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.3.5 Charged Particles in Electric Fields — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 32 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.3.5 Charged Particles in Electric Fields — IB PHYSICS SL notes: Describe and calculate the motion of charges accelerated and deflected in uniform electric fields.Force — an electric field exerts F = qE on a charge whether it moves or notE is in N C−1, which equals V m−1.Directi…

Acceleration
Speed change
Between parallel plates
Uncharged particles
Energy gain
Only V counts

Part 1 of 8Force and acceleration

Force and acceleration

Force

Force an electric field exerts F = qE on a charge whether it moves or not
E is in N C⁻¹, which equals V m⁻¹.

Direction

Direction the force is along the field for a positive charge and opposite to it for a negative charge

Acceleration

Acceleration a = qE/m from Newton's second law

Speed change

Speed change unlike the magnetic force, the electric force can change speed because it can have a component along the motion and so does work

Between parallel plates

Between parallel plates the field is uniform with E = V/d, directed from the positive to the negative plate

Uncharged particles

Uncharged particles a neutron feels no force and passes straight through undeflected

Part 2 of 8Acceleration along the field

Energy gain

Energy gain a charge released from rest through a p.d.
V gains kinetic energy qV

Speed

Speed qV = ½mv², so v = √(2qV/m)

Only V counts

Only V counts the result is independent of how the field is arranged

Electronvolt

1 eV = 1.6 × 10⁻¹⁹ J, the energy gained by an electron through 1 V
An electron accelerated through 2000 V has kinetic energy 2000 eV.

Applications

Applications electron guns in old televisions and X-ray tubes

Part 3 of 8Deflection at right angles to the field

Model

Model the motion is like a projectile under gravity, with qE replacing mg
Along the field the particle accelerates at a = qE/m.
Perpendicular to the field it moves at constant speed v_x.

Time in the field

Time in the field for plates of length ℓ, t = ℓ/v_x

Sideways velocity

Sideways velocity v_y = at = qEℓ/(mv_x)

Sideways displacement

Sideways displacement y = ½at² = qEℓ²/(2mv_x²)

Path

Path a parabola inside the field and a straight tangent line after it leaves
The straight line appears to come from the midpoint of the plates.

Deflection angle

Deflection angle tan θ = v_y/v_x

Part 4 of 8Direction and amount of deflection

Direction and amount of deflection

Positive charge

Positive charge it is deflected towards the negative plate

Negative charge

Negative charge it is deflected towards the positive plate

Force

Force it is the same at all points and always in the same direction

Greater mass

Greater mass the deflection is smaller

Greater charge

Greater charge the deflection is greater

Greater speed

Greater speed the deflection is smaller, since the particle spends less time in the field

Comparing particles

Comparing particles a particle with a much larger mass than charge ratio is much less deflected and hits the plate later, with a smaller curve
A boron nucleus has 5 times the charge but 11 times the mass of a proton, so it deflects less.

Part 5 of 8Comparing electric and magnetic deflection

Feature	Electric field	Magnetic field
Force	qE, along or against E	qvB, perpendicular to v and B
Acts on stationary charge?	Yes	No
Work done	Yes, speed changes	None, speed constant
Path (entry perpendicular to field)	Parabola	Circle

Gravity
- it is normally negligible for charged particles, because even in a weak field the electric force on an electron is enormously larger than its weight

Part 6 of 8Applications

1Accelerators

Accelerators electric fields give energy to charges

2Oscilloscopes

Oscilloscopes deflection plates steer an electron beam to draw a trace

3Inkjet printers

Inkjet printers charged droplets are deflected onto paper

4Electrostatic precipitators

Electrostatic precipitators charged dust is pulled onto collecting plates

Part 7 of 8Energy method

Work by the field

Work by the field qEd = qΔV, equal to the change in kinetic energy

Plates

Plates when a charge is released between plates, find the final speed from energy
Use E = V/d only if the force or time is also needed.

Massive droplets

Massive droplets gravity must be included when the particle is a massive charged droplet

Part 8 of 8More from the specification

Key terms

Term	Meaning
Electric field strength E	Force per unit positive charge, in N C⁻¹ or V m⁻¹.
Electronvolt	1.6 × 10⁻¹⁹ J, the energy gained by an electron through 1 V.
Uniform field	Field of constant size and direction, as between parallel plates.
Parabolic trajectory	Path of a charge entering perpendicular to a uniform field.
Electron gun	Device that accelerates electrons through a p.d. into a beam.

Avoid these mistakes

Giving the force on a negative charge in the direction of E — the force is opposite to E.

Using E = V/d for a non-uniform field — it applies only between parallel plates.

Treating electric deflection as circular motion — it is parabolic.

Forgetting to convert cm to m or eV to joules.

Including gravity when it is negligible, or ignoring it for a massive charged droplet.

Last updated: 15 October 2026