00:01
So the work energy theorem here tells us that the total work done by the external force, which we call that w -e -x -t, and the electric field, which we'll call w -sub -e together, is the change in kinetic energy.
00:17
So the work done by the electric field is going to be given by equation 17 -2b.
00:23
And we're told that the external work that's done externally is 15 times 10 minus 4 joules.
00:29
The final kinetic energy, when it reaches point b, is k e sub f.
00:34
And the charge here, q, is minus 6 .5 times 10 to minus 6 coulomes.
00:39
So our energy here, the work energy theorem, says that the external minus the work done by the electric field, is equal to the kinetic energy final minus the kinetic energy initial.
01:08
But the kinetic energy initial is zero, so we can just ignore that.
01:11
So therefore, the external work, w -e -x -t, minus the work done by the magnetic field is q times the change of potential, which would be v -a minus v -b, is equal to the final kinetic energy, k -e -f.
01:44
The f is the subscript there...