III. Which paragraph says that...?

1. Kinetic energy is required for orbital motion.

2. Friction at the point of contact will require the centre of mass to lower itself.

3. Each year the point where the Sun crosses the Equator moves slightly to the west.

4. Nutation represents by itself up and down oscillation.

5. Uniform circular motion is produced by the torque.

6. Inertial guidance is commonly used in navigation.

7. The wheel does not fall due to gyroscopic precession.

8. The axis of the Earth precesses due to the form of the planet.

9. To resist changes in the direction of the axis of spin, spinning at the maximum frequency is required.

IV. Answer the questions:

• Why doesn't the wheel fall in spite of the fact that gravity applies a downward frce on the wheel?

• What is the cause of mutation?

• Can you describe the motion of a spinning top?

• How does a gyroscope work?

• Why does the Sun apply its torque to the Earth?

• What causes the Earth's axis to precess?

• What does the precession of the equinoxes consist in?

• What other examples of gyroscopic precession can you give? Where else can it be used?

V. Speaking

• Make a short summary of the article (no more than 8 sentences).

You are taking part in a popular scientific programme about mechanics. You are an experienced scientist who is explaining the basics to the journalist. Role-play the conversation. Try to be as short and informative as possible.

Text IV Hydroelectric Power

Hydroelectric power is electricity produced from generators driven by water turbines that convert the 1. _____ energy in falling or fast-flowing water to 2._____ energy.

In the generation of 3. _____ power, water is collected or stored at a higher elevation and led downward through large 4. ____ or tunnels (penstocks) to a lower elevation; the difference in these two elevations is known as the head. At the end of its passage down the pipes, the falling water causes turbines to 5. _____ . The 6. _____ in turn drive generators, which 7. ______ the turbines' mechanical energy into electricity. Transformers are then used to convert the alternating voltage suitable for the 8. ______ to a higher voltage suitable for long-distance 9. ______ . The structure that houses the turbines and generators, and into which the pipes or penstocks feed, is called the 10. _____.

Powerhouse, generators, transmission, turbines, convert, hydroelectric, mechanical, pipes, potential, rotate.

A. Falling water is one of the three principal sources of energy used to generate electric power, the other two being fossil fuels and nuclear fuels. Hydroelectric power has certain advantages over these other sources: it is continually renewable owing to the recurring nature of the hydrologic cycle and produces neither atmospheric nor thermal pollution. Hydroelectric power is a preferred energy source in areas with heavy rainfall and with hilly or mountainous regions that are in reasonably close proximity to the main load centres. Some large hydro sites that are remote from load centres may be sufficiently attractive to justify the long high-voltage transmission lines. Small local hydro sites may also be economical, particularly if they combine storage of water during light loads with electricity production during peaks.

B. In certain coastal areas, such as the Rance River estuary in Brittany, France, hydroelectric power plants have been constructed to take advantage of the rise and fall of tides. When the tide comes in, water is impounded in one or more reservoirs. At low tide, the water in these reservoirs is released to drive hydraulic turbines and their coupled electric generators.

C. Hydroelectric power plants are usually located in dams that impound rivers, thereby raising the level of the water behind the dam and creating as high a head as is feasible. The potential power that can be derived from a volume of water is directly proportional to the working head, so that a high-head installation requires a smaller volume of water than a low-head installation to produce an equal amount of power. In some dams, the powerhouse is constructed on one flank of the dam, part of the dam being used as a spillway over which excess water is discharged in times of flood. Where the river flows in a narrow steep gorge, the powerhouse may be located within the dam itself.

D. In most communities, electric-power demand varies considerably at different times of the day. To even the load on the generators, pumped-storage hydroelectric stations are occasionally built. During off-peak periods, some of the extra power available is supplied to the generator operating as a motor, driving the turbine to pump water into an elevated reservoir. Then, during periods of peak demand, the water is allowed to flow down again through the turbine to generate electrical energy. Pumped-storage systems are efficient and provide an economical way to meet peak loads.

I Put the paragraphs into the right order. Explain your choice.

II Fill in the right terms:

• Falling water, fossil and nuclear fuels are _______.

• Water is collected or stored at a higher ______ and led downward through large pipes to a lower _______ .

• To create a high head it is necessary to raise the level of the water behind the _____.

• Transformers convert the _____ ______ suitable for the generators to a higher ______ suitable for transmission.

• The potential power produced from a volume of water is _______ to the working head.

• In mountainous areas ______ may be located within the dam itself.

• The turbine ______ water into an elevated reservoir.

• In coastal areas people have learnt to _____ ______ of the rise and fall of tides.

III Explain the following: a penstock, the working head, an off-peak period, to meet peak loads, to even the load on, a spillway, to take advantage of, a load centre.

IV Say whether the statements are true or false:

1. Water turbines are driven by generators.

2. The rising water level causes turbines to rotate.

3. In the generation of hydroelectric power, water should be moved downward to a lower level.

4. Transformers are used to convert the mechanical energy into electricity.

5. A high-head installation requires a bigger volume of water than a low-head installation to produce an equal amount of power.

6. Where the river flows in a narrow steep place, the powerhouse should be constructed on one flank of the dam.

7. Pumped-storage systems are used to vary electric power demand.

8. A powerplant should be as close to the main load centre as possible.

V Answer the following questions:

• What are the key “participants” of the process of generation of energy? What does it (the process) involve?

• What is the role of transformers? Why are they important?

• What is important to know about the location of powerplants?

• What equipment is used to meet the variation of electric power demand?

• What system is used in certain coastal areas?

• What are the advantages of falling water over other sources of energy?

VI You are an expert in hydroelectric power. You are taking a group of tourists on a tour around the powerplant you work for. Tell them about its location, structure and the way the plant operates.