Certain fish, such as the Nile fish (Gnathonemus), concentrate charges in their
ID: 1964716 • Letter: C
Question
Certain fish, such as the Nile fish (Gnathonemus), concentrate charges in their head and tail, thereby producing an electric field in the water around them. (See the figure .) 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 resistivities different from that of seawater, 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 chapter, 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 running directly from the head to the tail. We can model the seawater through which that field passes as a conducting tube of area 1.10 and having a potential difference of 3.10 across its ends. The length of a Nile fish is about 20.0 , and the resistivity of seawater is 0.130 .a) How large is the current through the tube of seawater?
b) Suppose the fish swims next to an object that is 10.0 long and 1.10 in cross-sectional area and has half the resistivity of seawater. This object replaces the seawater for half the length of the tube. What is the current through the tube now? (Hint: How are this object and the remaining water in the tube connected, in series or in parallel?)
c) How large is the change in the current that the fish must detect? (Hint: How are this object and the remaining water in the tube connected, in series or in parallel?)
Explanation / Answer
Firstly we must calculate the amount of force needed to produce a stretch of 0.60 cm: F = k*d where F is Force (N), k is the Spring Constant (N/m) and d is the Dispacement from its original position. (m) F = x This force is present as the current is at 90 degrees to the magnetic field. We can use the following formula to calculate the current through the wire: Current = F / B*l where B is the Magnetic Field Strength (T) and l is the Length of the wire in the Magnetic Field (m) Current = y