University Physics Volume 2

Summary

5.1Electric Charge

• There are only two types of charge, which we call positive and negative. Like charges repel, unlike charges attract, and the force between charges decreases with the square of the distance.
• The vast majority of positive charge in nature is carried by protons, whereas the vast majority of negative charge is carried by electrons. The electric charge of one electron is equal in magnitude and opposite in sign to the charge of one proton.
• An ion is an atom or molecule that has nonzero total charge due to having unequal numbers of electrons and protons.
• The SI unit for charge is the coulomb (C), with protons and electrons having charges of opposite sign but equal magnitude; the magnitude of this basic charge is $e≡1.602×10−19Ce≡1.602×10−19C$
• Both positive and negative charges exist in neutral objects and can be separated by bringing the two objects into physical contact; rubbing the objects together can remove electrons from the bonds in one object and place them on the other object, increasing the charge separation.
• For macroscopic objects, negatively charged means an excess of electrons and positively charged means a depletion of electrons.
• The law of conservation of charge states that the net charge of a closed system is constant.

5.2Conductors, Insulators, and Charging by Induction

• A conductor is a substance that allows charge to flow freely through its atomic structure.
• An insulator holds charge fixed in place.
• Polarization is the separation of positive and negative charges in a neutral object. Polarized objects have their positive and negative charges concentrated in different areas, giving them a charge distribution.

5.3Coulomb's Law

• Coulomb’s law gives the magnitude of the force vector between point charges. It is
$F→12(r)=14πε0q1q2r122r^12F→12(r)=14πε0q1q2r122r^12$
where $q1q1$ and $q2q2$ are two point charges separated by a distance r. This Coulomb force is extremely basic, since most charges are due to point-like particles. It is responsible for all electrostatic effects and underlies most macroscopic forces.

5.4Electric Field

• The electric field is an alteration of space caused by the presence of an electric charge. The electric field mediates the electric force between a source charge and a test charge.
• The electric field, like the electric force, obeys the superposition principle
• The field is a vector; by definition, it points away from positive charges and toward negative charges.

5.5Calculating Electric Fields of Charge Distributions

• A very large number of charges can be treated as a continuous charge distribution, where the calculation of the field requires integration. Common cases are:
• one-dimensional (like a wire); uses a line charge density $λλ$
• two-dimensional (metal plate); uses surface charge density $σσ$
• three-dimensional (metal sphere); uses volume charge density $ρρ$
• The “source charge” is a differential amount of charge dq. Calculating dq depends on the type of source charge distribution:
$dq=λdl;dq=σdA;dq=ρdV.dq=λdl;dq=σdA;dq=ρdV.$
• Symmetry of the charge distribution is usually key.
• Important special cases are the field of an “infinite” wire and the field of an “infinite” plane.

5.6Electric Field Lines

• Electric field diagrams assist in visualizing the field of a source charge.
• The magnitude of the field is proportional to the field line density.
• Field vectors are everywhere tangent to field lines.

5.7Electric Dipoles

• If a permanent dipole is placed in an external electric field, it results in a torque that aligns it with the external field.
• If a nonpolar atom (or molecule) is placed in an external field, it gains an induced dipole that is aligned with the external field.
• The net field is the vector sum of the external field plus the field of the dipole (physical or induced).
• The strength of the polarization is described by the dipole moment of the dipole, $p→=qd→p→=qd→$.
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