Module 5: Electricity

by Jeremiah N. Junatas, June 2009

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Table of Contents

• The Electrical Nature of Matter

• The Behavior of Electric Charges

• Electric Fields

• Electric Potential (Voltage)

• Current , Voltage, and Resistance

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A. The Electrical Nature of Matter

All atoms are made up of protons, neutrons, and electrons. The classical model of the atom consists of a positively charged nucleus made up of protons and neutrons, and a number of negatively charged electrons in orbit about the nucleus. The simplest form of the model pictures electrons as tiny particles which circle the nucleus in definite orbits similar to the orbits of the planets about the sun. Though this model is useful in visualizing electrical processes, it must be pointed out that it is quite inadequate for describing the details of atomic structure. These details can be explained with the methods of quantum mechanics, the branch of physics used to describe molecular and atomic phenomena. The usefulness of the orbital model of the atom is based upon the fact that the physical parameters of electrons are “quantized” and can take on only certain discrete values.

The electron is considered to have one “quantum” of charge. The proton also has one quantum of charge, but it is of the opposite polarity. The electron charge is designated “negative” and the proton charge “positive.” The neutron has no charge and is “neutral.” Electric charge is measured in units of coulomb, abbreviated C. The quantum of charge, positive or negative, has the value 1.6 x 10 – 19 coulombs. Hence, 1 C = 6.25 x 1018 of electrons.

The electron is the primary charge carrier in most electrical phenomena involving metal wires because it is the lightest and most mobile of the constituents of the atom. An electron has definite mass (me = 9.1 x 10 – 31 kg), so it can conveniently be considered to be a particle in ordinary electrical phenomena.

The mass of a proton is mp = 1.6726 x 10 – 27 kg while that of a neutron is mn = 1.6749 x 10 – 27 kg.

B. The Behavior of Electric Charges

Charges of the same polarity repel each other, while unlike charges experience a strong attractive force. The force between two charged particles is found to be proportional to the product of their charges and inversely proportional to the square of the distance between them. This experimental relationship is known as Coulomb’s Law:

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where Q1 and Q2 are two charges in coulombs, and r is the distance between them in meters. The parameter K is Coulomb’s constant with the value K = 9 x 109 N-m2/C2 in SI units. The electrostatic force, F, is the force on each charge. The forces on the two charges are, of course, equal in magnitude and opposite in direction, as required by Newton’s third law, and are directed along the line joining the two particles as shown in Figure 1.

(a) Unlike charges attract

(b) Like charges repel

Figure 1.

Example 1: What would be the force experienced by the charges of one coulomb each, which are one meter apart?

Solution: Applying Coulomb’s law:

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The positive sign indicates a repulsive force. Each of the charges is repelled with a force of 9 x 109 N. If one of the charges had been – 1 C, then the force would be 9 x 109 N attractive force on each charge.

C. Electric Fields

If you brought a small positive test charge near another concentration of positive charge, the test charge would experience a repulsive force. You could say that there is an “electric force field” around the concentration of charge which will repel any other positive charge but attract a negative charge. The electric field at any point is defined as the force per unit charge exerted upon any point test charge at that point. The direction of the electric field is then defined by relationship

[pic]Electric field

and its units are newton/coulomb in the SI system. This relationship for the...