The electric field produces a potential difference. If you place one electrode \(10 \mathrm{~m}\) below the surface of the water, you will measure the greatest potential difference if you place the second electrode A. At the surface.

B. At a depth of \(20 \mathrm{~m}\).

C. At the same depth \(10 \mathrm{~m}\) to the north.

D. At the same depth \(10 \mathrm{~m}\) to the east.

The ocean is salty because it contains many dissolved ions. As these charged particles move with the water in strong ocean currents, they feel a force from the earth's magnetic field. Positive and negative charges are separated until an electric field develops that balances this magnetic force. This field produces measurable potential differences that can be monitored by ocean researchers.
The Gulf Stream moves northward off the east coast of the United States at a speed of up to \(3.5 \mathrm{~m} / \mathrm{s}\). Assume that the current flows at this maximum speed and that the earth's field is \(50 \mu \mathrm{T}\) tipped \(60^{\circ}\) below horizontal.