course Phy 122
Ch.13 (26-30)26. What are the following temp. on the Kelvin scale: a)86? b)78? c)-100? d)5500? e)-459?
-K=? +273.15
K= (5/9 (?-32))+273.15
a) K=86? + 273.15 = 359.15K
b) K=(5/9(78?-32))+273.15 = 298.71K
c) K=-100?+273.15 = 173.15K
d) K=5500?+273.15 = 5773.15K
e) K=(5/9(-459?-32))+273.15 = 0.37K
Good, but you're using formulas rather than common sense. Be sure you understand the common sense of the formulas (as well you might). Common sense persists much longer than formulas, and common sense provides a building block for additional understanding.
27. Absolute zero is what temp on the Fahrenheit scale?
- 0K=?+273.15 ?= -273.15
?=9/5x(-273.15)+32 = -459.67?
28. Typical temp. in the interior of the Earth and Sun are about 4000? and 15x10^6?, a) What are these temps in Kelvins? b) What percent error is made in each case if a person forgets to change ? to K?
-K=?+273.15
a) K=4000?+273.15= 4273.15K and 15x10^6?+273.15 = 15000273.15K
b)4273.15-4000= 273.15 difference.
You need to calculate percent errors. 273 is about 6 or 7% of 4270 K, whereas it's only a small fraction of a percent of 1.5 * 10^6.
In fact for the latter temperature the temperature would have to be given to 6 significant figures before the 273-degree difference would have any significance at all.
29. If 3.00m^3 of a gas initially at STP is placed under a pressure of 3.20atm, the temp. of the gas rises to 38.0?. What is the volume?
-P1V1 / T1 = P2V2 / T2
1atm(3.00m^3) / 273K = 3.2atmV2 / 311K = 1.07m^3=V
Part of your statement is
3.2atmV2 / 311K = 1.07m^3.
These two quantities are not equal and you shouldn't be using = signs to indicate anything but equality.
However in this case your intent is clear. The equation 1atm(3.00m^3) / 273K = 3.2atmV2 / 311K has solution V2 = 1.07 m^3.
30. In an internal combustion engine, air at atmospheric pressure and a temp of about 20? is compressed in the cylinder by a piston to 1/9 of its original volume (compression ratio=9). Estimate the temp. of the compressed air, assuming the pressure reaches 40atm.
-P1V1 / T1 = P2V2 / T2 1atm(1m^3) / 293.15K = 40atm(1/9m^3) / T2
T2= 1305.88K
Good work. See my notes and let me know if you have questions.