How much water would be required to absorb this heat?

  • #1

Homework Statement


The United States generates about 5.7 x 10^16 J of electric energy a day. This energy is equivalent to work, since it can be converted into work with almost 100% efficiency by an electric motor.

(a) If this energy is generated by power plants with an average efficiency of 0.32, how much heat is dumped into the environment each day?

Answer --> 1.21e+17 J

(b) How much water would be required to absorb this heat if the water temperature is not to increase more than 2.7° C?

Answer --> 1.07e+13 kg.



The Attempt at a Solution



I don't see how it is possible to pump out more heat into the environment than work generated, especially at 0.32 efficiency. Can someone help me with this problem please? Thanks
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
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a) Efficiency = work output/work input.
In the problem, work output and efficiency is given. Find work input.
heat is dumped into the environment each day = work input - work output.
b) Q = m*c*change in temperature.
 
  • #3
Andrew Mason
Science Advisor
Homework Helper
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I don't see how it is possible to pump out more heat into the environment than work generated, especially at 0.32 efficiency. Can someone help me with this problem please? Thanks
Following on what rl.bhat has said, the work is generated from heat engines. The heat engines only convert a fraction (.32) of the heat flow into work. The rest is expelled as waste heat.

AM
 
  • #4
gneill
Mentor
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Following on what rl.bhat has said, the work is generated from heat engines. The heat engines only convert a fraction (.32) of the heat flow into work. The rest is expelled as waste heat.

AM
One might argue that virtually *all* of the generated power is eventually expelled as waste heat; mostly it is used to heat things, illuminate things, and do repetitive tasks in a friction-filled environment. Elevators go down as well as up!
 

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