1/9/07 184 Lecture 2 1

PHY 184PHY 184

Spring 2007Lecture 2

Title: Electric Charge

1/9/07 184 Lecture 2 2

AnnouncementsAnnouncements

PHY 184 section 2• Register your clicker in lon-capa!• Reading: Chapter 16 this week

Homework• Set #1 is open. Due next Tuesday, January 16, 8 am

Lecture slides:• www.pa.msu.edu/courses/phy184/

Honors Option will be announced Thursday.

Learning CenterAlways available for use!

Great place for assistance

PHY184 times so far:Monday: 14:00 – 18:00

Wednesday: 14:00 – 18:00Friday: 14:00 – 16:00

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Electric ChargeElectric 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)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.

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

Negative charge: an excess of electrons Positive charge: a deficit of electrons

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Measuring Charge: The Electroscope

Measuring Charge: The Electroscope

The glass and the plastic rod have opposite charge.

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Explanation of the DemosExplanation 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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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

Law of ChargesLaw of Charges

+

-

+

-

-

+

m2m1

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Static clingStatic cling

What is the force between an electrically charged object (q) and a neutral object (0)?

Observe: It is always attractive.

Why?+q

0+++++

- ----Polarization

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The Unit of ChargeThe 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 as1 C = 1 A s

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Charge of an ElectronCharge 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 = 1.60210-19 C

• A proton is a particle with q = +e

e = 1.602 x 10-19 C

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Coulomb of ChargeCoulomb 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

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

1819-

1024.6C 101.602

C 1

eN

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Charge ConservationCharge 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 QuantumElementary 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 AtomsStructure 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 AtomsDescription 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 ChargeExample - 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 ConductorsInsulators 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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SuperconductorsSuperconductors

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 SuperconductorsApplications 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

Magnetic Resonance Imaging - MRI

MRI stands for nuclear magnetic resonance imaging.

MRI produces high quality images of living tissue without causing any damage.

The quality of an MRI image (signal-to-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.

Magnetic Field = 1.5 T

Magnetic Field = 3.0 TYue 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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SemiconductorsSemiconductors 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

Modern computer chip with millions of transitors

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Summary …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 …Tomorrow …

Electrostatic charging The electric force - Coulomb’s Law

• Many examples