the potential difference in an equipotential surface is zero. so the work done in moving the charge across the equipotenial surface is also zero. according to this the test charge must then keep moving continuously over the equipotential surface w
The source charge and test charge when connected by a straight line, the straight line will always be pependicular to the any equipotential surface. Hence, when test charge moves towards the source charge it's no logner moving on equipotential surface.