Physics Unit 3 - Electricity

Electrostatics: Charge Concepts

By Marc Reif

Matter is composed of atoms, which we usually view as acting like extremely tiny, round, hard objects.† However, as you probably know, atoms are composed of smaller particles, electrons, protons, and neutrons.† There are a host of other, smaller particles that are known, but that are beyond the scope of this course.

Charge is a fundamental property of matter.† It has been found to be conserved in all situations; the total atomic charge in the universe remains constant.

There are two kinds of charge that have been identified by experiment. They have been given the names positive and negative.  Although there are not good reasons for these names, they have persisted for historical reasons.

Electrons have been found to be the carriers of negative charge in atoms. They are located in a "cloudlike" distribution far from the nucleus. Electrons are much less massive than protons and neutrons.

Protons are the carriers of positive charge in atoms. They are located in the nucleus of atoms. Protons have slightly less mass than neutrons, but far more than electrons.

Neutrons are also located in the nucleus of atoms. They do not have the property of charge.

The unit of charge is the Coulomb. The charge of each individual electron and proton is exactly equal in magnitude, even though it is opposite in sign. The absolute value of the charge on an electron or proton is 1.6 x10^-19 C.

Electric charge can be separated by friction. When this happens, electrons are removed from an object, leaving positive charge "exposed" The positive object becomes positive by an amount exactly equal to the amount of negative charge removed.

Two objects with like charge experience an equal and opposite force of repulsion (Newton's 3rd Law). Two objects with unlike charge experience an equal and opposite force of attraction (Newton's 3rd Law).

When charge is "conductedî"to an object, the two objects must come into contact, or close proximity. Electrons jump from one object to another.

A charged object can "induce" a charge in another object when it is brought near the object (charging by "induction").  The electrons in the object are displaced from their normal positions, separating charge temporarily, and allowing the objects to attract. The force between a charged object and the object it induces a charge in will always be attractive. Can you see why?

Matter is loosely classified as conductors and insulators. Conductors are materials (primarily metals) in which electrons move readily. Insulators are materials that hold electrons more tightly.

The electric or Coulomb force for point charges obeys Coulomb's law. It is Fe=kqq/R2, where k is a constant, found by experiment to be 8.99 x 109 N*m2/C2.

The similarity of Coulomb's law with Newton's law of Universal Gravitation is striking, and comes about because both can be viewed as the result of something emanating away from objects in all directions.

Spherical charged objects separated by a distance which is large compared to their radii act like point charges, but the electric force between any other shape charged object must be evaluated using the techniques of calculus.