A styrofoam container used as a picnic cooler contains a block of ice at 0°C. If 225 grams of ice melts in 1 hour, how much heat energy per second is passing through the walls of the container? (The heat of fusion of ice is 3.33 × 10^5 J/kg)
20.8 J (per second)
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The free-fall contraction of a molecular cloud
a. can be initiated by shock waves from supernovae. b. can be initiated by nearby spectral type G stars. c. can be initiated by the rotation of the cloud. d. causes the cloud to become transparent to ultraviolet radiation. e. causes the particles in the cloud to decrease the speed with which they move.
A car starting from rest accelerates at a constant 2.0 m/s2 for 10 s. It then travels with constant speed it has achieved for another 10 s. Then it finally slows to a stop with constant acceleration of magnitude 2.0 m/s2
How far does it travel after starting? A) 200 m B) 400 m C) 500 m D) 300 m
Estimate (a) the heat transfer coefficient for a spherical fuel droplet injected into a diesel engine at 80°C and 90 m/s. The oil droplet is 0.025 mm in diameter, the cylinder pressure is 4800 kPa, and the gas temperature is 944 K. (b) Estimate the time required to heat the droplet to its self-ignition temperature of 600°C.
GIVEN
• An oil droplet injected into a diesel engine
• Initial droplet temperature (To) = 80°C
• Injection velocity (Ud) = 90 m/s
• Droplet diameter (D) = 0.025 mm = 2.5 × 10–5 m
• Cylinder pressure = 4800 kPa
• Gas temperature (T?) = 944 K = 671°C FIND (a) The heat transfer coefficient ( h c) (b) The time (t) required for the drop to reach 300°C ASSUMPTIONS
• Radiative heat transfer is negligible
• The gas has the properties of air and behaves as an ideal gas
• Only the density of the gas is affected by pressure
• Variation of the thermal conductivity of the oil with temperature is negligible
• Fuel has the same properties as unused engine oil
SKETCH
A refrigerator
A) produces cold. B) causes internal energy to disappear. C) transfers internal energy from inside to outside. D) transfers heat into cold. E) none of the above