other words, the two particles gain energy in this process. The energy difference (final minus initial) will be equal to the work required to move them at a constant speed.

.

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