Certain fish, such as the Nile fish (Gnathonemus), concentrate charges in their
ID: 1392994 • 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. 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 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 running directly from the head to the tail. We can model the seawater through
which that field passes as a conducting tube of area 1.0 cm2 and having a potential difference of
3.0 V across its ends. The length of the Nile fish is about 20 cm, and the resistivity of seawater
is 0.13 ?
Explanation / Answer
resistance=resistivit*length/area
=0.13*0.2/10^(-4)=260 ohms
then current=voltage/resistance=3/260=11.538 mA
b)
they will be connected in series .
the second resistance=(0.13/2)*0.1/(10^(-4)=65 ohms
net resistance=65+260=325 ohms
so current=3/325=9.23 mA