Consider 0.8 grams of air (which may be assumed to be an ideal gas) at 295 K and

Question

1. Consider 0.8 grams of air (which may be assumed to be an ideal gas) at 295 K and 1.5 bar trapped in a rigid tank. An electrical heater is operated for 30 seconds. During this time the voltage across the heater is 12 V and 0.6 amps pass through the resistance element. During this process, 126 J of energy is lost via heat transfer through the tank walls.

1. Assuming the specific heats of air are constant (at the 300 K values (use table B.9)) determine the final temperature of the air. Hint: Use the prop. relationships. ans. 452 K
2. Determine the final pressure in the tank.

Explanation / Answer

Total heat supplied = V*I*t = 12*0.6*30 = 216 J

Heat loss = -126 J

Heat used to change in Internal energy = 216-126 = 90 J

Therefore

m*Cv*(T2-T1) = 90

0.8*0.717*(T2 - 295) = 90

T2 = 452 K

Since the volume is fixed

P2 = P1*(T2/T1) = 1.5*(452/295)

P2 = 2.3 bar

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