Physics question

Just did the June 2003 Physics unit 4 paper (AQA b) and I made a mistake on what appeared to be a simple question, and I dont know why.

"A turbine is linked to a generator that produces 800MW of electrical power. Calculate the efficiency of conversion of the internal energy of the steam into electrical enrgy."

I thought efficiency = output/input

From part (a) you know that 3.53GW is used to heat the water, so

efficiency = 800E6/3.53E9 = 22.7%

but apparently the answer is 18.5% (?). It says eff = 0.8/(0.8+3.53)

I dont understand why!

Reply 1

queen-of-pain

Its the useful energy divided by the total energy thats been produced.

Reply 2

Goat

queen-of-pain

Its the useful energy divided by the total energy thats been produced.

OK that would make sense but in the text book it says

efficiency = useful power output / power input

Isn't power input is different from total produced?

Reply 3

john !!

Your textbooks defines it more generally, "total energy thats been produced" is a bit ambiguous.

I don't understand why the useful power output would be used in the part of the equation that should represent the total power input.

Reply 4

Golden Maverick

Goat

"A turbine is linked to a generator that produces 800MW of electrical power. Calculate the efficiency of conversion of the internal energy of the steam into electrical enrgy."

I thought efficiency = output/input

From part (a) you know that 3.53GW is used to heat the water, so

efficiency = 800E6/3.53E9 = 22.7%

but apparently the answer is 18.5% (?). It says eff = 0.8/(0.8+3.53)

I dont understand why!

From part a are you getting the energy lost when the steam re-cools ar the actual energy used to heat it?