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Chapter 21: Electric Charge and Electric Field
- Encompasses both electricity and magnetism.
- Electromagnetic interactions involve particles that have electric charge, an attribute that is as
fundamental as mass
- Electrically charged objects are accelerated by electric forces.
- Charge is quantized and obeys a conservation principle.
- When charges are at rest in our frame of reference, they exert electrostatic forces on each other
- Electrostatic forces are governed by a simple relationship known as Coulomb’s law and are most
conveniently described by using the concept of electric field.
- When something is able to attract other objects, it has a net electric charge, or it has become
- Electrostatics are the interactions between electric charges that are at rest.
- There are exactly two kinds of electric charge
o The kind on the plastic rod rubbed with fur (negative)
o The kind on the glass rod rubbed with silk (positive)
- Two positive charges or two negative charges repel each other
- A positive charge and a negative charge attract each other
Electric Charge and the Structure of Matter:
- The structure of atoms has three particles
o Electrons (negative)
o Protons (positive)
o Neutrons (neutral/uncharged)
- Protons and neutrons are the combination of other entities called quarks which have charges of
+/- 1/3 and +/- 2/3 times the electron charge.
- It is impossible in principle to observe a quark in isolation.
- The nucleus of an atom is composed of protons and neutrons.
- The nucleus is held together by a strong nuclear force that overcomes the electric repulsion of
the protons. This force however has a short range and does not extend far beyond the nucleus
- The masses of the particles are as follows:
o Electron = 9.10938291(40) x 10-31 kg
o Proton = 1.672621777(74) x 10-27 kg
o Neutron = 1.674927351(74) x 10-27 kg
- The numbers in the parenthesis are uncertainties in the last two digits.
- Over 99.9% of the mass of any atom is concentrated in its nucleus
- The negative charge of the electron has about exactly the same magnitude as the positive
charge of the proton.
- The number of protons or electrons in a neutral atom of an element is called the atomic number
of the element
- If one or more electrons are removed from an atom, what remains is called a positive ion.
- A negative ion is an atom that has gained one or more electrons.
- This gain or loss of electrons is called ionization
Electric Charge Is Conserved:
- Principle of conservation of charge:
o The algebraic sum of all the electric charges in any closed system is constant
- Charge is not created or destroyed; it is merely transferred from one body to another
- Conservation of charge is thought to be a universal conservation law.
- Second important principle:
o The magnitude of charge of the electron or proton is a natural unit of charge
- Charge is quantized. Charge can’t be divided into amounts smaller than the charge of one
electron or proton. Thus the charge on any macroscopic body is always zero or an integer
multiple (negative or positive) of the electron charge.
Conductors, Insulators, and Induced Charges:
- Conductors permit the easy movement of charge through them
- Insulators do not permit the easy movement of charge through them
- Most metals are good conductors while most nonmetals are insulators
- Some materials called semiconductors are intermediate in their properties between good
conductors and good insulators
Charging By Induction:
- There is a technique in which a plastic rod can give another body a charge of the opposite sign
without losing any of its own charge. This is called induction.
o An uncharged metal ball is support on an insulating stand. When a negatively charged
rod is brought near it without actually touching it, the free electrons in the metal ball
are repelled by the excess electrons on the rod, and they shift towards the right, away
from the rod.
o They can’t escape the ball because the supporting stand and the surrounding air are
insulators, so we get excess negative charge at the right surface of the ball and
deficiency of negative charge at the left surface.
o These excess charges are called induced charges.
Electric Forces on Uncharged Objects:
- A charged body can exert forces even on objects that are not charged themselves.