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Figure 6

Thereare two typesof pressure:

absolute

and

relative

.

The absolute pressure

is that which includes, in

addition to the pressure that we generate through a

pump, the atmospheric pressure.

Thepressureof theair available tobeusedoutside the

receptacle is defined as

relative pressure

(or gauge

pressure) - i.e. that indicated by the pressure gauge.

Tomeasureatmosphericpressureweuse the

barometer

,

while tomeasure the pressure of a gas enclosed in a

container we use the

manometer (pressure gauge)

.

It ismadeof ametallic tube,withanelliptical section,

in the shape of a circumference.

By varying the relative pressure at the end

B

, the

tube varies its length. Point

A

, which is sensitive to

this change, reflects this on the scale as a result of

the rotary motion of a hinge. In this case, the gauge

doesn’t giveus any value since the pressuredifference

between the inside and the outside of the tube is

zero

.

Figure 7

Through this figurewe can observe that, by increasing

thepressureat theend

B

, the

A

end tends tostraighten

andmove the index, as the resistance created by the

external atmospheric pressure is overcome.

Thedifferencebetween the twopressures determines

the angular displacement of the index.

A

1

0

B

3

2

4

Fig. 6

2

4

0

1

3

Fig. 7

Boyle’s law

The state of a gas is described through three parameters:

Volume

,

Pressure

, and

Temperature

.

To understand the relationship between them, we study how a gas behaves by controlling one of these parameters

and observing the behaviour of the others. A particular characteristic of gas is its capacity to expand and occupy

the largest volume available. With the following examples, we study the change of pressure and volume at a

constant temperature.

Figure 8

Pos. 1

: a rigid transparent tubewith a constant section andwith a

U

shape is arranged in a vertical positionwith

the long arm exposed and the short arm connected to a tap. Filling themercury tube, the height of themercury in

the twoarmswill beequal, since theatmospheric pressure is actingonboth surfaces.We close the tap, the volume

of air present in the shorter arm, (subject to the atmospheric pressure) is denoted by

V

X

, with

x

as its height.

Pos. 2

: we pour additional mercury through the open armwhereby the volume of the air in the short arm halves.

We observe a difference in the level of mercury in the two arms of 76

cm

, i.e. 1

bar

.

Pos. 3

: continuing to add mercury, we observe that when the volume of the trapped air becomes a third of the

initial value, the difference in levels is 76

cm

+ 76

cm

= 152

cm

, i.e. 2

bar

.

1

16

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PHYSICS