1- Liquid helium is stored at its boiling-point temperature of 4.2 K in a spheri
ID: 2254638 • Letter: 1
Question
1- Liquid helium is stored at its boiling-point temperature of 4.2 K in a spherical container (radius 30cm). The container is a perfect blackbody radiator. The container is surrounded by a spherical shield whose temperature is 77 K ( the boiling temperature of liquid nitrogen). A vacuum exists in the space between the container and the shield. The latent heat of vaporization for helium is 2.1*10^4 J/kg. What mass of liquid helium boils away through a venting valve in one hour?
2- The cooling system of a particular car holds 15 liters of coolant. The system starts exactly full on a cool morning 5 degree C. The car is then started and the temperature in the cooling system rises to 80 degree C. Treat the coolant as pure water and determine how much coolant is pushed into the overflow reservoir?
Explanation / Answer
Net heat absorbed by helium container as black body in one hour
= 5.671*(10^-8)*4*pi*{(0.30)^2}*3600*[86^4 - 4.2^4] J = 12630 J
Amount of helium boiling away = 12630/21000 = 0.601 kg or 601 g
coefficient of Volume Expansion in 207x10^-6 Cu.M./K for water. Total water overflowed is 207x75x15 = 0.232 litres
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