Conductors and Insulators

• Some substances readily allow the passage of electricity through them — these are called conductors.
• Others do not allow electricity to pass easily — these are called insulators.
• Most substances fall into one of these two classes.
• Metals, human and animal bodies, and earth are conductors.
• Glass, porcelain, plastic, nylon, and wood are insulators.
• Insulators offer high resistance to the passage of electricity.

Those substances in which electric charge cannot flow are called ‘insulators' (or dielectrics). Glass, hard-rubber, plastics and dry wood are insulators. Insulators have practically no free electrons.

OR

The material in which electrons are tightly bound to the nucleus and thus not available for conductance is called an insulator.

OR

Substances which offer high resistance to the passage of electricity and do not allow electricity to pass through them easily, are called insulators.

Conductors are those through which electric charge can easily flow. Metals, human body, earth, mercury and electrolytes are conductors of electricity.

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The material through which electric charge can flow easily is called a conductor.

Substances whose resistance to the movement of charges is intermediate between conductors and insulators, are called semiconductors.

• Conductors have electric charges (electrons) that are comparatively free to move inside the material.
• In metals, only the negative charge (electrons) is free to move — the positive charge is immobile.
• The actual charge carriers in metals are called free electrons.
• The outer (valence) electrons of metal atoms leave their atoms and become free to move throughout the volume of the metal.
• These free electrons constantly collide with each other and with the residual positive ions, moving randomly in different directions.
• They have practically no affinity with their parent atoms.
• In an external electric field, free electrons drift against the direction of the field.
• As the intensity of the field increases, more and more free electrons cross through a section of the conductor.
• The free electrons, though not bound to any particular atom, are bound to the conductor as a whole and cannot escape from its surface unless external energy is supplied.

• In electrolytes, both positive and negative charges are free to move.
• The charge-carriers in an electrolyte are ions — it is the ions that move.
• Mercury and electrolytes are also classified as conductors of electricity.

• In insulators, electrons are tightly bound to the nuclei of their atoms.
• There are no free electrons to carry current.
• Insulators have a negligibly small number of free electrons.
• Glass, hard rubber, plastics, ebonite, mica, wax, paper, and dry wood are insulators.
• Insulators are also called dielectrics.

• Dielectrics fail to conduct electricity.
• However, when an electric field is applied, induced charges appear on the surface of the dielectric.
• Thus, dielectrics transmit electric effects but do not conduct electricity.

• When a charge is placed on a conductor, it readily spreads over its entire surface.
• When a charge is placed on an insulator, it stays in the same place.
• A nylon or plastic comb gets electrified when combing dry hair because the charge stays on it (it's an insulator).
• A metal spoon does not get charged on rubbing — the charge leaks through the body to the ground, as both are conductors.
• If a metal rod with a wooden or plastic handle is rubbed without touching its metal part, it shows signs of charging.

• Some substances lie between conductors and insulators — these are called semiconductors.
• Examples: Germanium (Ge) and Silicon (Si).
• Semiconductors are discussed in detail in later chapters.

• After studying the effects of static electric charge, it becomes important to utilize them for useful purposes.
• Capacitors are devices that store electric charge and can be used in various ways.
• The principles of capacitors and their uses are built upon the understanding of conductors and insulators.