I have been wondering why is the direction of force on dielectric in case b) is

Question

I have been wondering why is the direction of force on dielectric in case b) is going to be inwards even tough the result is saying that is is outwards.

I understand that there can be some fringing that will make it happen, but where is our formula making a mistake (this formula):
F = - dU/dx

I suppose that this system is not isolated (there is a voltage source connected) and i think that this should be the reason why we cannot apply F = - dU/dx. (But at the same time, this is confusing: all forces seem to be conservative and we should be able to apply the formula, and when we apply it we get a wrong result).

One more thing: how do we calculate the magnitude of the force?

Explanation / Answer

F = (I t)(d / t)B simplify...

The boat itself needs a reasonable force to accelerate it. The following calculations are based on my construction of the boat.

Use F = I d B

I = 3 A (short circuit current between electrodes)

d = 2.7 cm (0.027 m) (distance between electrodes)

B = 0.2 T ( S.I. units: 0.2 N/(A*m) measured)

therefore force on water (force propelling the boat) is:

F = (3.0 A)(0.027 m)(0.2 N/(A*m))

It is only natural that students in grade 12 physics don't have an intuitive idea of how much this force actually is, so to put things into perspective you ask the question:

what mass would this be if the force is due to gravity acting on that mass?

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