Page 156 - Physics

Physics for Secondary Schools

The simplest form of a manometre consists of a Atmospheric Pressure
U-shaped tube fi lled with a liquid of a known density. The surface of the earth is
The liquid is normally coloured to enhance visibility. surrounded by the atmosphere,
The manometre is fi lled with the liquid until both sides which is the layer of air consisting
of the tube are half-fi lled, as shown in Figure 7.23 (a).
of a mixture of gases. Atmospheric
Zero point pressure is a result of the weight
of this layer of gases. This is why
air pressure is usually referred
to as atmospheric pressure. The
atmospheric pressure on the
earth’s surface and objects on the
earth is approximately 1.01 x
Water Water 10 N/m or 1 atmosphere. We do
5
2
not experience this great pressure
because the fl uids in our bodies
(a) Pressure is equal (b) Pressure is higher exert pressure slightly greater than
in both arms in the left arm the atmospheric pressure.

Figure 7.23: Manometer Variation of atmospheric

pressure with altitude
When both ends of the tube are open, the pressure The pressure exerted by the
on both sides is equal. This water level is marked atmosphere decreases with an
as zero point as in Figure 7.23 (a). When one end increase in altitude from the
of the tube is connected to the unknown pressure mean sea level. For example,
P , the liquid level falls in one arm and rises in the atmospheric pressure is found to
1
other arm as shown in Figure 7.23 (b). The change be higher at the base and lower
in height Δh ( ) is caused by the change in pressure at the top of Mount Kilimanjaro

( ΔP) which can be obtained from the equation, as shown in Figure 7.24. This

variation is due to the fact that
P = P + Δh ( ) ρg.
1 2 the density of air is high at the
Since, a manometre measures pressure by mean sea level and decreases with
comparing it with atmospheric pressure, it is altitude (air exerts less pressure)
sometimes called a gauge pressure. above the mean sea level.

Task 7.3 The variation of pressure with

Demonstrate the use of a manometre in altitude has a practical impact on
measuring the pressure of a liquid. Measure human life. Mountain climbers
the pressure of different liquids using the may face nose bleeding at high
manometer. altitudes above the mean sea level

150
Student’s Book Form One

Physics Form 1 Final.indd 150 16/10/2024 20:57