Investigating Conductors & Insulators (Cambridge O Level Physics)

• The key difference between conductors and insulators is that:
  • Conductors allow charge carriers to freely move
  • Insulators do not allow charge carriers to move
• The reasons for this are to do with their internal structure

Conductors

• A conductor is a material that allows charge (usually electrons) to flow through it easily
• Examples of conductors are:
  • Silver
  • Copper
  • Aluminium
  • Steel
• Conductors tend to be metals

Relative Conductivity of Different Materials

Different materials have different properties of conductivity

• On the atomic scale, conductors are made up of positively charged metal ions with their outermost electrons delocalised
  • This means the electrons are free to move

• Metals conduct electricity very well because:
  • Current is the rate of flow of charged particles
  • So, the more easily electrons are able to flow, the better the conductor

Metallic Bonding and Delocalised Electrons

The lattice structure of a conductor with positive metal ions and delocalised electrons

Insulators

• An insulator is a material that has no free charges, hence does not allow the flow of charge through them very easily
• Examples of insulators are:
  • Rubber
  • Plastic
  • Glass
  • Wood

• Some non-metals, such as wood, allow some charge to pass through them
• Although they are not very good at conducting, they do conduct a little in the form of static electricity
  • For example, two insulators can build up charge on their surfaces. If those surfaces touch, this would allow that charge to be conducted away

The Gold-leaf Electroscope (GLE)

• To distinguish between conductors and insulators a Gold-leaf electroscope (GLE) can be used

Gold-Leaf Electroscope

The gold-leaf electroscope is a device used to demonstrate charge

• The GLE consists of
  • A metal plate attached to one end of a metal rod
  • At the other end of the rod a very thin leaf of gold foil is attached
  • The rod is held by an insulating collar inside a box with glass sides, allowing the gold leaf to both be seen and protected from draughts

• When the GLE is charged, the plate, rod and gold leaf have the same charge (either positive or negative)
  • Since the rod and leaf have the same charge, they repel, and the leaf sticks out to the side
  • When the rod and leaf are discharged (are neutral) the leaf hangs down

To Test Electrical Conductors and insulators

• Charge the plate of the GLE so that the gold leaf stands clear of the rod
• Carefully touch the plate of the GLE with the items being tested, for example:
  • Metals, such as: wire, paperclip, scissor blades
  • Non-metals, such as: paper, fingers, glass, graphite
  • Plastics, such as: plastic ruler, the handles of the scissors, finger in a plastic sandwich bag
  • Comparisons, such as: wet cloth, dry cloth; finger and finger in a plastic sandwich bag

• Record the observations each time
  • Leaf falls: material is a good conductor
  • Leaf remains in place: object is a poor conductor (good insulator)
  • Leaf falls slowly: material is a poor conductor

 

Alternative method

• An electronic charge detector can be used in place of the Gold-leaf Electroscope

Expected Results

• Overall, metals are very good conductors whilst non-metals tend to be good insulators

Conductors & Insulators Summary Table

Conductors	Insulators
Metals	Plastics
Graphite	Glass
	Most other non-metals