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1/9/07184 Lecture 21 PHY 184 Spring 2007 Lecture 2 Title: Electric Charge.

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Presentation on theme: "1/9/07184 Lecture 21 PHY 184 Spring 2007 Lecture 2 Title: Electric Charge."— Presentation transcript:

1 1/9/07184 Lecture 21 PHY 184 Spring 2007 Lecture 2 Title: Electric Charge

2 1/9/07184 Lecture 22 Announcements  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:  Honors Option will be announced Thursday. Learning Center Always available for use! Great place for assistance PHY184 times so far: Monday: 14:00 – 18:00 Wednesday: 14:00 – 18:00 Friday: 14:00 – 16:00

3 1/9/07184 Lecture 23

4 1/9/07184 Lecture 24 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.

5 1/9/07184 Lecture 25 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

6 1/9/07184 Lecture 26 Measuring Charge: The Electroscope The glass and the plastic rod have opposite charge.

7 1/9/07184 Lecture 27 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.

8 1/9/07184 Lecture 28 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 Charges m2 m1

9 1/9/07184 Lecture 29 Static cling  What is the force between an electrically charged object (q) and a neutral object (0)?  Observe: It is always attractive.  Why? +q Polarization

10 1/9/07184 Lecture 210 The Unit of Charge  The unit of charge is the coulomb, abbreviated C [named after Charles-Augustin de Coulomb ( )].  The coulomb is defined in terms of the SI unit for electric current, the ampere, abbreviated A [named after Andre-Marie Ampere ( )].  The ampere is a basic SI unit like the meter, the second, and the kilogram.  The unit of charge is defined as 1 C = 1 A s

11 1/9/07184 Lecture 211 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 =  C A proton is a particle with q = +e e = x C

12 1/9/07184 Lecture 212 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  Because a coulomb is a large amount of charge, everyday examples of static electricity typically involve 1 microcoulomb = 1  C = C 1 nanocoulomb = 1 nC = C 1 picocoulomb = 1 pC = C

13 1/9/07184 Lecture 213 Charge Conservation  Benjamin Franklin ( ) 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.

14 1/9/07184 Lecture 214 Elementary Charge Quantum  Electric charge is quantized.  The smallest charge observable is the charge of an electron.  Established by Robert Millikan ( ) in his famous oil drop experiment. electron charge =  e

15 1/9/07184 Lecture 215 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, 12 C has 6 protons, 6 neutrons, and 6 electrons.

16 1/9/07184 Lecture 216 Description of Atoms Atomic number = Z Mass number = A # electrons = Z (charge = -Ze) # protons = Z (charge = +Ze) # neutrons = N = A – Z Atomic mass = Z M p + N M n + Z M e – binding energy/c 2 Atomic mass  A M p

17 1/9/07184 Lecture 217 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 grams/mole. N(atoms) = M / AM N(electrons) = Z N atoms N(removed) = Q / e → fraction(removed) = N(re) / N(el) Answer: about 1 in 10 13

18 1/9/07184 Lecture 218 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

19 1/9/07184 Lecture 219 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.

20 1/9/07184 Lecture 220 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

21 1/9/07184 Lecture 221 NSCL Fly-by Link

22 1/9/07184 Lecture 222 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 T Yue Cao, Stephen Whalen, Jie Huang, Kevin L. Berger, and Mark C. DeLano, Human Brain Mapping 20:82–90(2003). (MSU Radiology)

23 1/9/07184 Lecture 223 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 Modern computer chip with millions of transitors

24 1/9/07184 Lecture 224 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 As  Law of charge conservation The total charge of an isolated system is strictly conserved.

25 1/9/07184 Lecture 225 Tomorrow …  Electrostatic charging  The electric force - Coulomb’s Law Many examples


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