Download Electric Charge - Purdue Physics

Electricity and Magnetism
 The second most important part of physics (after
mechanics, F=ma)
 Electricity and magnetism have been known for
thousands of years
• The ancient Greeks knew that a piece of amber rubbed
with fur would attract small, light objects
• The word for electron and electricity derived from the Greek word
for amber, ηλεκτρον
• Naturally occurring magnetic materials called lodestones
were used as early as 300 BC to construct compasses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Monday, January 13, 2014
The Fundamental Forces: 4 forces
Range:
Infinite
Infinite
10-18 m
0.1% of the diameter of a proton
10-15 m
diameter of medium-sized nucleus
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Monday, January 13, 2014
Fundamental Forces of Nature
 Unification of forces?
 E.g., electricity and magnetism were thought
different, but actually are “two sides” of the
same force
 EM & weak forces are also united - electroweak
 Some ideas how to unite electroweak & strong
 Gravity remains separate...
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Monday, January 13, 2014
Fundamental Forces of Nature
 Unification of forces?
3
Monday, January 13, 2014
The Four Forces
 The four fundamental forces work by exchanging elementary
particles (not necessarily “real”, might be “virtual”).
• Electromagnetic - photon
• Weak - W and Z bosons (observed in 1983)
• Strong – gluons (observed in 1979)
• Gravity - graviton (?) (has not been observed yet)
 Thus forces can act at a distance
 The Sun attracts the Earth from distance
• Magnet can attract metal
• Electric charges attract each other
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Monday, January 13, 2014
Action at distance: through “fields”
• Charges produce fields, fields act on other charges
electric field
electric charge (source of field)
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Monday, January 13, 2014
Action at distance: through “fields”
• Charges produce fields, fields act on other charges
another charge
electric field
electric charge (source of field)
4
Monday, January 13, 2014
Action at distance: through “fields”
• Charges produce fields, fields act on other charges
Force
another charge
electric field
electric charge (source of field)
4
Monday, January 13, 2014
Maxwell’s equations
(Not needed in this course)
∇ · E = ρ/�0
electric charge produces E-field
∇·B=0
no magnetic charge
∇ × B = µ0 J + µ0 � 0 ∂ t E
∇ × E = −∂t B
electric current and
changing E-field
produce B-field
changing B-field also
produce E-field
∇ = {∂x , ∂y , ∂z }
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Monday, January 13, 2014
 Permeability of vacuum, �0 , µ
, -0 misnomers, coming
from days of mechanical analogies, ether. (Maxwell
initially thought that EM fields are transported by
a substance - ether- with some mechanical
properties.)

�0 , µ0are unit-dependent constants (e.g. do not
appear in CGS units).
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Monday, January 13, 2014
Units - The Metric System
 International system of units (SI)
University Physics, Chapter 1
Monday, January 13, 2014
20
Electric Charge: experimental facts
 Experiment: objects can repel/attract each other.
 Electric force.
 Electric force between two objects is proportional to their
charges (concept of a charge is introduced)
 Two types of charges: “positive” and “negative”
 Same charges repel, opposite attract.
 Value of a charge does not depend on motion
 Value of a charge changes discontinuously: charge is
quantized.
 In a closed system charge is conserved
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Monday, January 13, 2014
Electric Charge
 Normally, things are electrically neutral.
 They have equal amounts of positive and negative charge and
are thus electrically neutral
 Rubbing two different things may charge them both (opposite
charges)
 Energy of electrons are a bit different in different materials
due to their chemical structure, trying to equate the energy,
some electrons flow from one material to another.
 Charges are not created, just separated!
 Negative charge: an excess of electrons
 Positive charge: a deficit of electrons
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Monday, January 13, 2014
Charge (2)
 Demo:
• If we rub a plastic rod with fur, the rod will become charged
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Monday, January 13, 2014
Measuring Charge: The Electroscope
Metal
Shield
Electric repulsion
Metal
deflection arm
Metal rod
Gravity
The glass and the plastic rod
have opposite charge
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Monday, January 13, 2014
Charging by Conduction
++++++++++++++++++
Electroscope
F
F
We brought charge onto the
electroscope through
contact with the electrode.
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Monday, January 13, 2014
Explanation of the Demos
 Explanation:
Electrons are transferred from the fur onto the plastic rod.
This rod now carries a negative charge.
When the rod touches the electroscope, some charges are
transferred to it, metal rods repulse.
The electroscope shows the presence of charge.
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Monday, January 13, 2014
Law of Charges
This result leads to the Law of Charges
• Like charges repel and opposite charges attract
+
+
+
-
-
 Note that electricity is different from gravitation, in
which the force is always attractive
m1
m2
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Monday, January 13, 2014
The Unit of Charge
 The unit of charge is the Coulomb, abbreviated C
[named after Charles-Augustin de Coulomb (1736 1806)].
 The Coulomb is defined in terms of the SI unit for
electric current, the ampere, abbreviated A
[named after Andre-Marie Ampere (1775 - 1836)].
 The ampere is a basic SI unit like the meter, the
second, and the kilogram.
 The unit of charge is defined as
1C=1As
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Monday, January 13, 2014
Charge of an Electron
 We can define the unit of charge in terms of the
charge of one electron
• An electron is an elementary particle with charge q =
-e where
•e = 1.602 · 10
-19
C
• A proton is a particle with q = +e
e = 1.602 · 10-19 C
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Monday, January 13, 2014
Coulomb of Charge
 A full Coulomb is a very large amount of charge!
• A lightning discharge can contain 10’s of Coulombs
 The number of electrons required to produce 1 Coulomb of
charge is
 Because a Coulomb is a large amount of charge, everyday
examples of static electricity typically involve
• 1 microCoulomb = 1 µC = 10-6 C
• 1 nanoCoulomb = 1 nC = 10-9 C
• 1 picoCoulomb = 1 pC = 10-12 C
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Monday, January 13, 2014
Example - Net Charge
 Suppose we want to create a positive charge of 10 µC on a block of copper
metal with mass 2.00 kg. What fraction of the electrons in the copper
block would we remove?
Electric forces are strong!
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Monday, January 13, 2014
Example - Net Charge
 Suppose we want to create a positive charge of 10 µC on a block of copper
metal with mass 2.00 kg. What fraction of the electrons in the copper
block would we remove?
NA
Electric forces are strong!
20
Monday, January 13, 2014
Charge Conservation
 Benjamin Franklin (1706 - 1790) introduced the idea of positive and
negative charge (amber or plastic is negative).
 Franklin also proposed that electric charge is conserved.
 For example, when a plastic rod is charged by rubbing it with a fur, charge
is neither created nor destroyed, but instead electrons are transferred to
the rod leaving a net positive charge on the fur.
 Law of charge conservation
• The total charge of an isolated system is strictly
conserved.
 This law adds to our list of conservation laws: conservation of energy,
conservation of momentum, and conservation of angular momentum
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Monday, January 13, 2014
Charge Conservation
 Benjamin Franklin (1706 - 1790) introduced the idea of positive and
negative charge (amber or plastic is negative).
 Franklin also proposed that electric charge is conserved.
 For example, when a plastic rod is charged by rubbing it with a fur, charge
is neither created nor destroyed, but instead electrons are transferred to
the rod leaving a net positive charge on the fur.
 Law of charge conservation
• The total charge of an isolated system is strictly
conserved.
 This law adds to our list of conservation laws: conservation of energy,
conservation of momentum, and conservation of angular momentum
16
Monday, January 13, 2014
Charge Conservation
 Benjamin Franklin (1706 - 1790) introduced the idea of positive and
negative charge (amber or plastic is negative).
 Franklin also proposed that electric charge is conserved.
 For example, when a plastic rod is charged by rubbing it with a fur, charge
is neither created nor destroyed, but instead electrons are transferred to
the rod leaving a net positive charge on the fur.
 Law of charge conservation
• The total charge of an isolated system is strictly
conserved.
 This law adds to our list of conservation laws: conservation of energy,
conservation of momentum, and conservation of angular momentum
16
Monday, January 13, 2014
Summary …
 There are two kinds of electric charge – positive and
negative.
 Law of Charges
• Like charges repel and opposite charges attract
 The unit of charge is the Coulomb defined as
• 1 C = 1 A•s
 Law of charge conservation
• The total charge of an isolated system is strictly conserved.
 Charge is quantized
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Monday, January 13, 2014
Charge is quantized
 Elementary charge: that of (-) electron, “e”.
 Protons are composed of quarks, with charges
1
q=± e
3
2
q=± e
3
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Monday, January 13, 2014
Electrostatic Charging
 There are three ways to charge an object
• static - rubbing different materials
• conduction
• induction
 Static charging
• Due to energy dis-balance in different materials, electrons can flow
from a neutral to a neutral object
+
++ +
++
+
++ ++
-
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Monday, January 13, 2014
Electrostatic Charging
 Charging by conduction
• We can charge an object by connecting a source of charge directly to
the object and then disconnecting the source of charge.
• The object will remain charged
– Conservation of charge
+
+++
+
+
++ +
++
+
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Monday, January 13, 2014
Static Cling
 What is the force between an electrically charged
object (q) and a neutral object (0)?
 (I mean object, collection of many atoms, not a single
neutral particle, like neutron)
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Monday, January 13, 2014
Quiz:
 Will the charged ball affect the neutral board?
• A: NO
• B: Yes, attract
• C: Yes, repulse
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Monday, January 13, 2014
Static Cling
 What is the force between an electrically charged
object (q) and a neutral object (0)?
 (I mean object, collection of many atoms, not a single
neutral particle, like neutron)
 Observe: It is always attractive
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Monday, January 13, 2014
Polarization of the material
 Free electrons tend to flow towards
the positive charge
 Closer-in the electric field and the
force is stronger
+q
--++
++
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Monday, January 13, 2014
Polarization of the material
 Free electrons tend to flow towards
the positive charge
 Closer-in the electric field and the
force is stronger
 Alternatively: dipoles aline
+
+
++
+
++-+- ++ -+ - + -
+q
--++
++
+q
12
Monday, January 13, 2014
Polarization of the material
 Free electrons tend to flow towards
the positive charge
 Closer-in the electric field and the
force is stronger
 Alternatively: dipoles aline
+
+
++
+
++-+- ++ -+ - + -
+q
--++
++
+q
12
Monday, January 13, 2014
Polarization of the material
 Free electrons tend to flow towards
the positive charge
 Closer-in the electric field and the
force is stronger
 Alternatively: dipoles aline
+
+
++
+
++-+- ++ -+ - + -
+q
--++
++
+q
Not to be confused with polarization of light (later in
this course)
Monday, January 13, 2014
12
Charging by induction
--++
++
+q
extra electrons
30
Monday, January 13, 2014
Charging by induction
--++
++
+q
extra electrons
30
Monday, January 13, 2014
Charging by induction
--++
++
+q
extra electrons
--++
++
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Monday, January 13, 2014
Charging by induction
--++
++
+q
extra electrons
--++
++
This will not work if no free electrons
+
+
++
+
++-+- ++ -+ - + -
+q
30
Monday, January 13, 2014
Insulators and Conductors
 The electronic structure of materials determines their ability to conduct
electricity
• “Conducting electricity” means the transport of electrons
 Materials that conduct electricity well are called conductors
• Electrons can move freely (i.e., some, very few, of the electrons)
• Metals
• Water with dissolved materials
 Materials that conduct electricity poorly are called insulators
• Electrons cannot move freely
• Glass
• Pure water
 Semiconductors: change conductivity
 Superconductors: conduct electricity without resistance
Ability to (not) conduct electricity: resistivity
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Monday, January 13, 2014
Coulomb’s Law
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
q1 q2 r12
F=k 2
r12 r12
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
q1 q2 r12
F=k 2
r12 r12
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
2
q1 q2 r12
F=k 2
r12 r12
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
r12
2
q1 q2 r12
F=k 2
r12 r12
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
r12
2
q1 q2 r12
F=k 2
r12 r12
F
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
r12
2
q1 q2 r12
F=k 2
r12 r12
F
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
r12
2
q1 q2 r12
F=k 2
r12 r12
F
32
Monday, January 13, 2014
Electric Force - Coulomb’s Law
 Consider two electric charges: q1
and q2
 The electric force F between
these two charges separated by
a distance r is given by
Coulomb’s Law
 The constant k is called
Coulomb’s constant and is given
by
 Coulomb’s law in vector form
+
1
r12
2
q1 q2 r12
F=k 2
r12 r12
F
32
Monday, January 13, 2014