Answer (5 pts each) object generates thermal energy under edy cditions, making i

Question

Answer (5 pts each) object generates thermal energy under edy cditions, making it hotter than its sroundings. It has a large thermal conductivity Time Time small convective heat transfer coefficient. The object's power is tuned off. How does the object's temperature change over time? Time 2. On a very cold winter day, a group of engineering students notices that quickly licking the metal end of an ice scraper left outside overnight caused their tongues to freeze to the metal surface. However, a quick lick of the plastic handle of the scraper didn't cause any freezing to occur. How can you explain this observation? -A. Metal is colder than plastic because it transfers energy to the atmosphere faster B. Metal is colder than plastic because metal is more dense and therefore retains cold better C. Metal is colder than plastic because plastic stores energy better - D. Metal conducts energy better than plastic, so energy moves away from the tongue faster when touching metal 3. In designing an effective fin to remove heat, which statement is correct? A·Thermal conductivity of the fin material should be as low as possible. B. Ratio of cross-sectional area to perimeter of the fin should be as high as possible. C. The length of the fin should be very, very long. D. Fins should be used when convective heat transfer coefficients are high E. None of the above. a counter-flow heat exchanger, 2 kg/s of hot water flows through the tubes and 8 kg/s of cold water flows on the shell side. Which flow will experience the largest temperature change? Cold water Hot water A. cold water B. hot water C. the change will be the same for both 5. A pipe with cold refrigerant is running through a hot room. Adding insulation to the pipe will always reduce the rate of heat transfer through the pipe (true/false). 6. (10 pts) Write the heat equation in Cartesian coordinates and label each term to indicate what it represents.

Explanation / Answer

1. Answer (A)

As soon as we switch off the objects power the temperature of the object starts decreasing untill it will become equilibrium with surrounding temperature. The rate at which the temperature decreases depends on the heat transfer coefficients (convective heat transfer coefficient) as given that, the convective heat transfer coefficient is low the temperature of the object drops gradually.

From the options, A,B,C,D,E

A,C,D can be considered for temperature drop with time. But from option A as the temperature drops the heat transfer to the surroundings also drops(since the q directly proportional to deltaT) so as the temp. drops the object takes long to get equilibrium with the surroundings.

And options C,D the drop can be seen sudden which is not possible.

2. Answer(D) Metal conducts energy faster than plastic so energy moves away from the tounge faster when touching the metal.

3. Answer could be E

Option C the length of the fin can long for effective heat transfer but not very very long..!!

4. Answer (Hot water)

Since mass of hot water is less it experiences largest temperature change

Formula Q= mass(m)*specific heat(cp)*(deltaT)

5. False

Reason: adding insulation always will not reduce the heat transfer , it does but only upto critical thickness of insulation.

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