Fish are sensitive to electric fields and currents. A voltage applied to electro
ID: 1592644 • Letter: F
Question
Fish are sensitive to electric fields and currents. A voltage applied to electrodes in the water of a stream or lake can temporarily stun fish so that they can be counted and measured—useful for wildlife managers who can then directly assess the size and composition of a population of fish. An investigator made an experimental station to determine the health effects of this electrical stimulation. Trout (average length 34 cm) were placed in a tank that was 68 cm long, 40 cm wide, and filled to a depth of 24 cm with water. A potential difference of 100 V was applied to electrodes at opposite ends of the tank. • What is the approximate magnitude of the potential difference between the head and the tail of a 34 cm trout aligned with the long axis of the tank? • If the tank is doubled in length to 136 cm but all other properties of the system stayed the same, how would this change the electric field across and the current through the tank?
Explanation / Answer
Given: A potential difference of 100 V was applied to electrodes at opposite ends of the tank, i.e. V = 100 V
Electric field along the long axis (L = 68 cm = 0.68 m) of tank is therefore,
E = V/L = 100 V / 0.68 m = 147.06 V/m
Now, approximate magnitude of the potential difference between the head and the tail of a 34 cm trout aligned with the long axis of the tank is:
Vtrout = E x Ltrout = 147.06 V/m x 0.34 m = 50 V
If length is doubled to 136 cm, electric field for the same potential difference will be halved as E = V/L. This would reduce the potential difference between head and the tail of trout to 25 V.
The current changes because resistance offered by water changes (which is proportional to length of water stream). So, if length is doubled, resistance offered by water doubles and hence current is halved.
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