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in a physics book I found the equation E = - grad(U) and in another one E=U/d with E:electric field, U:voltage, d:distance. The minus sign confuses me. Can anybody explain that to me, please?

I am thinking of a room with a positively charged ceiling and a negatively charged floor. So a positive charge will move towards the floor to reduce the potential energy. In that case, is the current I positive or negative?

Thanks a lot.

Basti