Theme D — Fields
D.2.1 Electric Charge — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 16 of 39 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
D.2.1 Electric Charge — IB PHYSICS SL notes: Describe the properties of electric charge, including quantisation and conservation, and calculate charge from current and from numbers of electrons.Charge — the property of matter responsible for the electric force, and a scalar quant…
- Neutral atoms
- Charged objects
- Sign of charge
- Quantitative form
- Coulomb (C)
- Elementary charge (e)
Part 1 of 7Two kinds of charge
Two kinds of charge
Charge
- Charge the property of matter responsible for the electric force, and a scalar quantity
Particles
- Particles protons carry positive charge, electrons carry negative charge and neutrons carry none
Neutral atoms
- Neutral atoms an atom has equal numbers of protons and electrons, so most everyday objects are neutral
Charged objects
- Charged objects gaining electrons makes an object negative and losing them makes it positive
- Protons are bound inside nuclei, so it is electrons that move between objects.
Sign of charge
- Sign of charge a surplus of electrons is negative, and charge can be written as a count of electrons times −e
Part 2 of 7Forces between charges
Rule
- Rule like charges repel and unlike charges attract
Strength
- Strength the force between charged particles is much stronger than gravity between them
Quantitative form
- Coulomb's law (D.2.4) gives its size
Part 3 of 7Unit and elementary charge
Coulomb (C)
- Coulomb (C) the charge carried by a current of one ampere in one second
- For a steady current Δq = IΔt.
- Δq is in C, I in A and Δt in s.
Elementary charge (e)
- Elementary charge (e) the magnitude of the charge of a proton or electron, e = 1.60 × 10−19 C
Quantisation
- Quantisation any observed charge is a whole-number multiple of e
- A body with charge q has N = q/e surplus electrons (or a deficit).
Size of a coulomb
- Size of a coulomb about 6.25 × 1018 electrons, so ordinary electrostatic charges are in µC or nC
Quarks
- Quarks they carry ±1/3 e and ±2/3 e but are never found alone, so free charges stay multiples of e
Current
- Current a current of one ampere is roughly six billion billion electrons passing each second
Part 4 of 7Conservation of charge
1Conservation of charge
- Conservation of charge the total charge of an isolated system stays constant
- Charge can be transferred but cannot be created or destroyed.
2Rubbing
- Rubbing when a glass rod is rubbed with silk, electrons move from the rod to the silk
- The rod becomes positive and the silk negative by exactly the same amount.
3Identical conducting spheres
- Identical conducting spheres when they touch, the total charge is shared equally between them
- Each ends with the average of the original charges.
4Nuclear reactions
- Nuclear reactions total charge before equals total charge after
- In beta decay a neutron (charge 0) becomes a proton (+e) and an electron (−e) plus an antineutrino, so the sum stays zero.
Part 5 of 7Conductors and insulators
Conductor
- Conductor some electrons are free to move through a metal, so charge spreads over it and can flow as current
Insulator
- Insulator charges are held in place, so charge placed on one spot stays there
- Plastic, glass and dry air are insulators.
Charging by friction
- Charging by friction insulators charge easily because the transferred electrons cannot flow away
Isolated metal sphere
- Isolated metal sphere the charge distributes uniformly over its surface because like charges repel and move as far apart as possible
Semiconductors
- Semiconductors they lie between conductors and insulators
Part 6 of 7Charge carriers
Metals
- Metals the moving carriers are electrons
Electrolytes and gases
- Electrolytes and gases both positive and negative ions move
Conventional current
- Conventional current it is defined as the flow of positive charge, whichever carriers are responsible
Part 7 of 7More from the specification
1Coulombic force quantification
- Coulombic force quantification applying Coulomb's inverse-square electrostatic law provides a fundamental theoretical means to calculate inter-charge repulsion or attraction.
2Electrostatic and magnetic fields interactions
- Electrostatic and magnetic fields interactions stationary charges establish radial electrostatic fields,
- whereas moving charged particles generate circulating magnetic fields and forces.
3Coulomb electrostatic forces and field lines
- Coulomb electrostatic forces and field lines vector field lines indicate the directional forces exerted on test charges positioned within ambient electric fields.
4Electric Field Lines Between Parallel Plates
- A uniform electric field is established between two oppositely charged parallel metal plates;
- electric field lines are parallel, equally spaced,
- and directed perpendicular to the plates from positive to negative.
5Electric Field Strength and Permittivity
- The electric field strength E between parallel conducting plates separated by distance d with potential difference V is E = V / d = σ / ε,
- where σ is surface charge density and ε is the permittivity of the intervening material.
6Comparison with Magnetic Fields
- Unlike electric field lines which begin on positive charges and terminate on negative charges, magnetic field lines form continuous closed loops around current paths, with flux density B dictating magnetic force magnitude.
7Uniform electric field
- Uniform electric field electric field lines between oppositely charged parallel plates are evenly spaced and directed positive to negative.
8Electric permittivity
- Electric permittivity permittivity describes how an insulating dielectric material between plates affects the resulting electric field strength.
9Electric field direction
- Electric field direction defined as the direction of the electrostatic force exerted on a positive test charge.
10Potential difference
- Work done by a unit charge passing through a component.
11Relative permittivity
- Ratio of the permittivity of a material to that of free space.
12Permittivity
- A property of a medium affecting the electric force within it.
13Parallel
- Components on separate branches, each with the full e.m.f.
14Field
- Region in which an object experiences a force at a distance
15Density
- Mass per unit volume.
16Distance
- Total length of path travelled, a scalar.
17Electric field
- Region of space in which a charge experiences a force.
18Field lines
- Lines showing the direction and strength of a magnetic field.
19Field strength
- G = Fg/m.
20W = 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 |
|---|---|
| Elementary charge e | The smallest free charge, 1.60 × 10⁻¹⁹ C. |
| Coulomb | Charge delivered by a current of 1 A in 1 s. |
| Quantisation | Charge exists only in whole multiples of e. |
| Conservation of charge | The total charge of an isolated system is constant. |
| Conductor | A material with charges free to move. |
| Insulator | A material in which charges stay in place. |
Avoid these mistakes
1
Believing that a charged object has gained protons — it is electrons that move.
2
Thinking charge is destroyed when opposite charges meet — the total is conserved.
3
Forgetting that charge is quantised and giving fractional electrons.
4
Confusing the coulomb (charge) with the ampere (current) — 1 A = 1 C s^{−1}.
5
Saying an insulator cannot hold charge — it holds it very well, in place.
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