Theme B — The Particulate Nature of Matter
B.3.1 Gas Pressure — IB Physics HL Revision Notes & Topical Questions
5 min read 8 parts Lesson 25 of 53 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
B.3.1 Gas Pressure — IB PHYSICS SL notes: Define pressure, calculate it from force and area and from depth in a fluid, and explain gas pressure in terms of molecular collisions.Pressure = the force applied per unit area, p = F/A.p is in pascals, with 1 Pa = 1 N m−2.F is the force…
- Direction of the force
- Large area
- Small area
- Other units
- Column of fluid
- Total pressure
Part 1 of 6Defining pressure
Pressure
Pressure the force applied per unit area, p = F/A.
- p is in pascals, with 1 Pa = 1 N m−2.
- F is the force perpendicular to the surface in N.
- A is the cross-sectional area on which the force acts, in m2.
Scalar
- Scalar pressure acts equally in all directions at a point in a fluid.
Direction of the force
- Direction of the force it is always at right angles (normal) to the surface.
Large area
- Large area a force spread over a large area gives a small pressure.
Small area
- Small area a force spread over a small area gives a large pressure.
Examples
- Examples a drawing pin goes into the surface rather than the finger, high heels press harder than flat shoes and snowshoes spread weight.
Other units
- Other units the atmosphere (1 atm ≈ 1.01 × 105 Pa), the bar (105 Pa) and mm Hg.
- (1 mm Hg ≈ 133 Pa)
Part 2 of 6Pressure in a fluid at depth
Cause
- Cause the layers above must be supported by the layers below.
Column of fluid
- Column of fluid weight ρAhg spread over area A gives Δp = ρgh.
- ρ is the density of the fluid, g = 9.8 N kg−1 and h is the depth.
Total pressure
- Total pressure in a liquid open to the atmosphere p = p0 + ρgh.
Depends only on depth
- Depends only on depth not on the shape of the container, and it is the same at the same level in a connected fluid.
Graph
- Graph pressure against depth is a straight line with intercept p0 and gradient ρg.
Part 3 of 6Measuring pressure
Manometer
- Manometer a U-tube of liquid with one arm open to the air;
- the level difference Δh gives the gauge pressure ρgΔh.
Mercury barometer
- Mercury barometer an inverted tube of mercury with a vacuum above it;
- atmospheric pressure supports a column of about 0.76 m.
Gauge pressure
- Gauge pressure tyre gauges show the pressure above atmospheric.
Absolute pressure
- Absolute pressure gauge pressure plus atmospheric pressure, which must be used in gas-law calculations.
Part 4 of 6Molecular picture of gas pressure
1Gas
- Gas a huge number of molecules moving randomly at high speeds, colliding with each other and the walls.
2Collision with the wall
- Collision with the wall the momentum of the molecule changes, and the change per unit time is a force on the wall.
3Average
- Average the enormous number of collisions averages to a steady force, and force per unit area is the pressure.
4More molecules in the same volume
- More molecules in the same volume more collisions per second and a higher pressure.
5Higher temperature
- Higher temperature faster molecules hit harder and more often, so the pressure rises.
6Smaller volume
- Smaller volume molecules reach the walls more often, so the pressure rises.
Part 5 of 6Atmospheric pressure
Origin
- Origin the weight of the air above.
Sea level
- Sea level about 1.01 × 105 Pa, equivalent to a 10 m column of water or 0.76 m of mercury.
Altitude
- Altitude it decreases, falling by about half at 5 km.
Not noticed
- Not noticed it acts equally in all directions and body fluids are at about the same pressure.
Use
- Use it is the starting point for the gas laws.
Part 6 of 6More from the specification
1Atomic collisions and moles of gas
- Atomic collisions and moles of gas the total number of gas moles determines collision frequency of energetic atoms, providing a means of pressure regulation.
2Thermodynamic temperature
- Temperature in kelvin, to which an ideal gas's internal energy is proportional.
3Constant
- Not changing.
4Theory
- A well-tested, wide-ranging explanation supported by evidence.
5Proportional
- Changing by the same factor as another quantity.
6Helium
- Product of the fusion of hydrogen in stars.
7Kinetic theory
- Particles in constant random motion.
8Kinetic energy
- K = ½mv2, energy of motion.
9Boltzmann constant
- K = R/NA, the gas constant per molecule
10Internal energy
- Total kinetic and potential energy of the particles in a system
11N = nNA
What the symbols mean: N = number of particles · n = amount (mol) · NA = Avogadro constant (6.02 × 1023 mol−1)
12Kinetic model assumptions
- Kinetic model assumptions molecules are point-like with negligible volume and exert no forces on each other except during collisions.
Key terms
| Term | Meaning |
|---|---|
| Pressure | Force per unit area acting at right angles to a surface |
| Pascal (Pa) | Unit of pressure, 1 N m−2 |
| Gauge pressure | Pressure above atmospheric pressure |
| Atmospheric pressure | Pressure due to the weight of the air above, about 1.01 × 105 Pa |
| Manometer | U-tube instrument that measures pressure differences using a liquid |
| Absolute pressure | Gauge pressure plus atmospheric pressure |
Avoid these mistakes
1
Using the total surface area instead of the area normal to the force — use the area on which the force acts.
2
Using gauge pressure in the gas laws — use absolute pressure.
3
Thinking pressure at depth depends on the container shape — it depends on depth and density.
4
Using cm2 without converting — convert to m2.
5
Saying pressure is a vector — it is a scalar.
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