# Tag Archives: Topic Pressure

## Boiling water at 60 °C in lab!

Water boils at 100°C. In the view below, the water is initially at around 60°C.

How is this possible?

**Explanation:**

At sea level, where the pressure is atmospheric pressure (approx 105 000 Pa), the boiling point of water is 100°C.

In the video as air is sucked from the sealed container, the pressure in the container decreases. This lowers the boiling point of the water to around 60°C, hence water will boil and bubbles are formed.

It is similar to boiling water at high altitude, like on top of Mount Everest (about 8 km high). Water will boil around 70°C to to lower atmospheric pressure at high altitude.

## Physics of Construction Lifting Truck

**1) Why are the 4 extended legs necessary?**

This is to **increase the area of base **of the truck to **increase the stability**. From our theory, as long as the weight acting vertically downward from the centre of gravity is within the area of base, there will be a restoring moment to bring the truck back to its original position, hence increases its stability.

* 2) In what situation will the truck topple? *(note the the centre of gravity (CG) of the truck is in general very low due to mass concentration at the base)

Basically, there are 2 situdation:

– if the load that is lifted is too heavy, the overall new CG of the truck and load might shift outside the area of base, hence causing the truck to topple. i.e. the clockwise moment created by the load is created than the anticlockwise moment created the weight of truck.

– If the angle of tilt is too much which increases the perpendicular distance from load to pivot. Likewise, the overall CG might be outside the area of base and there is a net clockwise moment.

*3) What is the purpose of the metal plate underneath the legs?*

The purpose is to increase the area of base. The whole weight of the truck is spread over the 4 legs. Since P = F/A, with a bigger base area, the pressure acting on the ground will be reduced. This is to minimise any damages done to the ground.

## N2004 P1 Q15 – Pressure P1V1 = P2V2

^{3}.

^{3 }B) 3 cm

^{3 }C) 12 cm

^{3 }D) 18 cm

^{3}

**Solutions: Option A**

P_{A}V_{A} = P_{B}V_{B}

(P_{atm} + P_{water}) V_{A} = (P_{atm}) V_{B}

(10 + 20) V_{A} = 10 x 6

V_{A} = 60 / 30

= 2 cm^{3}

## Using barometer to estimate height of mountain

At sea-level, the barometer which Pie is holding reads 760 mm of Hg. When he is at the top of Mount Pie, the same barometer reads 230 mm of Hg.

Given take the density of mercury to be 13.6 g cm^{-3} and the density of air, 1.23 x 10^{-3} g cm^{-3}.

Estimate the height of the mountain?

**Solutions: 5860 m**

Pressure difference in mercury, Pmercury = pgh

= 13600 x 10 x (0.76 – 0.23)

= 72080 Pa

Pressure difference, Pair = pgh

72080 = 1.23 x 10 x h

h = 5860 m

In general, barometer can be used to measure altitude as height increases, height of mercury column will decreases. Of course same calibration has to be done. It is not practical to bring around a mercury barometer as its liquid, heavy and poisonous etc.

An aneroid barometer (no mercury inside though) is used to measure altitude more accurately.

## Pressure – Manometer and how mercury levels in both limbs move as pressure changes

The diagram shows a manometer connected to a gas cylinder of large volume. The atmospheric pressure is 76 cm Hg.

Due to the pressure of the gas cylinder, the mercury level in the left limb is at 30 cm while the mercury level in the right limb is at 0 cm.

Actually there is a short-cut. If atmospheric pressure remains constant, when the pressure of the gas cylinder decreases by 20 cm Hg, the difference in mercury levels between the left and right limbs decreases from 30 cm Hg to 10 cm Hg.

If you understand this, straight away you will be able to tell that the mercury height difference between left limb and right limb is 10 cm Hg.

You can then consider the markings on the manometer. When the drop in pressure is 20 cm Hg, note that mercury level on one limb will increase by 10 cm and the other limb will decrease by 10 cm. Shared equally!

## Liquid Pressure Video

Liquid pressure, **P = ρgh**.

**ρ** is the density of the liquid (kg/m^{3}) ,

**g** is the gravitational field strength (10 N/kg), and

**h** is the vertical height from the surface of the liquid (m)

In this video, since the last outlet is the lowest (greatest height from the liquid surface), the pressure is the greatest. Hence water ejects out with the greatest force and longest distance.