Question

The Hoover Dam in the U.S produces 2 x 10 ^9 Watts of electricity and is composed of 7 x 10^9 kg of concrete. Concrete requires 1MJ of energy to produce 1 kg

1) How much energy did it take to produce the dam?

2) How long is the "energy payback time" for the dam?

Show your work.

Answer 1

1)
`7 x 10^(15) J`

2)
`40.5` days

Explanation:

1) First of all we use
`1MJ=1x10^(6)J` so the total energy will be
`7x10^(9) kg * 1x10^(6) (J)/(kg) =7x10^(15)J`.

2)Then we use
`1W=1(J)/(s)` and
`Time=(7 x 10^(15) J )/(2x10^(9)J/s) =3.5x10^(6)s` or
`3.5x10^(6)s*(1h)/(3600s)*(1day)/(24h) =40.5 days`