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I was looking at past exam papers and found a question that I can't understand. It is basically about current flow but the diagram is really confusing me. The diagram is the following: Diagram

The question is the following one: Question

So far I am okay, but when I reached question e.) part iii, a lot of things are confusing me. Basically, the procedure is to find the electric field due to the two objects by using the current density, and then integrate this electric field to find the voltage, and finally the resistance. The problem is that I don't know if the current density is positive or negative, as in this question in the answer, they take it as positive and in other question as negative, why do they change the sign before integrating? Is there a convention for this kind of questions? the answer is this one, (After finding the voltage, they just replace to find the resistance but I am okay with that, what confuses me is the procedure shown):enter image description here

Also, If possible, can you explain to me why they use the thickness of the plate as the height of the cylinder?. I would really appreciate if you guys can help me, and sorry if I am not clear in something...

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