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Physics 103 Spring 2008 Exam 2 section 006 Name ___________________________A

1

The exam is closed book and closed notes. Choose the answer that is closest to the given answer. ρ = m/V; p = F/A; ph = ρgh; 1 atm = 1.013 x105 Pa R = 8.313 J/mol⋅K 1 BTU 1.055103. J= 1 cal 4.187J=

T(0C) = 95 [T(0F) - 32]; T(0F) =

59 T(0C) + 32; T(K) = [T(0C) + 273]; 1 m = 100 cm

1 Liter = 10-3 m3 Vcube = a3 Acircle = πr2 1 cal = 4.186 J , 1 hr = 3600 s σ = 5.67x10-8

W/m2K4

PV = nRT 2

22

1

11

TVP

TVP

= ; T - temp. in kelvins,

heat lost = heat gained cwater=Ckg

J4186 0⋅; Avogadro Number= NAv= 6.02x1023 /mole N = nNav

Thermodynamics: ∆U = Q – W W = P∆V Qh = W + Qc hQWe =

hcTT

1e −= WQ

COP c=

Q m c. ΔT.cc Q mL. P k A. ΔTL

.kk P σ A. e. T4 T 04.σσ W F Δy.ΔyΔy W P A. Δy.PP W P ΔV.PP

ΔU U f U iU iU i ΔU Q WQ WQ W U 32n. R. T.nn ΔU 3

2n. R. ΔT.nn P

A T h T c.

ΣR i

T cT c RL ik i

L iL i in ft2 10 F. h.

Btu

P k A. ΔTL

. ft2 hr.

BTU59. 0.176 s

3

kg=

R values for some common Building Materials: Wood shingles 0.87 Brick 4.00 Concrete block 1.93 Styrofoam 1" thick 5.0 Fiber glass batting 3.5" thick 10.9 Flat glass 0.125" thick 0.89 (ft2 0F h) / BTU Thermal conductivities Al 238 Cu 397 Air 0.0234 Concrete 1.3 Ice 1.6 Water 0.6 Wood 0.10 (J/s m 0C)

As a student at NJIT I ___________________________________, will conduct myself in a professional manner and will comply with the provisions of the NJIT Academic Honor Code. I also understand that I must subscribe to the following pledge on major work submitted for credit as described in the NJIT Academic Honor Code:

On my honor, I pledge that I have not violated the provisions of the NJIT Academic Honor Code.

Physics 103 Spring 2008 Exam 2 section 006 Name ___________________________A

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1. A refrigerator has a surface area of 5.3 m2. It is lied with 0.075-m-thick insulation whose thermal conductivity is 0.03 W/mC0. The interior temperature is kept at 50C, while the temperature at the outside surface is 250C. How much heat per seconds is being removed from the unit? A) 42.4 J/s B) 70.8 J/s C) 124 J/s D) 145 J/s E) 280 J/s 2 A 5-gallon container of water (approximately 20 kg) having a temperature of 100°C is added to a 50-gallon tub (approximately 200 kg) of water having a temperature of 10°C. What is the final equilibrium temperature (in °C) of the mixture? a. 54 b. 36 c. 18 d. 66 e. 14 3. A 5-kg piece of lead (specific heat 0.03 cal/g ⋅ °C) having a temperature of 80°C is added to 500 g of water having a temperature of 20°C. What is the final equilibrium temperature (in °C) of the system? a. 79 b. 26 c. 54 d. 34 e. 20 4. How much heat (in kilcalories) is needed to convert 1.00 kg of ice at 0°C into steam at 100°C? a. 23.9 b. 79.6 c. 564 d. 643 e. 720 5. The R value of fiberglass batting, 3.5 inches thick, is 11 ft2 °F h/BTU. What is the thermal conductivity (in BTU/ft ⋅ °F ⋅ s) a. 7.4 × 10–6 b. 2.7 × 10–2 c. 8.9 × 10–5 d. 1.4 × 10–4 e. 3.6 × 10–3

6. A slab of concrete and an insulating board are in thermal contact with each other. The temperatures of their outer surfaces are 68°F and 50°F. Determine the rate of heat transfer (in BTU/ft2 h) if the R values are 1.93 and 8.7 ft2 °F h/BTU, respectively. a. 9.7 b. 2.5 c. 5.3 d. 1.7 e. 28

Physics 103 Spring 2008 Exam 2 section 006 Name ___________________________A

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7. A wall is constructed of a 2 inch layer of fiberglass board (R = 8) and six inches of fiberglass batting (R= 19). If the temperature on the outside surface of the fiberglass board is 50°F and the temperature on the inside surface of the fiberglass batting is 68°F, what is the temperature (in °F) at the interface? (The units of R are ft2 °F h/BTU.) a. 62 b. 58 c. 55 d. 65 e. 52 8. Gas in a container expands at a constant pressure of 3 atm. Find the work done (in J) by the gas if the initial volume is 5 liters and the final volume is 10 liters. a. 0 b. 150 c. 15

d. 1500 e. 1.5 9. A certain quantity of an ideal gas initially at temperature T0, pressure P0, and volume V0 is compressed to one-half its initial volume. As shown, the process may be adiabatic (process 1), isothermal (process 2), or isobaric (process 3). a. It is greatest for process 1. b. It is greatest for process 3. c. It is the same for process 1 and 2 and less for process 3. d. It is the same for process 2 and 3 and less for process 1. e. It is the same for all three processes. 10. Calculate work (from the PV diagram) of a gas being compressed at constant pressure. a 8 J b 10 J c 2 J d 4 J e 6 J 11. A radiator has an emissivity of 0.7 and its exposed area is 1.5 m2. The temperature of the radiator is 1000C and the surrounding temperature is 220C. What is the heat flow rate from the radiator? A) 600 W B) 700 W C) 800 W D) 900 W E) 1000 W 12. A layer of fiber glass insulation in an attic is 3.5 inch thick. The R value of the insulation is 10.9. The owner of the house decided to put the second 3.5 inch layer of the fiber glass insulation to lower heating bills. The new value of R will be: a. 2*R b. 3/2*R c. (1/R)^2 d. R^2 e. 2/R

Physics 103 Spring 2008 Exam 2 section 006 Name ___________________________A

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13. A super-insulated house is at a temperature of 20°C. The temperature outside is 0°C. The surface area of the house

is 200 m2, and the emissivity is 1. Approximately how much energy is radiated (in W) per second? A) 20 000 W B) 2000 W C) 200 W D) 2 W E) 0.2 W 14. A Carnot engine takes 2000 J from a hot reservoir at 500 K, does some work, and discards some heat to cold reservoir at 350 K. The work done by the engine is closest to A) 3600 J B) 3000 J C) 2400 J D) 1200 J E) 600 J 15. A refrigerator has a coefficient of performance of 4.0. When removing 2.4 × 104 J from inside the refrigerator, how much energy is sent into the environment? a) 9.6 × 104 J b) 3.0 × 104 J c) 1.8 × 104 J d) 0.60 × 104 J e) 0.20 × 103 J 16. A gas in a cylinder is at a pressure of 1*105 Pa and the piston has an area of 0.1 m2. As energy is slowly added to the gas by heat, the piston is pushed up a distance of 4 cm. Calculate the work done by expanding gas on the surroundings assuming the pressure remains constant. a 50 J b 100 J c 200 J d 300 J e 400 J 17 A box with a total surface area of 1.3 m2 and a wall thickness of 5 cm is made of an insulating material. An 11 W electric heater inside the box maintains the inside temperature at 23.8 0C above the outside temperature. Find the thermal conductivity of the insulating material. a 0.02 W/m2C b 0.03 W/m2C c 0.04 W/m2C d 0.05 W/m2C e 0.06 W/m2C 18 The brick wall k = 0.99 W/m2 0C of a building has dimensions of 4.5 m by 9 m and is 11 cm thick. How much heat flows through the wall in a 6 h period when the average inside and outside temperatures are, respectively, at 160 C and 30C? a 34 MJ b 46 MJ c 59 MJ d 73 MJ e 104 MJ

Physics 103 Spring 2008 Exam 2 section 006 Name ___________________________A

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1A 2C 3D 4E 5A 6D 7C 8D 9A 10A 11B 12A 13A 14E 15B 16E 17A 18E 10. 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 × 105 J/kg).

a. J b. J c. J d. J e. J