The harbour authority on a remote island tests an underwater generator beside a narrow channel. Fig. 1 shows the arrangement during an outgoing tide. A stro...
Question 1 Report
The harbour authority on a remote island tests an underwater generator beside a narrow channel. Fig. 1 shows the arrangement during an outgoing tide. A strong water current rotates the rotor. The rotor drives a generator in a sealed box above the blades, and the electrical energy is sent by cable to charge a battery bank for harbour lights. The current reverses about six hours later, so the blades can turn in the opposite direction.
(a) State the energy source that makes the water current move. [1]
(b) Describe the energy changes from the moving water to the battery. [2]
(c) Calculate the charge transferred to the battery in 45 minutes when the current in the cable is 18 A. [3]
(d) Explain why this generator cannot provide a constant power output throughout a day. [2]
Answer Details
(a) The water current is driven by tidal energy, caused by the gravitational attraction of the Moon. [1]
(b) Kinetic energy of the moving water transfers to kinetic energy of the rotor. [1] The generator transfers this to electrical energy, which is stored as chemical energy in the battery. [1]
(c) Convert the time to seconds:
\[t=45\times60=2700\text{ s}\]
Use \(Q=It\):
\[Q=18\text{ A}\times2700\text{ s}=48\,600\text{ C}\]
The charge transferred is \(48\,600\text{ C}\). [3]
(d) The speed of the tidal current changes during a tide. [1] At slack water the flow, and so the generator power, is very small or zero. [1]
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