Chapter 5, Problem 1RQ

Interpretation Introduction

Interpretation:

The principle behind the working of barometer and manometer has to be explained.

Concept Introduction:

The force acting per unit area is called as Pressure. Differential pressure is indicated by measuring devices i.e. in relative with the atmosphere pressure. This is known as gauge pressure. The calculated pressure can either be positive or negative with respect to the atmospheric pressure. Vacuum is generally known as a negative gauge pressure.

Atmospheric pressure is measured by using an instruments called as barometer and manometer.

Explanation of Solution

A barometer consists of Mercury column that is tipped inverted and positioned in a dish containing Mercury. The Mercury in the column moves to and fro. The measure of atmospheric pressure is done by the height of the column. There is a downward force produced by the weight of the Mercury, this downward force pushes the mercury to fall out of the column.  But, there is repulsive force that keeps Mercury in the column. This repulsive force is due to the atmospheric gas particles that collide with the surface of the Mercury in the dish; this makes the mercury to get pushed up in the column. The level of Mercury in the column stays constant, when the two repulsive forces are same in strength to each other. The Mercury’s constant height is supported by the atmosphere is a measure of pressure of the atmosphere.

A simple barometer is illustrated in the figure 1,

Figure 1: Simple Barometer

To explain the working of Manometer

A manometer also has two repulsive forces against each other. A force is exerted by the gas molecules in the flask. The repulsive force is seen on the further region of the Mercury filled U tube; that is force exerted by atmospheric gases. The measure of the variation in pressure between the gas in the flask and the atmosphere gives the difference in height of the Mercury in the U tube. The greater or lesser the gas in the flask to the atmosphere pressure can be determined by measuring the difference in the height of the Mercury.

Illustration of simple manometer is shown in figure 2,

Figure 2: A simple manometer

Conclusion

The principle and working of barometer and manometer are explained.