كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

Theme E — Nuclear and Quantum Physics

E.5 Fusion and Stars — IB Physics SL Revision Notes & Topical Questions

5 min read 10 parts Lesson 22 of 30 1 quiz

📚 IB Physics SL Hub / Topical Revision

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.

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Lesson summary

E.5 Fusion and Stars — IB PHYSICS SL notes: Explain fusion as the energy source of stars and use the HR diagram, stellar evolution and parallax to describe stars.Nuclear fusion = the joining of two light nuclei to form a heavier nucleus.The product has a higher binding energy per…

The product has a higher binding energy per nucleon
In stars four hydrogen nuclei fuse to one helium nucleus
Nuclei need high kinetic energy
The strong nuclear force has a very short range
Such conditions exist in the cores of stars
On the main sequence these balance

Part 1 of 8Nuclear fusion

1Nuclear fusion

Nuclear fusion the joining of two light nuclei to form a heavier nucleus.

2The product has a higher binding energy per nucleon

The product has a higher binding energy per nucleon so mass decreases and energy ΔE = Δmc² is released.
This holds for nuclei lighter than iron-56.

Per unit mass of fuel, fusion releases more energy than fission.

4In stars four hydrogen nuclei fuse to one helium nucleus

In stars four hydrogen nuclei fuse to one helium nucleus releasing a huge amount of energy.

Part 2 of 8Conditions for fusion

1Nuclei need high kinetic energy

Nuclei need high kinetic energy to overcome electrostatic repulsion between protons.

Very high temperature (millions to about 100 million kelvin) and very high pressure and density are needed.

3The strong nuclear force has a very short range

The strong nuclear force has a very short range so nuclei must come extremely close.

4Such conditions exist in the cores of stars

Such conditions exist in the cores of stars or in particle accelerators.

Part 3 of 8Stellar equilibrium

Gravity pulls the material of a star inward.

Gas pressure and radiation pressure from fusion push outward.

3On the main sequence these balance

On the main sequence these balance and the star is stable.

A hotter core raises the outward pressure and the star expands; a cooler core lowers it and the star contracts.

Part 4 of 8Hertzsprung–Russell diagram

The HR diagram plots luminosity (relative to the Sun) against surface temperature.

Temperature increases to the left; luminosity increases upward.

2Main sequence

Main sequence a band from hot luminous stars at top left to cool dim stars at bottom right.

3Red giants and red supergiants

Red giants and red supergiants cool but very luminous, so very large.

4White dwarfs

White dwarfs hot but dim, so very small.

5The diagram shows only stable phases

The diagram shows only stable phases transitions are short.

Part 5 of 8Evolution of stars

A nebula collapses under gravity and heats up as a protostar.

Hydrogen fusion starts and the star joins the main sequence.

Core hydrogen runs out and the outer layers swell, giving a red giant or supergiant.

Low mass: the outer layers are ejected as a planetary nebula and a white dwarf remains.

High mass: fusion reaches iron, then a supernova leaves a neutron star or black hole.

The path depends on the initial mass of the star.

Part 6 of 8Distance and radius

Distance and radius

Stellar parallax gives the distance to nearby stars

Stellar parallax gives the distance to nearby stars d (pc) = 1 / p (arcsecond).

1 pc

3.26 ly — and the method works up to about 100 pc.

Radius follows from luminosity and temperature

L = 4πr²σT⁴, with T from Wien's law.
What the symbols mean: L = luminosity, the total power emitted (W) · σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴ · r = radius of the star (m) · A = surface area (m²) · T = surface temperature (K)

Part 7 of 8Fission compared with fusion

Feature	Fission	Fusion
Process	Heavy nucleus splits	Light nuclei join
Nuclei involved	A much greater than 60	A much less than 60
Conditions	Thermal neutrons	Very high temperature and density
Energy per unit mass	Lower	Higher
Where	Nuclear reactors	Stars

Part 8 of 8More from the specification

Solar neutrino detection on Earth

Solar neutrino detection on Earth capturing solar electron neutrinos in deep underground detectors on Earth validates core thermonuclear proton-proton fusion chains.

Nuclear fusion in stars

Nuclear fusion in stars hydrogen nuclei fuse into helium at high temperature and pressure, with the gravitational collapse balanced by radiation pressure

Emission and absorption spectra

Emission and absorption spectra hot gas gives emission lines,
- while cooler gas in front of a continuous spectrum gives dark absorption lines at the same wavelengths

Spectral analysis

Spectral analysis lines in a stellar spectrum identify its elements, temperature and motion by Doppler shift

Photons

Photons atoms emit a photon of energy hf when an electron moves down between energy levels

Stellar radius

Stellar radius from L = 4 pi r² sigma T⁴, the radius follows from luminosity and surface temperature measured from the continuous spectrum

Emission spectrum

The set of discrete wavelengths emitted by excited atoms of an element.

Energy level

A discrete energy an electron in an atom can have

Continuous spectrum

Spectrum with no gaps between the regions.

Spectrum

Range of colours in white light.

Absorption

The conversion of wave energy to internal energy of the medium.

Reaction

The partner force acting on the other object.

Photon

A quantum of electromagnetic energy, E = hf
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10⁻³⁴ J s) · f = frequency (Hz)

Wavelength

Distance between a point on one wave and the same point on the next wave

Key terms

Term	Meaning
Nuclear fusion	Joining of two light nuclei to form a heavier nucleus with release of energy.
Main sequence	Band on the HR diagram of stars fusing hydrogen in their cores.
Red giant	Large, cool, luminous star that has left the main sequence.
White dwarf	Hot, dense, dim remnant of a low-mass star.
Supernova	Explosion of a massive star at the end of its life.
Stellar parallax	Apparent shift of a nearby star against distant stars as Earth orbits the Sun.
Parsec	Distance at which a star has a parallax angle of one arcsecond.

Avoid these mistakes

Saying stars burn fuel chemically — their energy comes from nuclear fusion.

Believing fusion releases energy for every nucleus — beyond iron it needs an energy input.

Reading the HR diagram with temperature rising to the right — it rises to the left.

Thinking a star's fate is fixed by its colour — it is set by its mass.

Using parallax for very distant stars — the angles become too small to measure.

Last updated: 18 October 2026