# Gauge Pressure (teacher completely skip this)

## Homework Statement

A tire contains air at a gauge pressure of 4.86x10^4 Pa at a temperature of 25*C. After nightfall the temperature drops to -10*C. Find the new gauge pressure in the tire. Pa

## Homework Equations

Pa_i/T_i = Pa_f/T_f
T = T_c + 273

## The Attempt at a Solution

4.86x10^4/298 = Pa_f/263

Pa_f = 4.29x10^4

This is incorrect. I read the book and it said something about absolute pressure - atomospheric pressure (1.01x10^5). The problem i have is how do i find the absolute pressure? Also am i going into the right approach on solving the problem?

or

Am i completely off on what I am doing. Mainly i need help on figuring out how to get the absolute pressure.

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AlephZero
Homework Helper
Gauge pressure is the pressure difference above atmospheric pressure. For example if you fitted a new car tyre and didn't inflate it, the gauge pressure (as measured by most pressure gauges - hence the name) would be zero. But the absolute pressure inside the tire isn't zero, it is the same as atmospheric pressure.

In your gas law equation you need absolute pressure and absolute temperature. You used abs. temperature correctly.

Use the standard value for atmospheric pressure to convert gauge pressure to absolute.

andrevdh
Homework Helper
The gauges at a fuel station reads "gauge pressure". That is the guage reads zero pressure if a tire is completely deflated. The guages therefore reads only pressure above the local atmospheric pressure. To get the absolute pressure one only need to add the atmospheric pressure to the guage pressure (hope it make sense).

if i understand this correctly the absolute pressure is

4.86x10^4 + 1.01x10^5

andrevdh
Homework Helper
That is correct.