Quiz Question

Bob Bidonde

Bob Bidonde

Well-known member
You jack your Model "A" so all 4 wheels are off of the ground. Then you replenish the tires to 35 PSIG. When you put the car back onto the ground, what happens to the gauge pressure of the tires?:)
 
We often changed tire pressures on drag slicks with the car jacked up and never had the pressures change once lowered from the jack. These tires were running low pressures (12-20 psi). Of course, tire pressures did chance with temperature changes.
 
To tell the truth, I have never checked my tire pressures with the tires off the ground (except when first mounting them on the wheels).. I always check my tire pressure when on the ground as they would be while driving.
 
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The pressure would only change if the internal volume of the tire changed, which it doesn’t really. It flattens but sidewalls push out to compensate.
 
alexiskai said:
The pressure would only change if the internal volume of the tire changed, which it doesn’t really. It flattens but sidewalls push out to compensate.
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From a practical standpoint, I agree. I think Bob's quiz was more theory than practical. From a theory basis, where the absolute internal pressure is >3x the absolute external pressure (35+15=50 external, 15 external) I think it's reasonable to treat the inflated tire casing as a constant volume, hence no increase in internal pressure.
 
PV=nRT

P= pressure (absolute pressure)
V= volume
n= the number of moles of gas
R= the gas constant
T= temperature (absolute temperature, Kelvin or Rankine)

n and R are constant, so you can just plug in ratios. Twice the temperature will result in twice the pressure or volume.

You have to add 459.67 degrees to your Fahrenheit reading to get the absolute temperature in Rankine. So if you are 0 degrees F you would have to go to 460 F to double the temperature.

As Jay Jay said above, absolute pressure is the real pressure. 35 psi is relative to atmospheric pressure, about 15 psi. So the absolute pressure is 50 psi.

Having the volume would double the absolute pressure.
 
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nkaminar said:
PV=nRT

P= pressure (absolute pressure)
V= volume
n= the number of moles of gas
R= the gas constant
T= temperature (absolute temperature, Kelvin or Rankine)

n and R are constant, so you can just plug in ratios. Twice the temperature will result in twice the pressure or volume.

You have to add 459.67 degrees to your Fahrenheit reading to get the absolute temperature in Rankine. So if you are 0 degrees F you would have to go to 460 F to double the temperature.

As Jay Jay said above, absolute pressure is the real pressure. 35 psi is relative to atmospheric pressure, about 15 psi. So the absolute pressure is 50 psi.

Having the volume would double the absolute pressure.
Click to expand...

At work we differentiate the two as Atmospheric pressure (psia) and Gauge Pressure (psig)
 
old31 said:
Bob, see what you started. It's all hot air, and now my head hurts from NK's PV=nRT.
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For a fixed amount of gas, the "n" (number of moles of gas) falls out of the equation. R is always constant. So it becomes P1V1/T1=constant value=P2V2/T2. In the case of the tire, assuming that temperature doesn't change, you can rearrange the equation to P1V1/V2=P2. Or, as the volume of the tire would change from initial condition (in the air, V1) to final condition (on the ground, V2) the pressure will change. If the volume of the tire decreases because of the footprint on the ground then V1>V2 and P2>P1. But I don't believe the volume would change.

We smart-*** undergraduate engineers-to-be used to call PV=nRT the "pervert equation" (giggle giggle, ignoring the n somehow). Its proper name is actually the "Ideal Gas Law". Goes to show how times change in 50+ years. Saying "pervert" on a campus these days would likely get you kicked out of school.
 
Big Hammer hit the nail on the head as usual. The air molecules scatter the blue light but not the warm colors, yellow and red. The red and yellow come straight through during sunrise and sunset and get reflected off the clouds to produce the beautiful sunsets and sunrises. I get up before the sun on most mornings and enjoy watching the sun come into view as the my planet rotates towards the east. Some mornings are spectacular. Always best with a cup of espresso and steamed milk.
 
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Depends on how many feet above sea level ?? Also which hemisphere you live in ??/ & maybe which way up the valve is when inflating , Wet & cold in NZ
 
You are from New York and since the temperature outside was 104 degrees, when you lowered the tire to the ground and the rubber came in contact with the hot pavement, of course the pressure inside the tire would go up as the air inside the tire was heated up from the outside pavement temperature.

Brad in Maryland
 
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