Chapter 1, Problem 1.3DP

A child opens the door of the freezer that is turned off, to observe its operation. If the ambient temperature is 30°, what will be the behavior of the internal temperature of the freezer when it is opened and why? a. does not change, since there is no material between the environment and the interior of the freezer for heat exchange b. is increasing, because the convection between the environment and the air inside the refrigerator does not transfer heat from the outside to the center c. does not change, since there is no contact between the surface of the meat and the external environment d. increases, because sunlight falls on the meat, increasing the energy. Then, when turning on the freezer again, it is observed that the back of the freezer gradually warms up, and remains warm during the operation of the machine. The amount of thermal energy emitted by the freezer must be: a. equal to 0; the heating is due to a problem with the freezer b. Less than the energy drawn from inside the…

You are working with an imagined cubical pot of water, 1 meter on each side, sitting on a stove. Heat is applied to the bottom of the pot from the stovetop. The pot has been on the stove for a while, so the stovetop is hot and some heat has already been added to the water. Assume the water at the bottom of the pot is held at 99 degrees C , while the water at the top of the pot is cold, 4 degrees C. Compute the heat delivered by that 1 g of water at 99 °C into the layer of water at the top of the pot, assuming the latter is 1 meter square, 1 cm deep, and 4 °C. Start by solving the calorimetry problem (Section 14-4 in the text) to determine the final temperature of the top layer plus hot 1 gram from below. Then use that to determine how much heat was delivered in the hot water

Build a spreadsheet to do the step by step method for estimating the temperature of unprotected steel work exposed to the ASTM E119 Standard Fire.  Assume a convective heat transfer coefficient of 25 W/m2K and an emissivity of 0.5. Assume the specific heat to remain constant at 600 J/kgK. Use a time step of 30 seconds and an ambient temperature of 20°C. Provide an Excel plot showing the curves of the fire and a steel member through a time of 60 minutes with F/V ratios of: a)25 m-1 b)100 m-1 c)200 m-1 d)300 m-1 Note your plot should contain 5 curves. One for the fire and one for each of the F/V values. Also on the spreadsheet, highlight in green, the cells identifying the temperature of the unprotected steel member with an F/V of 200m-1 at the following times: a)3 minutes b)10 minutes c)20 minutes d)50 minutes

Model the following heat transfer situations. Specify heat flows and storages. Try to write down the mathematical expressions. (i) Solar heating of the road surface (ii) A steam pipe passing through an open space between two buildings (iii) Heat transfer from a person in a warm room in the cold season (iv) Pressure cooker-warming up-cooling down (v) Pressure cooker-steady conditions (vi) A rod with one end in a furnace and the remaining surface in atmosphere (vii) A wire carrying current, exposed to air (viii) A water heater (electrical) with hot water being drawn out with cold water admission. (ix) Cake being baked in an oven or a fruit placed in a refrigeration (x) A frying pan placed on a stove. 1.2 Choose the correct statement in each question. (i) A pipe carrying steam at about 300°C traverses a room, the air being still at 30°C. The major fraction of the heat loss will be by (a) conduction to the still air (b) convection to the air (c) radiation to the surroundings…

1. Discuss all the processes involved in engineering design 2. The resistance of a copper wire used to make a motor is 0.5 Ω at 25 oC and 0.8 Ω at the motor’s operating temperature. Determine the operating temperature and the rise in temperature of the motor, given that copper has a temperature constant of 234.5 oC. 3. What are the possible energy sources and energy transfer systems of a purely mechanical machine?

A metal spoon and a wooden spoon are left overnight in a room.  Why do we feel colder when we touch the metal spoon than when we touch the wooden one? a. Since metals usually have a lower heat capacity than non-metals, the metal spoon loose more heat to the room and consequently has a lower temperature b. Metals release heat faster than non-metals because of their higher thermal conductivity c. This is merely psychological since we are conditioned to think of metals as “cold” d. Metals absorb heat faster than non-metals because of their higher thermal conductivity

Suppose you have two kettles – a plug-in electric kettle and a stovetop kettle. The electric kettle uses electricity from a natural gas fired powerplant, while you boil water in your stovetop kettle on a natural gas burner. a) Based on the following information, which of these kettles demonstrates a higher energy efficiency (i.e., which is more efficient at using energy from fuel to heat water)? Please calculate the efficiencies of each kettle and express answers as percentages. ​ -The natural gas power plant converts chemical energy of the natural gas to electricalenergy with 58% efficiency. -High-voltage power lines from the power plant to your house convey electricity with92% efficiency. -The electric kettle converts electrical energy to thermal energy in the water with 85%efficiency (the other 15% heats up the kettle itself). -The stove burner converts chemical energy of natural gas to thermal energy with 98%efficiency (the other 2% is light). -The stovetop kettle transfers 35% of…

a) The number of kilocalories in food is determined by calorimetry techniques in which the food is burned and the amount of heat transfer is measured. How many kilocalories per gram are there in a 5.00-g peanut if the energy from burning it is transferred to 0.500 kg of water held in a 0.100-kg aluminum cup, causing a 54.9C temperature increase? (b) Compare your answer to labeling information found on a package of peanuts and comment on whether the values are consistent.

Latent heat of fusion is the heat given up as a substance freezes. The latentheat of fusion for water is 144 British thermal units per pound (Btu/lb). Thismeans that 144 Btu are given up for each pound of water that freezes. Howmany British thermal units of heat are given up when 4,000 pounds of waterfreeze?