A heat engine uses resistors to boil water as part of its operation. There are 3
ID: 1272425 • Letter: A
Question
A heat engine uses resistors to boil water as part of its operation. There are 3 resistors with resistances 25? 10? and 15?, connected in series. The resistors are connected to a 240 V power source.
a. What is the equivalent resistance of the three resistors?
b. How much electric power is dissipated in the resistors (all together)?
c. The engine turns water into steam and then heats up the steam to 250 C. How much heat do you have to add to .5 kg of water, starting at 25 C to turn it completley to steam at 250 C?
d. The engine extracts 1200 kJ of heat from the steam each cycle, and does 345 kJ of work. What is the engines efficiency?
Explanation / Answer
a) Rnet = 25+10+15 = 50 ohms
b) P = V^2/Rnet = 240^2/50 = 1152 Watts
c) Q = m*c*dT + m*Lf + m*C*dT
= 0.5*4186*(100-25) + 0.5*2240*10^3 + 0.5*1996*(250-100)
= 1.43*10^6 J
d) n = 345/1200
= 0.2875 or 28.75%