Elements of an Atom

  • All matter is composed of molecules which are made up of a combination of atoms. Atoms have a nucleus with electrons orbiting around it.
  • The nucleus is composed of protons and neutrons (not shown). Most atoms have an equal number of electrons and protons.
  • Electrons have a negative charge (-).
  • Protons have a positive charge (+).
  • Neutrons are neutral.
  • The negative charge of the electrons is balanced by the positive charge of the protons.
  • Electrons are bound in their orbit by the attraction of the protons.
  • These are referred to as bound electrons.

Free electrons

  • Electrons in the outer band can become free of their orbit by the application of some external force such as movement through a magnetic field, friction, or chemical action.
  • These are referred to as free electrons.
  • A free electron leaves a void which can be filled by an electron forced out of orbit from another atom.
  • As free electrons move from one atom to the next an electron flow is produced.
  • This is the basis of electricity.

Conductors, Insulators and Semiconductors

  • An electric current is produced when free electrons move from one atom to the next.
  • Materials that permit many electrons to move freely are called conductors.
  • Copper, silver, aluminum, zinc, brass, and iron are considered good conductors.
  • Copper is the most common material used for conduc- tors and is relatively inexpensive.


  • Materials that allow few free electrons are called insulators.
  • Materials such as plastic, rubber, glass, mica, and ceramic are good insulators.
  • An electric cable is one example of how conductors and insulators are used.
  • Electrons flow along a copper conductor to provide energy to an electric device such as a radio, lamp, or a motor.
  • An insulator around the outside of the copper conductor is provided to keep electrons in the conductor.


  • Semiconductor materials, such as silicon, can be used to manufacture devices that have characteristics of both conductors and insulators.
  • Many semiconductor devices will act like a conductor when an external force is applied in one direction.
  • When the external force is applied in the opposite direction, the semiconductor device will act like an insulator.
  • This principle is the basis for transistors, diodes, and other solid-state electronic device.

Electric Charges
Neutral state of an atom

  • Elements are often identified by the number of electrons in orbit around the nucleus of the atoms making up the element and by the number of protons in the nucleus.
  • A hydrogen atom, for example, has only one electron and one proton.
  • An aluminum atom (illustrated) has 13 electrons and 13 protons.
  • An atom with an equal number of electrons and protons is said to be electrically neutral.

Positive and negative charges

  • Electrons in the outer band of an atom are easily displaced by the application of some external force.
  • Electrons which are forced out of their orbits can result in a lack of electrons where they leave and an excess of electrons where they come to rest.
  • The lack of electrons is called a positive charge because there are more protons than electrons.
  • The excess of electrons has a negative charge.
  • A positive or negative charge is caused by an absence or excess of electrons.
  • The number of protons remains constant

Attraction and repulsion of electric charges

  • The old saying, “opposites attract,” is true when dealing with electric charges. Charged bodies have an invisible electric field around them.
  • When two like-charged bodies are brought together, their electric field will work to repel them.
  • When two unlike-charged bodies are brought together, their electric field will work to attract them.
  • The electric field around a charged body is represented by invisible lines of force.
  • The invisible lines of force represent an invisible electrical field that causes the attraction and repulsion.
  • Lines of force are shown leaving a body with a positive charge and entering a body with a negative charge.

Coulomb’s Law

  • During the 18th century a French scientist, Charles A. Coulomb, studied fields of force that surround charged bodies.
  • Coulomb discovered that charged bodies attract or repel each other with a force that is directly proportional to the product of the charges, and inversely proportional to the square of the distance between them.
  • Today we call this Coulomb’s Law of Charges.
  • Simply put, the force of attraction or repulsion depends on the strength of the charred bodies, and the distance between them.

1.Bunty B. Bommera
2.Dakshata U. Kamble

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