In a hydroelectric dam, water falls 33.0 m and then spins a turbine to generate electricity. What is
ΔU of 1.0 kg of water? Suppose the dam is 80% efficient at converting the water's potential energy to electrical energy. How many kilograms of water must pass through the turbines each second to generate 45.0 MW of electricity? This is a typical value for a small hydroelectric dam.

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Answer:

[tex]m = 1.74 \times 10^5 kg/s[/tex]

Explanation:

As we know that potential energy is given as

[tex]U = mgh[/tex]

here we know that

m = 1 kg

h = 33 m

now we have

[tex]U = 1(9.8)(33)[/tex]

[tex]U = 323.4 J[/tex]

Part b)

Now we need total power given as

[tex]P = 45 MW[/tex]

so we have

[tex]P = \frac{\Delta U}{t}[/tex]

here we know that it is 80% efficient so we can write

[tex]P = 0.80(\frac{\Delta U}{\Delta t})[/tex]

[tex]45 \times 10^6 = 0.80(m)(323.4)[/tex]

[tex]m = 1.74 \times 10^5 kg/s[/tex]

The change in the potential energy of the water is 323.4 J.

The mass of water that must pass through the turbine each second is 173,933.2 kg/s.

The given parameters;

  • height of the waterfall, h = 33 m
  • mass of the water, m = 1 kg

The change in the potential energy of the water is calculated as follows;

ΔU = mgh

ΔU = 1 x 9.8 x 33

ΔU = 323.4 J

The efficiency of the turbine is calculated as follows;

[tex]Eff = \frac{0utput\ power }{1nput\ power } \times 100\% \\\\Eff = \frac{P_0}{P_1} \times 100\%\\\\Eff = \frac{45 \times 10^6 }{P_1} \times 100\% \\\\80\% = \frac{45 \times 10^6}{P_1} \times 100\% \\\\0.8 = \frac{45 \times 10^6}{P_1} \\\\P_1 = \frac{45 \times 10^6}{0.8} \\\\P_1 = 56,250,000 \ J/s \\\\[/tex]

The mass of water that must pass through the turbine each second is calculated as follows;

[tex]\frac{mgh}{t} = P_1\\\\\frac{m}{t} = \frac{P_1}{gh} \\\\\frac{m}{t} = \frac{56,250,000}{9.8 \times 33} \\\\\frac{m}{t} = 173,933.2 \ kg/s[/tex]

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