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An electric power plant uses energy from burning coal to generate steam at 450∘C. The plant is cooled by 20∘C water from a nearby river. If burning coal provides 100 MJ of heat, what is the theoretical minimum amount of heat that must be transferred to the river during the conversion of heat to electric energy? A. 100 MJ. B. 90 MJ. C. 60 MJ. D. 40 MJ.

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Answer:

40 MJ (D)

Explanation:

Quantity of heat (Qh) = 100 MJ

temperature of steam (Th) = 450°c = 450 + 273 = 723 K

emperature of water (TI) = 20 °c = 20 + 273 = 293 k

efficiency = (Qh-Qi)/Qh = (Th-Ti)/Th

 [tex]\frac{100x10x^{6}-Qi }{100x10^{6}} = \frac{723-293}{723}[/tex]

[tex]100x10^{6}[/tex] - Qi= 0.5947 x [tex]100 x 10 ^{6}[/tex]

[tex]100x10^{6}[/tex] - (0.5947 x [tex]100x10^{6}[/tex]) = Qi

Qi = 40.5 MJ equivalent to 40 MJ (D)

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