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Class 10 Class 12

In terms of band theory, what is the difference between a conductor and a semi-conductor?

Semiconductor: In instrinsic semiconductors, the energy gap, Eg, is relatively small (≈ = 1 eV). At absolute zero, conduction band is empty and valence band is full. Therefore, the solid behaves as an insulator at very low temperatures. However, at room temperature, some electrons from the top of valence band acquires enough thermal energy to jump into the conduction band. They leave behind a number of holes. Both electrons and holes movement contribute to conduction.
In extrinsic semi-conductor of n-type, the donor energy level is close to the bottom of conduction band.
In p-type the acceptor energy level is close to the top of valence band.

Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Amorphous solids: Polyurethane, teflon, cellophane, polyvinyal chloride, fibre glass.
Crystalline solids: naphthlene, benzoic acid, potassium nitrate, copper.

Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show clevage property?

Refractive index of a solid which have the same value along all directions are isotropic in nature. It would not show cleavage property.

Why are solids rigid?

Solids are rigid because the internuclear distance between the  molecules of a solid are very less and closely packed and their positions are fixed due to the strong forces of attraction between them.

Why is glass considred a super-cooled liquid?

Glass is amorphous solids and  have a tendency to flow, though very slowly. Therefore, these are called pseudo solids or super cooled liquids .

Why do solids have a definite volume?

A solid have a definite volume because the inter molecular distance  between its molecules are fixed.

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