In a hydroelectric power station, water is allowed to fall at a rate of 2000 kJ s-¹ on a turbine kept 100 m below the water level. Calculate (a) the potential energy of water falling every 1 s at the highest point (b) the kinetic energy of this water when it falls through a height of 25 m (c) the power output in 1 s if 80% of the initial potential energy is converted into electrical energy. (Take g = 10 m s-²)
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Answer:
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Explanation:
To calculate the values requested, we can use the formulas for potential energy (PE), kinetic energy (KE), and power (P).
Given:
Rate of water falling (water flow rate) = 2000 kJ/s
Height of fall (highest point to turbine) = 100 m
Height of fall (from highest point to 25 m) = 25 m
Efficiency of conversion = 80% = 0.8
Acceleration due to gravity (g) = 10 m/s²
(a) Potential Energy (PE) of water falling every 1 s at the highest point:
PE = mgh
Here, m is the mass of the water falling per second.
Mass of water falling per second = rate of water falling = 2000 kJ/s
1 kJ = 1000 J
Mass of water falling per second = 2000 kJ/s × (1000 J/kJ) = 2,000,000 J/s
PE = (2,000,000 J/s) × (100 m) = 200,000,000 J/s = 200 MW (megawatts)
Therefore, the potential energy of water falling every 1 second at the highest point is 200 MW.
(b) Kinetic Energy (KE) of water falling through a height of 25 m:
KE = (1/2)mv²
Here, m is the mass of the water falling and v is the velocity of the water at the lower point.
To find v, we can use the equation:
v² = u² + 2gh
where u is the initial velocity (which is assumed to be 0 as the water starts from rest).
v² = 0 + 2 × 10 m/s² × 25 m
v² = 500 m²/s²
v ≈ 22.36 m/s
Mass of water falling = rate of water falling = 2000 kJ/s = 2,000,000 J/s
KE = (1/2) × (2,000,000 J/s) × (22.36 m/s)²
KE ≈ 5,000,000 J/s = 5 MW (megawatts)
Therefore, the kinetic energy of the water when it falls through a height of 25 m is 5 MW.
(c) Power output in 1 second:
Power output = Efficiency × PE
PE = 200 MW (as calculated in part a)
Power output = 0.8 × 200 MW = 160 MW
Therefore, the power output in 1 second is 160 MW.
Note: It's important to remember that these calculations are based on the given values and assumptions provided in the question.