Question

7. Certain fish, such as the Elephant Fish (Mormyridae),
concentrate charges in their head and tail, thereby pro-
ducing an electric field in the water around them. This
field creates a potential difference of a few volts between
the head and tail, which in turn causes current to flow in
the conducting seawater. As the fish swims, it passes near
objects that have a resistivity different from that of sea-
water, which in turn causes the current to vary. Cells in the skin of the fish are sensitive to this current and can detect changes in it. The changes in the current allow the fish to navigate. (In the next few chapters, we shall investigate how the fish might detect this current.) Since the electric field is weak far from the fish, we shall consider only the field in the vicinity of the fish. If the fish is 20 cm long (certain members of the family can reach 1.5 m), we can model the seawater through which that field passes as a conducting tube around the fish of area 4.0 cm2 which has a potential difference of 3.0 V between its ends. The resistivity of seawater is 0.13 olm·m. For this model let’s assume that the fish has negligible width so that it does not impact the conductivity of the tube.

(b) Suppose the fish swims along the side of a vertical rock face such that one quarter of the tube is now taken up by the rock. The rock has a metal content that gives it twice the conductivity of seawater. What is the difference in current that the fish detects? (Hint: How are the rock and the remaining water in the tube connected, in series or in parallel?)

Answer 1