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 D — Fields

D.2.7 Electric Potential — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 22 of 39 1 quiz

📚 IB Physics HL 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

D.2.7 Electric Potential — IB PHYSICS HL notes: Define electric potential, use V = kQ/r for point charges and spheres, and add potentials from several charges.Electric potential — the work done per unit charge in taking a small positive test charge from infinity to a defined poin…

Depends on
Conducting sphere
Inside a charged sphere
Positive charge
Negative charge
Field lines

Part 1 of 8Definition

Electric potential

Electric potential the work done per unit charge in taking a small positive test charge from infinity to a defined point

Unit

Unit joule per coulomb, J C⁻¹, which is the volt

Scalar

Scalar it has no direction but a sign matching the charge

Reference

Reference zero at infinity, as for gravitational potential

Depends on

Depends on the size of the charge and the distance from it

Part 2 of 8Potential of a point charge

Equation

V = kQ/r = Q/(4πε₀r)

Q the charge producing the potential, with its sign

r the distance from the centre of the point charge

Conducting sphere

Conducting sphere the same equation applies with the charge treated as concentrated at the centre

Inside a charged sphere

Inside a charged sphere the potential is constant

Sign

V has the sign of Q, unlike gravitational potential which is always negative

Part 3 of 8Positive and negative charges

Positive charge

V is positive, rises as a test charge moves closer and falls with distance
Energy must be supplied to push a positive test charge closer

Negative charge

V is negative, falls as a test charge moves closer and rises towards zero with distance
Energy is released as a positive test charge moves closer

Field lines

Field lines potential always decreases in the direction of the field lines

Free charges

Free charges a free positive charge accelerates towards lower potential and a free negative charge towards higher potential

Part 4 of 8Combining potentials

Superposition

V = kQ₁/r₁ + kQ₂/r₂ + …, adding algebraically with signs

Equal charges

Equal charges a point equidistant from two equal positive charges has double the potential of one

Dipole

Dipole at the midpoint V = 0 although the field is not zero, because fields add as vectors

Part 5 of 8Potential difference and work

1Potential difference

ΔV = V_f − V_i

2Point charge

ΔV = kQ(1/r_f − 1/r_i)

3Work by an external agent

W = qΔV, where q is the charge being moved

4Field

Field it does the negative of this work

Part 6 of 8Potential and field compared

Quantity	Electric potential	Electric field strength
Type	Scalar	Vector
Unit	V or J C⁻¹	V m⁻¹ or N C⁻¹
Point charge	kQ/r, falls as 1/r	kQ/r², falls as 1/r²
Information	Energy per unit charge	Force per unit charge

Part 7 of 8More from the specification

1Zero reference ground on Earth

Zero reference ground on Earth physically connecting conducting shells to the planet Earth establishes a stable zero volt reference electric potential.

2Electric Potential and Equipotential Surfaces

An equipotential surface connects points of identical electric potential;
- between parallel plates, equipotentials form parallel planes perpendicular to electric field lines, requiring zero work to move a charge across the surface.

3Potential Gradient and Field Strength

The magnitude of the electric field strength E equals the negative gradient of the electric potential graph with respect to distance (E = -dV/dr), directed toward regions of lower potential.

4Electrostatic vs Magnetic Forces

While electric forces act on charges parallel to field lines between charged plates, moving charges in a magnetic field experience Lorentz forces perpendicular to both velocity and magnetic field lines.
- (electric current)

5Electric Potential and Equipotential Surfaces

An equipotential surface connects points of identical electric potential;
- between parallel plates, equipotentials form parallel planes perpendicular to electric field lines, requiring zero work to move a charge across the surface.

6Potential Gradient and Field Strength

The magnitude of the electric field strength E equals the negative gradient of the electric potential graph with respect to distance (E = -dV/dr), directed toward regions of lower potential.

7Electrostatic vs Magnetic Forces

While electric forces act on charges parallel to field lines between charged plates, moving charges in a magnetic field experience Lorentz forces perpendicular to both velocity and magnetic field lines.
- (electric current)

8Relative permittivity

Ratio of the permittivity of a material to that of free space.

9Equipotential

A surface of constant potential

10Parallel

Components on separate branches, each with the full e.m.f.

11Permittivity

A property of a medium affecting the electric force within it.

12Current

Flow of electrons in a circuit.

13Uniform

Same density throughout, so the weight acts at the centre.

14Density

Mass per unit volume.

15Magnetic field

Area around a magnet where its effects can be detected.

16Equipotential line

A line joining points of equal gravitational potential.

Part 8 of 8More from the specification

W = mg
What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg⁻¹ on Earth)

Key terms

Term	Meaning
Electric potential	The work done per unit charge in taking a small positive test charge from infinity to the point.
Volt	One joule per coulomb.
Potential difference	The work done per unit charge moving a charge between two points.
Scalar	A quantity with magnitude but no direction.

Avoid these mistakes

Treating potential as a vector and adding with directions.

Dropping the sign of Q in V = kQ/r.

Confusing potential (J C⁻¹) with potential energy (J).

Believing V = 0 means E = 0, or the reverse.

Using r from the surface rather than from the centre of a charged sphere.

Last updated: 18 October 2026