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How is the pressure of a gas measured, and what is atmospheric pressure?

Describe how a manometer and a mercury barometer measure gas and atmospheric pressure

A focused answer to the O-Level Physics outcome on gas pressure. Gas pressure from particle collisions, the manometer for measuring gas pressure, the mercury barometer for atmospheric pressure, and reading pressure from a liquid column.

Generated by Claude Opus 4.87 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to explain how a gas exerts pressure, and to describe two instruments: the manometer, which measures the pressure of a gas supply, and the mercury barometer, which measures atmospheric pressure. Both work from the liquid-column idea p=hρgp = h\rho g, so this dot point builds directly on liquid pressure.

The answer

How a gas exerts pressure

A gas is made of fast-moving particles travelling in all directions. They constantly collide with the walls of the container, and each collision pushes on the wall. The huge number of collisions per second, spread over the wall area, produces a steady pressure. Squeeze the gas into a smaller volume and the particles hit the walls more often, so the pressure rises.

The manometer

A manometer is a U-shaped tube partly filled with liquid (often mercury or water). One arm connects to the gas supply; the other is open to the atmosphere.

  • If the gas pressure equals atmospheric pressure, the liquid sits level in both arms.
  • If the gas pressure is greater, it pushes the liquid down on its side and up on the open side; the difference in levels hh gives the excess pressure as hρgh\rho g.

pgas=patmosphere+hρgp_{\text{gas}} = p_{\text{atmosphere}} + h\rho g

(when the open side is higher). The manometer measures the difference between the gas pressure and atmospheric pressure.

The mercury barometer

A mercury barometer measures atmospheric pressure. A long tube is filled with mercury and inverted into a dish of mercury. The mercury falls until the column's pressure balances the atmospheric pressure pushing down on the dish, leaving a vacuum at the top. The vertical height of the mercury column then measures atmospheric pressure:

patmosphere=hρgp_{\text{atmosphere}} = h\rho g

At sea level the column is about 0.76 m0.76\ \text{m} (760 mm) of mercury, which equals about 1.0×105 Pa1.0 \times 10^5\ \text{Pa}.

Why mercury

Mercury is used because it is very dense, so the column is short (under a metre). A water barometer would need a tube over 10 m10\ \text{m} tall, because water is far less dense and a taller column is needed to balance the same pressure.

Examples in context

Example 1. Weather forecasting. A barometer's reading rises and falls with the weather. High atmospheric pressure (a tall mercury column) usually means settled, fine weather, while a falling reading often warns of an approaching storm, which is why barometers have long been used in forecasting.

Example 2. A gas cylinder gauge. The pressure gauge on a gas cylinder is a kind of manometer reading how much the gas pressure exceeds the surrounding atmosphere. When the gauge reads zero the cylinder is not empty of gas, it has simply fallen to atmospheric pressure, so no more gas will flow out on its own.

Try this

Q1. Explain, in terms of particles, how a gas exerts pressure on its container. [2 marks]

  • Cue. Fast-moving gas particles collide with the walls; the many collisions per second over the area produce the pressure.

Q2. A mercury column of height 0.70 m0.70\ \text{m} (ρ=13 600 kg m−3\rho = 13\,600\ \text{kg m}^{-3}, g=10 N kg−1g = 10\ \text{N kg}^{-1}) balances the atmosphere. Find the atmospheric pressure. [2 marks]

  • Cue. p=hρg=0.70×13 600×10=95 200 Pap = h\rho g = 0.70 \times 13\,600 \times 10 = 95\,200\ \text{Pa}.

Q3. State why mercury, not water, is used in a barometer. [2 marks]

  • Cue. Mercury is much denser, so the column is short (under a metre); a water column would need to be over 10 m10\ \text{m} tall.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA U-tube manometer connected to a gas supply shows the mercury higher on the open side than on the gas side, with a difference in levels of 0.15 m0.15\ \text{m}. Mercury has density 13 600 kg m−313\,600\ \text{kg m}^{-3} and g=10 N kg−1g = 10\ \text{N kg}^{-1}. Find the pressure difference between the gas and the atmosphere.
Show worked answer →

Pressure difference =hρg=0.15×13 600×10=20 400 Pa= h\rho g = 0.15 \times 13\,600 \times 10 = 20\,400\ \text{Pa} (about 2.0×104 Pa2.0 \times 10^4\ \text{Pa}).

Because the open side is higher, the gas pressure is greater than the atmospheric pressure by this amount.

Markers reward using hρgh\rho g with the difference in mercury levels as hh, the correct value in pascals, and the statement that the gas pressure exceeds the atmospheric pressure since the open arm is higher.

Original4 marks(a) Explain, in terms of particles, how a gas exerts pressure on the walls of its container. (b) State what happens to the gas pressure if the gas is squeezed into a smaller volume at the same temperature, and why.
Show worked answer →

(a) Gas particles move quickly in all directions and collide with the container walls. Each collision exerts a tiny force on the wall, and the many collisions per second over the wall area produce the gas pressure.

(b) The pressure increases. In a smaller volume the same number of particles hit the walls more often (more collisions per second per unit area), so the pressure is larger.

Markers reward pressure as the result of particle collisions with the walls, and the explanation that a smaller volume means more frequent collisions and so higher pressure.

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