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Theme B — The Particulate Nature of Matter

B.5.4 Electrical Conductors and Insulators — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 45 of 53 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

B.5.4 Electrical Conductors and Insulators — IB PHYSICS SL notes: Explain, in terms of charge carriers, why materials behave as conductors, semiconductors or insulators.Conductor — a material that allows charge (usually electrons) to flow through it easilyExamples are silver, cop…

  • Why metals conduct well
  • Effect of temperature
  • Charge stays put
  • Contact with a conductor
  • Light-dependent resistor
  • Free-electron density

Part 1 of 7Conductors

Conductors
Conductor
  • Conductor a material that allows charge to flow through it easily
    • (usually electrons)
  • Examples are silver, copper, aluminium and steel.
  • Conductors tend to be metals and different materials have different conductivities.
Structure
  • Structure a metal is a lattice of fixed positive ions surrounded by a sea of delocalised electrons
  • Each atom gives up one or more outer electrons, which move randomly through the lattice.
  • A p.d. creates an electric field that gives these electrons a net drift.
Why metals conduct well
  • Why metals conduct well current is the rate of flow of charge and a metal contains a very large number of delocalised electrons that move easily
  • These carriers are always present, so metals conduct at all temperatures.
  • Only electrons move; the positive ions stay fixed in the lattice.
Effect of temperature
  • Effect of temperature heating a metal raises its resistance
  • The lattice ions vibrate with larger amplitude, so electrons collide with them more often.
  • The number of free electrons stays the same; only the collision rate rises.

Part 2 of 7Insulators

Insulator
  • Insulator a material with almost no free charge carriers, so charge cannot flow through it easily
  • Examples are rubber, plastic, glass and dry wood.
  • All electrons are tightly bound to their atoms or molecules, so none are free to drift.
Use
  • Use insulators cover wires and handles for safety, while copper cores give low resistance
Breakdown
  • Breakdown a very strong electric field can still tear electrons free from an insulator
  • A spark in air occurs at a field of about 3 × 106 V m−1.

Part 3 of 7Static charge on insulators

Charge stays put
  • Charge stays put an insulator cannot conduct charge away, so it can hold static charge on its surface
  • Rubbing a polythene rod with a cloth transfers electrons, leaving the rod negative and the cloth positive.
  • Charge is transferred, never created.
Contact with a conductor
  • Contact with a conductor when a charged insulator touches a conductor, charge can be transferred to the conductor
Earthing
  • Earthing a conductor held in the hand loses its charge to earth
  • A metal rod can be charged by rubbing only if it is insulated from earth.

Part 4 of 7Semiconductors

Semiconductor
  • Semiconductor a material with few free carriers at low temperature whose conductivity lies between a conductor and an insulator
Effect of temperature
  • Effect of temperature heating frees more electrons from their bonds, so the resistance falls
  • This is the opposite of a metal.
  • The effect is used in thermistors.
Doping
  • Doping adding small amounts of impurity atoms raises the number of carriers dramatically
  • Doped semiconductors are the basis of diodes and transistors.
Light-dependent resistor
  • Light-dependent resistor photons free extra carriers, so its resistance falls in bright light

Part 5 of 7Carrier numbers and resistivity

Free-electron density
  • I = nAvq, so a smaller n gives a smaller current for the same drift speed and field
  • A metal has n of about 1028 to 1029 m−3.
  • Pure silicon at room temperature has n of about 1016 m−3.
  • An insulator has effectively no free electrons.
Resistivity values
  • Resistivity values they span more than twenty orders of magnitude
  • Copper is about 10−8 Ω m.
  • Pure silicon is about 103 Ω m.
  • A good insulator such as polythene exceeds 1014 Ω m.
Reporting data
  • Reporting data conductivity is quoted at a stated temperature, usually 20 °C, because heating changes resistance

Part 6 of 7Explaining observations with carriers

Explaining observations with carriers
State what the charge carriers are.
Say whether their number changes or whether the collision rate changes.
Conclude about the current or the resistance.
1Metal at higher temperature
  • Metal at higher temperature the number of carriers is unchanged but collisions increase, so resistance rises
2Semiconductor at higher temperature
  • Semiconductor at higher temperature the number of carriers increases, so resistance falls

Part 7 of 7More from the specification

More from the specification
Conductors versus Insulators and Free Electrons
  • Electrical conductors possess a high concentration of free delocalised electrons that drift when an electric field is applied,
  • whereas electrical insulators have tightly bound electrons preventing conduction.
Definition of Electric Potential Difference
  • Potential difference (V) between two points is defined as the electrical work done per unit charge moving between those points: V = W/Q,
  • where 1 volt equals 1 joule per coulomb.
Series and Parallel Circuit Rules
  • In a series circuit, current is identical through each connected component and potential differences sum;
    • in a parallel circuit, each branch experiences the same potential difference while total current divides.
Electrical Power and Heating in Components
  • Electrical power delivered to a circuit component is calculated using P = VI = I2 R = V2 / R, representing the rate of electrical energy dissipation as thermal energy.
Conventional current
  • Current drawn from the positive to the negative terminal.
Source
  • Reservoir at TH providing heat QH
Potential difference
  • Work done by a unit charge passing through a component.
Internal resistance
  • Resistance inside a cell that causes lost p.d.
Electrical energy
  • Energy carried by electrons in a circuit.
Energy transferred
  • W = V Q, in joules.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
Combined resistance
  • Single resistance that could replace several resistors.
Circuit diagram
  • Drawing of a circuit using standard symbols.
Light intensity
  • How bright the light falling on a surface is.
Series
  • Components in one loop, sharing the e.m.f. between them.
Parallel
  • Components on separate branches, each with the full e.m.f.
Component
  • Part of a vector along one axis.
Battery
  • Two or more cells joined together.
Components
  • Perpendicular parts of a vector that add to give the vector.
Ammeter
  • Component that measures current.
Terminal
  • Connection point of a cell, labelled + or −.
Voltmeter
  • Instrument that measures p.d., connected in parallel.

Key terms

TermMeaning
ConductorMaterial with many free charge carriers and low resistivity.
InsulatorMaterial with almost no free charge carriers and very high resistivity.
SemiconductorMaterial whose carrier number rises sharply with temperature or doping.
DopingAdding impurity atoms to a semiconductor to change its conductivity.
Delocalised electronsOuter electrons of a metal that are free to move through the lattice.
Static chargeCharge held on an insulator that cannot flow away.

Avoid these mistakes

1
Saying insulators have no electrons — they have electrons, but all are bound to atoms.
2
Thinking positive charge moves in a solid metal — only electrons move, the positive ions are fixed in the lattice.
3
Stating that every material conducts worse when heated — semiconductors conduct better.
4
Saying a charged object has more protons — it has gained or lost electrons.
5
Assuming a conductor cannot be charged — it can, but only if it is isolated from earth.

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