PHYS 1107 Lecture Notes - Lecture 8: Electric Dipole Moment, Neve Electronics, Even And Odd Functions
V6.01 BQRS F’10 1
Chapter 21: Electric Charge and Electric Field
Introduction
!
Four fundamental forces of nature (Fewer if they can be “unified”)
• Long range " gravity and electromagnetism
• Short range " strong (color) and weak (flavor) nuclear force
!
Electromagnetism common in our daily lives
• Solid objects held together by electric interactions
• You don’t sink through your chairs because of electric interactions
!
Electromagnetic interactions involve particles which have electric charge
Electric Charge (21-1)
!
Electric charge is a fundamental attribute of matter
!
Ancient Greeks observed amber rubbed with wool attracted other objects
• Electric derived from Greek word
ηλεκτρον
meaning amber
!
Empirical observations (in low humidity to minimize surface adsorption of water):
• Two rubber (or amber) rods rubbed with fur repel
• Two glass (plexiglas) rods rubbed with silk repel
• Glass (rubbed with silk) attracts rubber rod (rubbed with fur)
• The fur attracts the rubbed rubber rods
• The silk attracts the rubbed glass rods
!
Conclusion (from many experiments): There are two kinds of electric charge
• Positive charge on rubbed glass rod and fur
• Negative charge on rubbed rubber rod and silk
• Named by Benjamin Franklin
Unfortunate choice: Makes electron negative. We now know that electrons are most mobile charge carriers, so
current (defined as flow of positive charge) moves opposite to the actual charge carriers. Thanks Ben!
• Charges of same kind repel, different kind attract (unlike gravity)
• Rubbing two different materials always charges one positive, the other
negative.
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V6.01 BQRS F’10 2
!
Charge quantization
• Every observable amount of electric charge is always an integer multiple of
a fundamental or basic unit denoted by
e
(Milliken oil drop experiment)
• The magnitude of charge of the electron or proton is the basic unit of
electric charge: electron has charge
e
−
and proton has charge
e
+
• The SI unit of electric charge is called the Coulomb (C)
• Fundamental unit of charge 19
1.602176462(63) 10eC
−
=×
# Very unusual to encounter a 1C charge in electrostatic problems
since it is such a huge amount of charge
# In practice, the Coulomb is defined as one Ampère-second
• Remarkable: all electrons have identical charge (presumed time invariant)
• Atom
# heavy dense core (nucleus) of protons and neutrons (zero charge)
bound by the (short-ranged) strong nuclear force (order 15
10 m
−
)
# “cloud” of light electrons bound by electric attraction (order 10
10 m
−
)
# usually equal number of protons and electrons (total charge zero)
# can lose electrons " positive ion
# can gain electrons " negative ion
• Aside: protons and neutrons are combinations of subatomic particles
known as quarks (Murray Gell-Mann, George Zweig 1963)
# u quark
2
3
e+ d quark
1
3
e
−
# proton = uud =
12
333
2
ee ee−++ =
# neutron = udd =
12
3
1
33
0
eee−+−
=
# hordes of other particles are known to exist:
,,,,
K
π
ρ
∆
…
# quarks never exist in isolation (only inside protons, etc.)
• Antiparticles have exact opposite electric charge
# Positron (antiparticle of electron) has charge e
+
# Antiproton has charge e
−
• in bulk matter we will often treat charge as continuous in this course
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V6.01 BQRS F’10 3
!
Charge conservation
• The algebraic sum of all the electric charges in any closed system is
constant with time
• Even in high-energy interactions in which particles and antiparticles are
created or annihilated, the total net charge remains constant
!
When two objects are rubbed - charge is neither created nor destroyed
• Charge just moves from one material to another: electron binding strength
varies by material; rub electrons “on” or rub them “off” a surface
• Positively charged objects have slightly fewer electrons than protons
• Negatively charged objects have slightly more electrons than protons
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Document Summary
Four fundamental forces of nature (fewer if they can be unified : long range " gravity and electromagnetism, short range " strong (color) and weak (flavor) nuclear force. Electromagnetism common in our daily lives: solid objects held together by electric interactions, you don"t sink through your chairs because of electric interactions. Electromagnetic interactions involve particles which have electric charge. Electric charge is a fundamental attribute of matter. Ancient greeks observed amber rubbed with wool attracted other objects: electric derived from greek word meaning amber. Conclusion (from many experiments): there are two kinds of electric charge: positive charge on rubbed glass rod and fur, negative charge on rubbed rubber rod and silk, named by benjamin franklin. We now know that electrons are most mobile charge carriers, so current (defined as flow of positive charge) moves opposite to the actual charge carriers.
