In the rainy season, the Amazon flows fast and runs deep. In one location, the river is \(23 \mathrm{~m}\) deep and moves at a speed of \(4.0 \mathrm{~m} / \mathrm{s}\) toward the east. The earth's \(50 \mu \mathrm{T}\) magnetic field is parallel to the ground and directed northward. If the bottom of the river is at \(0 \mathrm{~V}\), what is the potential (magnitude and sign) at the surface?