January 29, 2019

On the basis of electrical conductivity, materials are divided into three parts which are following:

  1. Insulators: In this type of material, the valence band electrons are tightly attached to their nucleus, because of this there is no free electron in conduction band. The energy gap between conduction and valence band of the material is large. Insulators has negative resistance-temperature coefficient because with increase in temperature some electron from valence band go to the conduction band. Glass, rubber, oil, asphalt, fiberglass, porcelain, ceramic, quartz are some example of insulators.
  2. Conductors: In this type of material, the valence band electrons are loosely attached to their nucleus due to this there are plenty of free electrons are present in the conduction band. Also, the valence and conductance band are overlap. Conductors has positive resistance-temperature coefficient. All metals are good conductor of electricity. Silver, Copper, Annealed copper, Gold and Aluminium are top best conductor of electricity.
  3. Semi-conductors: A type of material which has properties of both conductor and insulator. The energy band gap is lower than insulators but higher than conductors. At lower temperature valence band of semi-conductors are fully filled but as there is increase in temperature electrons start jump into the conduction band. Ge, Si, Se are some semi-conductor materials.

Superconductors are those conductors whose resistance can be absolute zero at low temperature. Conductors are cooled using liquid nitrogen until the value of resistance becomes zero. Examples of superconductors are Al,Hg, Ga and many more.

Questions asked in competitive examinations:

  1. The resistance of insulations, in general,———- with temperature rise.
    1. Decreases
    2. Increases rapidly
    3. Increases slowly
    4. Does not change

  2. Which of the following materials is semiconductor
    1. Chromium
    2. Selenium
    3. Bismuth
    4. Silica

  3. Which of the following is the best conductor of electricity?
    1. Warm water
    2. Salt water
    3. Cold water
    4. Distilled water

  4. Which of the following insulating materials has the least affinity for moisture?
    1. Cotton
    2. Paper
    3. Asbestos
    4. Mica

  5. Which of the following has the greatest mobility?
    1. Positive ion
    2. Negative ion
    3. Electron
    4. Hole

  1. A medium behaves like dielectric when the
    1. Displacement current is just equal to the conduction current
    2. Displacement current is less than the conduction current
    3. Displacement current is much greater than the conduction current
    4. Displacement current is almost negligible

  2. The forbidden energy gap in silicon at 300K is
    1. 1.41eV
    2. 1.1eV
    3. 0.785eV
    4. 0.72ev

  3. Which one of the following materials has the highest dielectric strength?
    1. Polystyrene
    2. Marble
    3. Cotton
    4. Transformer oil

  4. Material used for insulators are
    1. Porcelain or toughened glass
    2. Silica
    3. Cement
    4. Clay

  5. Metals approach super conductivity conditions
    1. Near absolute zero temperature
    2. Near room temperature
    3. Under the conditions of higher pressure and temperature
    4. Near boiling temperature

  6. Mobility of electrons is highest in
    1. Silicon
    2. Germanium
    3. Galium Arsanic
    4. Carbon

  7. Superconductivity is destroyed
    1. At high temperature
    2. At high magnetic field
    3. In presence of magnetic impurities
    4. All of these

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