Download PHY 184 lecture 2

PHY 184
Spring 2007
Lecture 2
Title: Electric Charge
1/9/07
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
1/9/07
www.pa.msu.edu/courses/phy184/
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Electric Charge
 Everyday example: When walking on a carpet on a dry
winter’s day and then touching a door knob, one often
experiences a spark
• This process is called charging
• Charging: negatively charged electrons move from the atoms and
molecules of the carpet to the soles of our shoes, to the body
• Spark: The built-up charge discharges through the metal of the door
knob.
 Similar phenomenon involving wind, rain and ice produces
lightning.
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Charge (2)
 Normally objects around us do not seem to carry a net charge.
 They have equal amounts of positive and negative charge and are thus
electrically neutral.
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Negative charge: an excess of electrons
Positive charge: a deficit of electrons
 Demo:
• If we rub a plastic rod with fur, the rod will become charged
• If we bring two charged plastic rods together, they will repel each other
• If we rub a glass rod with silk, the rod will become charged
• If we bring together a charged plastic rod and a charged glass rod, they
will attract each other
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Measuring Charge: The Electroscope
The glass and the plastic rod
have opposite charge.
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Explanation of the Demos
 Explanation:
Electrons are transferred from the fur onto the plastic
rod. This rod now carries a negative charge.
Electrons are transferred from the glass rod onto the silk.
The glass rod now carries a positive charge (electrons are
missing).
The electroscope shows the presence of charge.
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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
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m2
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Static cling
 What is the force between an electrically charged
object (q) and a neutral object (0)?
 Observe: It is always attractive.
 Why?
Polarization
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- -+ 0 +
+q
++
+
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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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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 x 10-19 C
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Coulomb of Charge
 A full coulomb is a very large amount of charge!
• A lightning discharge can contain 10’s of coulombs
• Demo - Wimshurst machine
 The number of electrons required to produce 1 coulomb
of charge is
1C
18
Ne 

6
.
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
10
1.602  10-19 C
 Because a coulomb is a large amount of charge, everyday
examples of static electricity typically involve
• 1 microcoulomb = 1 C = 10-6 C
• 1 nanocoulomb = 1 nC = 10-9 C
• 1 picocoulomb = 1 pC = 10-12 C
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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.
The total charge is constant.
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Elementary Charge Quantum
 Electric charge is
quantized.
 The smallest charge
observable is the charge
of an electron.
 Established by Robert
Millikan (1868 - 1953) in
his famous oil drop
experiment.
electron charge = -e
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Structure of Atoms
 Atoms are electrically neutral.
 Atoms are composed of a
positively charged atomic nucleus
surrounded by negative electrons.
 The atomic nucleus is composed
of positively charge protons and
electrically neutral neutrons.
 The number of protons is the
same as the number of electrons.
 For example, 12C has 6 protons, 6
neutrons, and 6 electrons.
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Description of Atoms
Atomic number = Z
Mass number = A
# electrons = Z
(charge = -Ze)
# protons = Z
(charge = +Ze)
# neutrons = N = A – Z
Atomic mass = Z Mp + N Mn + Z Me
– binding energy/c2
Atomic mass  A Mp
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Example - Net Charge
 Suppose we want to create a positive charge of 10 C on a block of
copper metal with mass 2.00 kg. What fraction of the electrons in the
copper block would we remove?
The atomic mass of copper is 63.55 grams/mole.
N(atoms) = M / AM
N(electrons) = Z Natoms
N(removed) = Q / e
→
fraction(removed) = N(re) / N(el)
Answer: about 1 in 1013
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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 of the electrons)
• Metals
• Water with dissolved materials
 Materials that conduct electricity poorly are called
insulators
• Electrons cannot move freely
• Glass
• Pure water
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Superconductors
 Some materials conduct electricity with no
resistance.
 Mainly metals at very low temperatures (~ temp. of
liquid helium).
 Persistent currents: Once electrons in a
superconductor are put in motion, there is nothing
to stop the motion --- no resistance.
 In a normal metal, some electrons are moving but
there is resistance, i.e., energy loss.
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Applications of Superconductors
 MSU Superconducting Cyclotrons
• World’s first superconducting cyclotrons
• K500 Superconducting Cyclotron, 1982
• K1200 Superconducting Cyclotron, 1989
• The magnets in the accelerator are electromagnets made
with superconducting wire.
• The MSU cyclotrons produce beams to study
• The origins of the elements
• The structure of exotic nuclei
• The properties of nuclear matter
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NSCL Fly-by
Link
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Magnetic Resonance Imaging - MRI
 MRI stands for nuclear magnetic resonance imaging.
 MRI produces high quality images of living tissue without
causing any damage.
Magnetic Field = 1.5 T
 The quality of an MRI image (signalto-noise) is proportional to the the
magnitude of the magnetic field
• High field mean high quality images
 Superconducting magnets can produce
up to four times the magnetic field of
a room-temperature magnet.
Yue Cao, Stephen Whalen, Jie Huang, Kevin L. Berger, and Mark C. DeLano,
Human Brain Mapping 20:82–90(2003). (MSU Radiology)
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Magnetic Field = 3.0 T
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Semiconductors
 Semiconductors are materials that can be switched
between being an insulator and being a conductor.
 Semiconductors are the backbone of modern
electronics and computers.
Replica of first transitor
in 1947
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Modern computer chip with
millions of transitors
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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.
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Tomorrow …
 Electrostatic charging
 The electric force - Coulomb’s Law
• Many examples
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