CK-12-Physics - Intermediate

12.2. Measuring Pressure http://www.ck12.org

atmospheric pressure. For example, a completely deflated tire still has pressure on it. Atmospheric pressure is
pushing on both the inside and outside of the tire. What matters is how much additional pressure we put in when
the tire is inflated.

When dealing with liquids, we typically refer to the pressureP=ρghas the gauge pressurePg. The total pressure
Ptis thereforePt=Patm+Pg=Patm+ρgh.

Check Your Understanding

1. A tire pressure gauge reads 32 pounds per square inch (psi) when measuring tire pressure. What is the total
   pressure in pounds per square inch?

Answer: Atmospheric pressure is 101,325 Newtons per square meter. To convert, we use:

• 1 pound = 4.45 newtons


• 1 inch = 0.0254 meters

Patm= 101 , 325

N

m^2

×

(

1 lbs

4 .45 N

)

×

(

0 .0254 m

1 in

) 2

= 14. 7

lbs

in^2

Pt=Patm+Pg= 14. 7

lbs

in^2

+ 32

lbs

in^2

= 46. 7

lbs

in^2

.

2. The bottom of Lake Pontchartrain has a gauge pressure of 3. 9 × (^104) mN 2. What is the total pressure in units of
   standard atmospheres of pressure, atm?
   Answer:To find the pressure in the units of atm, we convert frommN 2 to atm, then adding in the atmospheric pressure
   of 1.0 atm.
   Pg=

(

3. 9 × 104

N

m^2

)

×

(

1 atm

1. 01 × (^105) mN 2

)

= 0 .39 atm

Pt=Patm+Pg= 1 .0 atm+ 0 .39 atm= 1 .39 atm

.