23. Convert an absolute pressure of 7.00 x 10^5 N/m^2 to gauge pressure in lb/in

Question

23. Convert an absolute pressure of 7.00 x 10^5 N/m^2 to gauge pressure in lb/in^ 2. (This value was stated to be just less than 90.0 lb/in^2 in Example 13.9. Is it?) 24. Suppose a gas-filled incandescent light bulb is manufactured so that the gas inside the bulb is at atmospheric pressure when the bulb has a temperature of 20.0 degree C . (a) Find the gauge pressure inside such a bulb when it is hot, assuming its average temperature is 60.0 degree C (an approximation) and neglecting any change in volume due to thermal expansion or gas leaks. (b) The actual final pressure for the light bulb will be less than calculated in part (a) because the glass bulb will expand. What will the actual final pressure be, taking this into account? Is this a negligible difference? 5. Large helium-filled balloons are used to lift scientific equipment to high altitudes. (a) What is the pressure inside such a balloon if it starts

Explanation / Answer

23. 7 x 10^5 N/m^2 = 7 x 10^5 x 0.2248 lbs / 39.37^2 inch^2

= 101.52 lbs / inch^2

and 1 atm = 14.70 lbs/in^2

so gauge pressure = 101.52 - 14.70 = 86.82 lbs/in^2

24. Using PV = nRT

P/T = nR/V = constant

1 atm / (273 + 20) = P' / (273 + 60)

P' = 1.1365 atm (absolute pressure )

Gauge pressure = 0.1365 atm = 0.1365 x 14.70 = 2 lbs/in^2

b) volume will expand.

coefficient of lineare expansion of bulb is of order of 10^-6 m K

and deltaV = V x 10^-6 x (60 - 40)

V for bulb is also very small.

so deltV will be very very small.

so this is negligible./

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