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Theme D — Fields

D.2.3 Static Electricity — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 18 of 39 1 quiz

📚 IB Physics HL Hub / Topical Revision
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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.2.3 Static Electricity — IB PHYSICS SL notes: Explain charging by friction, contact and induction, and describe the effects, dangers and applications of static charge.Static electricity — electric charge that stays at rest on an object instead of flowing as a currentMechanism —…

  • Only electrons move
  • Any substances
  • Conditions
  • Circuit symbol
  • Role in charging
  • Charging by contact

Part 1 of 9Charging by friction

Static electricity
  • Static electricity electric charge that stays at rest on an object instead of flowing as a current
Mechanism
  • Mechanism when two different insulators are rubbed, electrons transfer from the material with the weaker hold on its outer electrons to the other
  • The loser becomes positive and the gainer negative, with equal and opposite charges.
  • This agrees with conservation of charge.
Examples
  • Examples a polythene rod rubbed with a cloth gains electrons and becomes negative, and glass rubbed with silk loses electrons and becomes positive
Only electrons move
  • Only electrons move an object becomes positive by losing electrons, never by gaining positive charge
Any substances
  • Any substances friction charging also occurs with liquids, for example liquid flowing in a pipe
Conditions
  • Conditions dry weather and good insulators help the charge remain, while moisture in humid air provides a conducting film that lets it leak away

Part 2 of 9Earthing

Earthing
  • Earthing connecting a conductor to the ground so that charge can flow to or from it
  • An earthed charged body discharges until its potential is 0 V.
Circuit symbol
  • Circuit symbol the earth symbol marks a point kept at 0 V
Safety
  • Safety appliances are kept at 0 V by an earthed copper conductor, because current takes the path of lower resistance
  • Built-up charge flows through the copper rather than through a person.
Role in charging
  • Role in charging earthing lets electrons leave or arrive, and it does not add charge itself

Part 3 of 9Charging by contact

1Charging by contact
  • Charging by contact charge is transferred when two objects touch
  • A charged object touching an uncharged conductor passes charge until they reach the same potential.
2Result
  • Result the conductor ends with the same sign of charge as the charged object
3Identical spheres
  • Identical spheres they share the charge equally
4Transfer from a charged insulator to an earthed conductor
  • Transfer from a charged insulator to an earthed conductor a large potential difference drives electrons across when they touch
  • A shock from a doorknob is an example.

Part 4 of 9Charging by induction

1Electrostatic induction
  • Electrostatic induction separation of charge in a conductor caused by a nearby charged object without contact
  • It is not the same as electromagnetic induction.
2Redistribution
  • Redistribution a negative rod repels electrons in a neutral conductor to the far side, leaving the near side positive
3Steps for a conducting sphere
  • Steps for a conducting sphere follow this order
  • Bring a charged rod near the sphere without touching it.
  • Earth the far side so electrons can leave or arrive.
  • Remove the earth connection first.
  • Remove the charged rod.
4Result
  • Result the sphere keeps a net charge of the opposite sign to the rod
5Contact versus induction
  • Contact versus induction induction gives the opposite sign and contact gives the same sign

Part 5 of 9Attraction of neutral objects

Conductor
  • Conductor induced separation of charge explains why a charged comb picks up scraps of paper
Insulator
  • Insulator the molecules are polarised, with their electrons shifted slightly, so the side nearest the charged object carries the opposite charge
Net force
  • Net force the nearer opposite charge is attracted more strongly than the farther like charge is repelled, because force depends on distance
Test for charge
  • Test for charge a charged body always attracts a neutral one, but repulsion is a sure test for charge because only like charges repel

Part 6 of 9Detecting charge

  • Gold-leaf electroscope
    • a metal stem with a thin leaf
    • A charged object near or touching the cap gives the stem and leaf the same charge, so the leaf rises.
    • More charge gives a greater divergence.
    • A neutral object does not cause divergence.
    • A rod of opposite sign brought close reduces the divergence.

Part 7 of 9Discharge

Leakage
  • Leakage charge leaks away through humid air
Breakdown
  • Breakdown a spark occurs when the field is strong enough to ionise the air, about 3 × 106 V m−1 in dry air
  • A large potential difference between objects can cause a spark or lightning.
Current in a discharge
  • I = Δq/Δt over a brief time

Part 8 of 9Dangers and applications

1Fire risk
  • Fire risk a spark can ignite fuel vapours, flour dust or flammable liquids
  • Tankers and aircraft are earthed before refuelling, using a bonding line.
  • Lightning can also cause electrocution.
2Electronics
  • Electronics workers wear wrist straps to avoid damaging components
3Lightning conductors
  • Lightning conductors sharp points discharge buildings steadily because the field is strongest at points
4Photocopiers and laser printers
  • Photocopiers and laser printers charged drums attract toner powder
5Spray painting
  • Spray painting charged droplets are attracted to an earthed object for an even coat
6Dust precipitators
  • Dust precipitators charged dust is removed from chimney gases

Part 9 of 9More from the specification

1Triboelectric charging and electrostatic fields
  • Triboelectric charging and electrostatic fields electron transfer between rubbed insulating surfaces produces static charge imbalances, generating strong localized electrostatic fields and forces.
2Deflection by magnetic fields and charges
  • Deflection by magnetic fields and charges static electric charges experience Lorentz forces when propelled through magnetic fields, providing a means of charge identification.
3Coulomb's Law and Permittivity
  • The electrostatic force between two stationary point charges is proportional to their magnitudes and inversely proportional to the square of their separation, modulated by the electric permittivity of the medium: F = (1 / (4*pi*epsilon)) * (|q1*q2| / r2.
4Electric Field Strength
  • Defined as the electrostatic force experienced per unit positive charge (E = F / q), possessing both magnitude and direction pointing along the line of action for a positive test charge.
    What the symbols mean: E = electric field strength (N C−1) · F = force on the charge (N) · Q or q = charge (C)
5Uniform Field Between Parallel Plates
  • Oppositely charged parallel conducting plates establish a uniform electric field characterized by parallel, evenly spaced field lines of magnitude E = V / d.
6Electric Field Line Density
  • The relative density of electric field lines in a diagram visually communicates local field strength, originating perpendicularly on positive charges and terminating on negative charges.
7Relative permittivity
  • Ratio of the permittivity of a material to that of free space.
8Permittivity
  • A property of a medium affecting the electric force within it.
9Coulomb
  • Charge passing a point when a current of 1 A flows for 1 s.
10Parallel
  • Components on separate branches, each with the full e.m.f.
11Density
  • Mass per unit volume.
12Constant
  • Not changing.
13Uniform
  • Same density throughout, so the weight acts at the centre.
14Field lines
  • Lines showing the direction and strength of a magnetic field.
15Field strength
  • G = Fg/m.
16W = mg
What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)

Key terms

TermMeaning
Static electricityElectric charge at rest on an object.
InductionRedistribution of charge in a conductor by a nearby charged object, without contact.
EarthingConnecting a conductor to the ground so that charge can flow to or from it.
PolarisationSlight displacement of charge in the atoms of an insulator in an electric field.
ElectroscopeA device that detects charge by the repulsion of a leaf from a stem.

Avoid these mistakes

1
Saying that protons move when an object is charged by friction.
2
Thinking that a neutral object cannot be attracted by a charged one.
3
Confusing charging by induction (opposite sign) with charging by contact (same sign).
4
Believing that earthing adds charge — it lets electrons leave or arrive.
5
Assuming that attraction proves an object is charged — only repulsion indicates like charges.

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