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4.2.4Electricity and Magnetism

Investigating Conductors and Insulators — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.2.4 Investigating Conductors and Insulators — IGCSE PHYSICS notes: Describe tests that sort materials into conductors and insulators and explain the difference using free electrons.Conductor = a material that allows charge (usually electrons) to flow through it easily.Examples …

  • No sharp line
  • Good conductor
  • Poor conductor
  • Good insulator
  • Improvement
  • Semiconductors

Part 1 of 7Conductors and insulators

Conductor
Conductor a material that allows charge (usually electrons) to flow through it easily.
  • Examples are silver, copper, aluminium, steel and graphite.
  • The best conductors tend to be metals.
Insulator
Insulator a material with no free charges that does not allow charge to flow easily.
  • Examples are rubber, plastic, glass and dry wood.
No sharp line
  • No sharp line silicon conducts a little, and salt solutions and damp skin conduct much better than dry skin.

Part 2 of 7Why metals conduct

Why metals conduct
Structure
  • Structure a metal is a lattice of positive ions with outer electrons that are delocalised.
Free electrons
  • Free electrons when a battery is connected, they drift in one direction and form an electric current.
Insulators
  • Insulators the electrons are held tightly by their atoms, so almost none are free to move.
Uses
  • Uses good conductors make wires and plug pins, and insulators coat cables and form plug cases to prevent shocks and short circuits.

Part 3 of 7Test with a gold-leaf electroscope

1
Touch the plate with a finger to make sure the electroscope is uncharged.
2
Charge a polythene rod by rubbing it with a cloth.
3
Touch the rod to the plate so the leaf stays raised.
4
Touch the plate with the material to be tested and record how fast the leaf falls.
5
Repeat for different materials such as metal, plastic, glass, graphite, wood, fabric and rubber.
Good conductor
  • Good conductor the leaf falls quickly because charge flows away.
Poor conductor
  • Poor conductor the leaf falls slowly.
Good insulator
  • Good insulator the leaf does not move.
Improvement
  • Improvement an electrometer gives numerical comparisons.

Part 4 of 7Test with a simple circuit

Test with a simple circuit
Connect a cell, a lamp and two leads in series, leaving two clip ends free.
Touch the clips to the two ends of the material, such as a coin, a pencil lead, a plastic ruler or a wooden spoon.
Observe the lamp: if it lights, the material conducts; if it stays off, it is an insulator.
Ammeter
  • Ammeter using an ammeter instead of a lamp is more sensitive, because a poor conductor may pass a current too small to light a lamp.
Fair test
  • Fair test keep the same cell, clip separation and sample size and clip onto bare surfaces.

Part 5 of 7Choosing materials

Copper
  • Copper the standard wire material because it conducts well, is ductile and is cheaper than silver.
Aluminium
  • Aluminium lighter, so used for overhead power lines, often around a steel core for strength.
Graphite
  • Graphite conducts but is brittle, so it is used in pencil leads and brushes rather than wires.
Water
  • Water pure water is a poor conductor but tap and sea water conduct because of dissolved ions, so electrical equipment must be kept dry.

Part 6 of 7Static and current electricity

Insulators
  • Insulators hold static charge on their surfaces.
Conductors
  • Conductors carry current.
Semiconductors
  • Semiconductors conduct in between, with conduction that changes with temperature or light, as in thermistors and LDRs.

Part 7 of 7More from the specification

More from the specification
Electrostatic deflection of like charges
  • Electrostatic deflection of like charges suspended charged rods carrying identical static polarities mutually repel, demonstrating repulsive electric force between identical charges.
Electrostatic Charging of Insulating Materials
  • Rubbing an insulating acetate strip with a cloth transfers negative electrons by friction,
  • giving the rod an electrostatic charge and an electric field.
Electrical Conductors vs Insulators
  • Conductors contain delocalised electrons that permit electric charge to flow freely,
  • whereas insulators have tightly bound electrons and high electrical resistance.
Potential Difference and Energy Transfer
  • Potential difference (V = W/Q) represents the electrical energy transferred per unit charge moved between two points in a circuit.
Direct vs Alternating Electrical Supplies
  • Direct current (DC) flows continuously in one direction from a battery,
  • whereas alternating current (AC) repeatedly reverses direction with a fixed frequency.
Electromotive force
  • Energy transferred from other forms to electrical energy per unit charge.
Gold-leaf electroscope
  • A device in which a charged gold leaf falls when photoelectrons discharge the plate.
Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Resistance
  • Opposition to current, R = V / I, in ohms (Ω).
Terminal
  • Connection point of a cell, labelled + or −.
Component
  • Part of a vector along one axis.
Potential difference
  • Work done by a unit charge passing through a component.
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)
Electric field
  • Region of space in which a charge experiences a force.
Alternating current
  • Current that continually reverses direction.
Field lines
  • Lines showing the direction and strength of a magnetic field.

Key terms

TermMeaning
ConductorMaterial that allows charge to flow through it easily.
InsulatorMaterial that does not allow charge to flow easily.
Free electronsDelocalised electrons in a metal that are free to move.
SemiconductorMaterial that conducts better than an insulator but worse than a metal.
ElectrometerElectronic instrument that measures electric charge.

Avoid these mistakes

1
Saying insulators have no electrons — they have electrons, but these are not free to move.
2
Writing that the lamp lighting shows the material is an insulator — lamp on means conductor.
3
Forgetting to keep the test fair — use the same cell and clip gap.
4
Assuming water never or always conducts — pure water is poor but impure water conducts well.

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