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Unformatted text preview: Week 12— Problems 12—1. a) Show that in a gas or liquid, near Earth, the pressure change in going from
height y to (y +Ay) Where Ay << 1m is given by
AP = ——d gAy  (I)
Where (1 is the density. .
b) Explain why Eq(l) can be used to calculate the pressure change for any Ay in a
liquid but not in a gas. 12—2. In a hydraulic press the diameters
of the pistons P1 and P2 are 1cm and
300m, respectively. If you push down
with a force f =50 N on P1, what force
F will be projected by P2? 123. What is temperature? Develop the deﬁnition by taking two thermodynamic
systems which are isolated from the surroundings but separated from one another
by a conducting wall. Each system contains a certain amount of gas. 124. At what temperature do the Celsius and Fahrenheit scales have the same _
numerical value? x 12—5. What is heat? What is the difference between speciﬁc heat and latent heat? 126. A copper sphere of radius 2.0000m is placed over a hole of radius 1.999cm in an
aluminum plate at 20°C. The expansion coefﬁcients are aw = 17 x 10'6 C”1 and a,” = 24 x 10‘6 C“. At what common temperature will the sphere pass through the
hole? Why? 12—7. A Pyrex glass beaker [a = 3.2 x10”6C—] J of volume IOOCm3 is ﬁlled to the brim with water [,6 = 2.1 x10“4 C ‘1] at 20°C. If you put it in a microwave oven and let
the temperature rise to 90°C, how much water will spill out? 12—8. A ZSOg lead ball at 210°C is placed in a 90g aluminum calorimeter that contains
200g of liquid at 20°C. If the ﬁnd temperature is 25°C, what is the speciﬁc heat of the liquid[C,,b =130J/kg——C, CA3 = 900J/kg—C]? 129. 20g of ice at ~5°C are dropped into a calorimeter containing 100g of water at
20°C. Assuming that the mass of the calorimeter is negligible, calculate the ﬁnal temperature Ciw = 2000J/kg  °C L. = 334x103J/kg,C : 4186J/kg —~ °C. 9 Ice water 1210. What is the difference between conduction and convection? 1211. For ﬂow of heat by conduction in the steady state, the heat current is written as:
I ﬁrmﬂ At AX
Why is there a negative sign on the right side of this equation? 12—12. 1213. 1214. A 1m long copper bar of cross—
section lcm x 20m has one end in a steam chamber at 100°C and the
other dipping in an ice bath at 0°C.
If the thermal conductivity of copper
is 39 J/m—sec°C, how much ice will melt in 10 secs? [13,06 =334x103J/kg]
(assume zero heat loss to surroundings) Two spheres are made of the same material (Identical emissivities). Their
diameters are 1m and 4m, respectively. (i) If they are at the same temperature,
which one emits more heat radiation per sec and by what factor? (ii) By what factor would you increase the temperature of the smaller one so it
emits the same amount of heat per see as the larger one? ' When an object of surface area A and temperature T; is sitting inside an enclosure
at temperature T2 the heat exchange per see is:
DQ 4 4
— = Aea T ~ T
N (I 2) >
Where 6 is emissivity and a" is the Stefan—Boltzmann constant. Show that if (T1 T2) is much less than T1 or T2 this leads to Newton’s law of cooling: rate of
cooling is proportional to (TIT2). ...
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 Spring '10
 Shawhan
 Physics

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