If only a small potential difference applied to a material is needed to move charges, the material in which they are moving is said to be a good conductor. If on the other hand, no (or very little) charge flows, even with a high potential difference, the material is called an insulator.
These terms, conductor and insulator are relative because even an ‘insulator’ will conduct if the potential difference is large enough. For example, the air is generally regarded as an insulator, but it will conduct electricity in the form of lightning during thunderstorms.
Conductors and insulators differ in their resistance to the current.
Generally, metals are good conductors of an electric current. The non-metal graphite is also a conductor and is used in batteries. Water solutions of salts and strong acids are also conductors of electricity. These solutions are referred to as electrolyte solutions. They are present in batteries and electrolytic capacitors.
Solids
In their normal state atoms are electrically neutral. Each atom contains an equal number of electrons (–ve) and protons (+ve). The electrons occupy a series of shells around the nucleus. In some situations, neutral atoms can gain or lose electrons. If they lose electrons they form positive ions. If they gain electrons they form negative ions.
In a metal, some of the electrons in the outer shell become free of their atom. The atoms become positively charged ions. The free electrons move between these positive ions. In a metal, the current depends on the motion of these mobile electrons through the solid material.
Liquids
In an electrolyte solution, the flow of a currency depends on the movement of positive and negative ions. When an ionic salt dissolves in water, the positive and negative ions become mobile in the solution. For example, a sodium chloride solution contains positive sodium and negative chloride ions. When placed in a circuit between a positively charged electrode and a negatively charged electrode, the positive ions migrate toward the negative terminal, and the negative ions move toward the positive terminal.
Gases
Gases are normally insulators. They do not conduct electricity under normal conditions. Atoms and molecules in a gas are usually neutral. When they are irradiated with high energy ultraviolet light, or heated to a high temperature (e.g. above 3000°C), the atoms are stripped of some of their outer electrons and become positively charged ions. The electrons will move freely in the gas, eventually recombining with positively charged ions. Conduction through a gas is due mainly to the directed flow of the free electrons.
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