ELECTRICAL PROPERTIES OF SOLIDS

On the basis of electrical properties, solids are divided into three parts:

(1)CONDUCTORS: - The substances which allow the electric current to pass through them are called conductors.

They are of two types.

(i)Metallic Conductors: - The conductors which conduct electric current without undergoing any chemical change are known as metallic conductors. In metallic conductors current is carried by free mobile electrons.E.g. Cu, Fe, Ag etc.

(ii)Electrolytic Conductors or Electrolytes: - The conductors which allow electric current to pass through them in their molten state or in their aqueous solution are known as electrolytic conductors or electrolytes. In electrolytes current is carried by free ions.

Electrolytes are of two types.

(a)Strong electrolytes:-The electrolytes which completely dissociate into ions are called strong electrolytes.E.g. H₂SO₄ ,NaCl, HCl etc

(b)Weak electrolytes: - The electrolytes which do not completely   dissociate into ions are called weak electrolytes.E.g. CH₃COOH ,NH₄OH ,ZnCl etc.

(2)INSULATORS:-The substances which do not allow the electric current to pass through them are called insulators.E.g. Rubber, plastic, wood etc

(3)SEMICONDUCTORS:-The substances whose conductivity lies in-between conductors and insulators are known as semiconductors.E.g. Si, Ge etc.Types of semiconductors.

Two types.

(1)Intrinsic   semi conductors:-The semi-conductors which behave as insulators at low temperature but conduct electric current at higher temperature due to electronic defects are called intrinsic semi-conductors.E.g. Si, Ge etc

(2)Extrinsic semi conductors:-The semi-conductors which are obtained by doping are known as extrinsic semi-conductors. They are of two types:

(i)p-type SEMI-CONDUCTORS :- The semi-conductors whose conductivity is enhanced by doping with electron deficient impurities are called  p-type semi-conductors.E.g. When Silicon crystals are doped with electron deficient Aluminium impurity we get p-type semiconductors.   

(ii)n-type SEMI-CONDUCTORS :-The semi-conductors whose conductivity is enhanced by doping with electron rich impurities are called n-type semi-conductors. E.g. when silicon crystals are doped with electron rich phosphorous impurity we get   n-type semiconductors.   

 Q: - How does electrical conductivity of semi-conductors vary with temperature?

Ans: - With the increase in temperature the electrical conductivity increases .

Q: - How does electrical conductivity of metallic conductors vary with temperature?

Ans:-With the increase in temperature electrical conductivity decreases  .      

Q: - How does current pass through the metallic conductors and electrolytic conductors?

Ans: -Metallic solids: due to the flow of electrons .

Electrolytes: due to the flow of ions.

Q: - How does electrical conductivity of  electrolytes  vary with temperature?

Ans :- With the increase in temperature the electrical conductivity increases .

Q; - Which one has high electrical conductivity? Metallic or electrolytic conductors?

Ans :-Metallic conductors.

Q: - What is the effect of temperature on the conductivity of metallic conductors and electrolytic conductors?

Ans :-conductivity of metallic conductors decreases whereas that of electrolytes increases with the increase in temperature.

Q:- Why does the conductivity of  metallic conductors decrease and electrolytes increase with  rise in temperature?

Ans :-  In case of conductors with the increase in temperature the kernels start vibrating and offer resistance to the flow of electrons .Hence, their conductivity decreases. On the other hand in case of electrolytes  with the increase in temperature the kinetic energy of  ions increases as a result of which   conductivity enhances.

Q: - What is the effect of temperature on the conductivity of conductors and semi-conductors?

Ans: - In case of conductors with the increase in temperature the kernels start vibrating and offer resistance to the flow of electrons .Hence, conductivity decreases. In semi-conductors with the increase in temperature more electrons can jump from valence band  to the conduction band. Hence, conductivity increases.

Q :- What is superconductivity ?

Ans :- At very low temperatures  some solids do not offer any resistance  to the flow of electricity. This is known as superconductivity.E.g Mercury becomes superconducting at 4 Kelvin.

Q:- What is dielectric constant?

Ans:- It is  a physical property of the solvent which measures the polar nature of the solvent. The dielectric constant helps to estimate the solubility of the polar and non-polar and ionic substances in the solvent. It is denoted by (epsilon) and it is given by:

If  epsilon is large F will be small that is force of attraction will be small and consequently small energy is sufficient to break the crystal lattice into ions and ultimately to dissolve .Ionic and polar compounds are readily soluble in the solvents of high dielectric constant.

     Q: - Explain the electrical properties of solids on the basis of band model.

BAND MODEL

According to this model the substances consist of two types of bands with smaller or larger energy gaps between them.

(1)Valence band: It is the outermost filled energy band.

(2)Conduction band: It is the empty band in which electrons can jump from filled valence band.

CONDUCTORS: - In this case both the bands overlap or there is a very small energy gap between these two bands and hence electrons can easily jump into conduction band from the filled valence band. Hence conductors have high electrical conductivity.

SEMI-CONDUCTORS:- In this case energy gap is small and the electrons can be excited from valence band to conduction band by increasing temperature or by doping.

INSULATORS: - In this case the energy gap between the valence band and the conduction band is very large. Hence electrons from valence band cannot jump into the conduction band. That is why insulators do not conduct electricity.

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