Absolute Pressure Formula: Definition and Examples

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Absolute pressure is an important concept and the term is related to pressure and is used to gain an idea about the amount of air pressure in a given system. Although there are many other methods and ways of calculating air pressure, the absolute pressure formula finds applications in many important industries, research purposes and weather forecasting. One can find a plethora of measurement methods used to define, calculate, and determine different kinds of pressure. Absolute pressure is one such scientific method widely used.

Read more: Vapor pressure

Table of Content

What is Absolute Pressure?
Absolute zero pressure
Absolute Pressure Formula
Absolute Pressure vs Atmospheric Pressure
Things to Remember
Sample Questions

Key Terms: Absolute Pressure, Absolute pressure formula, Atmospheric pressure, Vacuum, Absolute zero pressure,

What is Absolute Pressure?

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Absolute pressure gives the air pressure in relation to the zero pressure or vacuum. In other words, it is the total pressure in a place over and above the pressure of vacuum. Thus, this term relates the total pressure with that of vacuum, hence, it is known as absolute pressure.

The absolute pressure formula for fluids combines the terms atmospheric pressure and gauge pressure.
The unit of this physical quantity is the same as that of other measurement methods- pounds per square inch (psi).
Practically the absolute pressure is measured by a special device in which the sensor has a diaphragm with a vacuum behind it.
Hence, the pressure calculated by this device is considered to be absolute pressure.

If the reference frame is gauge pressure then the value of the atmospheric pressure is considered. The gauge won't reveal the presence of air pressure if it is not in a vacuum, value must be adjusted by using the absolute pressure formula.

The video below explains this:

Absolute Pressure Detailed Video Explanation:

Read more:

Relevant Concepts
Pressure Formula	Relation Between Bar and Atm	Combined Gas Law
Barometer	Pressure System	Mole fraction

Absolute zero pressure

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The pressure of vacuum or zero pressure in any space is termed as Absolute zero pressure. In outer space there is vacuum, hence the pressure there is Absolute zero Pressure.

Absolute Pressure Formula

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According to the above definition any pressure above the zero pressure level is Absolute Pressure. In the frame of reference of gauge pressure it is the sum of Gauge pressure and Atmospheric pressure. Thus, the formula for Absolute Pressure is given by:

P _abs = P_atm + P _gauge

Here: P _abs = absolute pressure, P _atm = atmospheric pressure, and P _gauge = gauge pressure.

Knowing the atmospheric pressure and gauge pressure in an area, one can easily calculate the Absolute pressure of that system.

Absolute Pressure vs Atmospheric Pressure

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In a perfect vacuum, the point of zero is the complete absence of pressure. This is referred to as Absolute Zero Pressure.

Absolute Pressure is the pressure measured from absolute zero pressure. It is a value (if not zero) then depends upon the atmospheric pressure and the gauge pressure (when viewed from the frame of reference of gauge pressure. This physical quantity can be used to study pressure at any place (in space and also in other planets).

On the other hand, Atmospheric Pressure is the pressure caused by the Earth’s atmosphere (unless one happens to be on another planet). This pressure is commonly affected by altitude, wind velocity, and temperature. This parameter of measuring pressure is restricted to the earth’s atmosphere.

Both the physical quantities of pressure have the same S.I unit of pressure- pascal.

Things to remember

Absolute Pressure is the pressure calculated over the pressure of vacuum or zero pressure. The pressure of a vacuum is referred to as Absolute zero pressure. It is the sum of Gauge pressure and Atmospheric pressure.
The Absolute Pressure formula is written as follows:-
P _abs = P _atm + P _gauge

Here: P _abs = absolute pressure, P _atm = atmospheric pressure, and P_gauge = gauge pressure.

Absolute Pressure is different from atmospheric pressure as it is that physical quantity that gives the value of total pressure above its starting point , that is, zero pressure.
Absolute pressure is used in the study of weather and changes in climate and also in industries for various purposes.
The pressure of vacuum or zero pressure in any space is termed as Absolute zero pressure.

Sample Questions

Ques. What do you mean by absolute pressure? How is it calculated? (1 mark)

Ans. Absolute pressure is the total pressure in a place above the zero pressure. This pressure is calculated to know the pressure in a region with relation to the pressure of vacuum.

Ques. What can we do with the Gauge Pressure and Absolute Pressure Formula? (2 marks)

Ans. By knowing gauge pressure and the absolute pressure of a region or area in space we can easily calculate the atmospheric pressure of that region or area with the help of the Absolute pressure formula:

P _abs = P _atm + P _gauge

Here: P_abs = absolute pressure, P_atm = atmospheric pressure, and P _gauge= gauge pressure.

Ques. A gauge pressure detecting device measures the p_gauge reading as 31 psi. If the atmospheric pressure is 14.2 psi. Calculate the absolute pressure that corresponds to p_gauge reading (3 marks)

Ans. P_atm (Atmospheric pressure) = 14.2 psi

P_gauge (Gauge pressure) = 31 psi

Absolute pressure (p_abs) = p_atm + p_gauge

= 14.2 psi + 31 psi

p_gauge= 45.2 psi

Hence, the absolute pressure in the given situation is 45.2 psi.

Ques. The psi pressure instrument gives the reading as 35.8 psi. If the atmospheric pressure is 15 psi, calculate the corresponding gauge pressure. (5 marks)

Ans. Atmospheric pressure patm = 15 psi

Absolute pressure pabs = 35.8 psi

The Gauge pressure is

p_gauge = 35.8 psi – 15 psi

gauge = 20.8 psi.

Hence, the atmospheric pressure in the given situation is 20.8 psi.

Ques. Determine (a) gauge pressure and (b) the absolute pressure of water at a depth of 9 m from the surface and the atmospheric pressure is 10N/m²10N/m²(g=10m/s²g=10m/s²) (5 marks)

Ans. The gauge pressure of any fluid is given in terms of product of the density of the fluid and depth below the surface by the following expression:

P=ρghP=ρgh

The absolute pressure is defined in terms of the gauge pressure and the atmospheric pressure by the following expression:

P _abs = P _atm + P _gauge

Find out the gauge pressure and absolute pressure of water. The depth to find these values is given as:

h=9m

Therefore, we have

P_gauge=ρgh=1000×10×9=90000N/m²

Now, to find out the absolute pressure of water at the given depth. This pressure is equal to the sum of the gauge pressure of water and the atmospheric pressure on the water surface. The value of atmospheric pressure as

P_atm=10N/m²

Therefore, calculate the absolute pressure of water in the following way.

P_absolute = P_atm+ P_gauge

=90000+10

=90010N/m²

P _abs = P _atm + P _gauge

90000+10=90010N/m²

Thus, the gauge pressure of water is 90000N/m² and the absolute pressure of water is 90010N/m².

Ques. A pressure gauge measures the reading as 29 psi. If the atmospheric pressure is 13 psi. Find out the absolute pressure that corresponds to this gauge pressure reading. (5 marks)

Ans. Atmospheric pressure, P_atm=31psi

Gauge pressure, P_gauge=14.2psi

Now applying the formula,

Absolute pressure, P _abs = P _atm + P _gauge

P_abs=13psi+29psi

P_abs=42psi

Thus Absolute pressure will be 42 psi.

Ques. The psi pressure instrument gives the reading as 36.8 psi. If the atmospheric pressure is 16 psi, then calculate the corresponding gauge pressure. (5 marks)

Ans. Atmospheric pressure P_atm=16psi

Absolute pressure P_abs=36.8psi

The Gauge pressure is by the absolute pressure formula:

P _abs = P _atm + P _gauge

P _gauge = 36.8psi – 16psi

= 20.8 psi.

Thus Gauge pressure will be 20.8 psi.

Ques. What are the applications of absolute pressure? (2 marks)

Ans. Absolute pressure has some important applications. They include:

It is used in absolute pressure sensors to measure barometric pressure variations due to changes in high and low atmospheric pressure created by weather patterns.
Another application is determining altitude by measuring change of absolute pressure as a function of elevation.

Ques. What are the ways by which we can calculate the atmospheric pressure? (4 marks)

Ans. There are two ways by which we can calculate the atmospheric pressure:

Absolute pressure formula, by knowing the absolute pressure and the gauge pressure one can easily find out the Barometric pressure or atmospheric pressure. It is the most used method as it gives accurate results.
By mercury gauge: It is a type of manometer which uses mercury as the working fluid. The height of the mercury column tells the atmospheric pressure. This method does not give 100% accurate readings because there are chances of change in nature and composition of mercury used.

Ques. What is gauge pressure and how is it measured? (2 marks)

Ans. Gauge pressure is the pressure of a system above atmospheric pressure. Gauge pressure is zero-referenced against ambient air (or atmospheric) pressure, Gauge pressure varies according to height above sea level as well as to weather conditions. It can be easily calculated by subtracting the absolute pressure by the atmospheric pressure.

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Absolute Pressure Formula: Definition and Examples

What is Absolute Pressure?

Absolute Pressure Detailed Video Explanation:

Absolute zero pressure

Absolute Pressure Formula

Absolute Pressure vs Atmospheric Pressure

Things to remember

Sample Questions

CBSE CLASS XII Related Questions

1.
Write any two features of nuclear forces.

2.
Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.
A tank is filled with a liquid to a height of \( 12.5 \, \text{m} \). The apparent depth of a needle lying at the bottom of the tank is measured to be \( 9.0 \, \text{m} \). Calculate the speed of light in the liquid.

4.
What is displacement current (\( i_d \))? Considering the case of charging of a capacitor, show that \( i_d = \varepsilon_0 \frac{d\Phi_E}{dt} \). What is the value of \( i_d \) for a conductor across which a constant voltage is applied?

5.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

6.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

Similar Physics Concepts

Comments

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Absolute Pressure Formula: Definition and Examples

What is Absolute Pressure?

Absolute Pressure Detailed Video Explanation:

Absolute zero pressure

Absolute Pressure Formula

Absolute Pressure vs Atmospheric Pressure

Things to remember

Sample Questions

CBSE CLASS XII Related Questions

1. Write any two features of nuclear forces.

2.Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.A tank is filled with a liquid to a height of \( 12.5 \, \text{m} \). The apparent depth of a needle lying at the bottom of the tank is measured to be \( 9.0 \, \text{m} \). Calculate the speed of light in the liquid.

4.What is displacement current (\( i_d \))? Considering the case of charging of a capacitor, show that \( i_d = \varepsilon_0 \frac{d\Phi_E}{dt} \). What is the value of \( i_d \) for a conductor across which a constant voltage is applied?

5.Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

6.Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

Similar Physics Concepts

Comments

SUBSCRIBE TO OUR NEWS LETTER

1.
Write any two features of nuclear forces.

2.
Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.
A tank is filled with a liquid to a height of \( 12.5 \, \text{m} \). The apparent depth of a needle lying at the bottom of the tank is measured to be \( 9.0 \, \text{m} \). Calculate the speed of light in the liquid.

4.
What is displacement current (\( i_d \))? Considering the case of charging of a capacitor, show that \( i_d = \varepsilon_0 \frac{d\Phi_E}{dt} \). What is the value of \( i_d \) for a conductor across which a constant voltage is applied?

5.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

6.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]