In this video http://www.youtube.com/watch?v=elJUghWSV… from 7:10 to 10:00, Salman Khan calculates the work to approximate a +charged particle towards a +charged plate.
I disagree on what he says, that 10N is enough to move the particle down.
I mean, if the field pushes it up with 10N and somebody pushes it down with 10N too, obviously the charge won't move! Neither upwards nor downwards!!
Am I right or wrong?
I disagree on what he says, that 10N is enough to move the particle down.
I mean, if the field pushes it up with 10N and somebody pushes it down with 10N too, obviously the charge won't move! Neither upwards nor downwards!!
Am I right or wrong?
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Yes!! the answer should be 10.000000000000000000000000000000001 N
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If the field pushes up with 10 N and someone pushes down with 10 N, then there is no net force, and the particle will continue doing whatever it was doing before (Newton's First Law). So if the particle is already moving, it will continue to move.
What Salman is doing is assuming the particle is already moving with an arbitrarily small speed. This gets him out of worrying about accelerating the particle to speed it up, and then decelerating it again at the end to slow it down. The work done in those two acceleration processes will cancel, anyway, so the result (how much work is done) will be the same as assuming the arbitrarily small speed.
So, you're right that it will take more than 10 N to get the particle moving if it is at rest. But you are wrong if you think a force of greater than 10 N must be applied after that to keep it going.
What Salman is doing is assuming the particle is already moving with an arbitrarily small speed. This gets him out of worrying about accelerating the particle to speed it up, and then decelerating it again at the end to slow it down. The work done in those two acceleration processes will cancel, anyway, so the result (how much work is done) will be the same as assuming the arbitrarily small speed.
So, you're right that it will take more than 10 N to get the particle moving if it is at rest. But you are wrong if you think a force of greater than 10 N must be applied after that to keep it going.