PHY 184

1. PHY 184 Spring 2007 Lecture 2 Title: Electric Charge 184 Lecture 2

2. Announcements • PHY 184 section 2 • Register your clicker in lon-capa! • Reading: Chapter 16 this week • Homework • Set #1is open. Due next Tuesday, January 16, 8 am • Lecture slides: • www.pa.msu.edu/courses/phy184/ • 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 – 16:00 184 Lecture 2

3. 184 Lecture 2

4. 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. 184 Lecture 2

5. 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 184 Lecture 2

6. Measuring Charge: The Electroscope The glass and the plastic rod have opposite charge. 184 Lecture 2

7. 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. 184 Lecture 2

8. Law of Charges - - - + + + m1 m2 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 184 Lecture 2

9. Static cling +q - - - 0 - + - Polarization + + + + • What is the force between an electrically charged object (q) and a neutral object (0)? • Observe: It is always attractive. • Why? 184 Lecture 2

10. 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 1 C = 1 A s 184 Lecture 2

11. 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 184 Lecture 2

12. 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 = 10-6 C • 1 nanocoulomb = 1 nC = 10-9 C • 1 picocoulomb = 1 pC = 10-12 C 184 Lecture 2

13. 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. 184 Lecture 2

14. 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 184 Lecture 2

15. 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. 184 Lecture 2

16. 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 184 Lecture 2

17. 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 184 Lecture 2

18. 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 184 Lecture 2

19. 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. 184 Lecture 2

20. 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 184 Lecture 2

21. NSCL Fly-by Link 184 Lecture 2

22. Magnetic Resonance Imaging - MRI Magnetic Field = 1.5 T Magnetic Field = 3.0 T • 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. Yue Cao, Stephen Whalen, Jie Huang, Kevin L. Berger, and Mark C. DeLano, Human Brain Mapping 20:82–90(2003). (MSU Radiology) 184 Lecture 2

23. Semiconductors Modern computer chip with millions of transitors Replica of first transitor in 1947 • Semiconductors are materials that can be switched between being an insulator and being a conductor. • Semiconductors are the backbone of modern electronics and computers. 184 Lecture 2

24. 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. 184 Lecture 2

25. Tomorrow … • Electrostatic charging • The electric force - Coulomb’s Law • Many examples 184 Lecture 2