1 1/9/07184 Lecture 21

2 1/9/07184 Lecture 22 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.

3 1/9/07184 Lecture 23 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

4 1/9/07184 Lecture 24 Cargas del mismo signo se repelen y de signo opuesto se atraen  Nótese que la electricidad es algo diferente de la gravitación …. “Ley de las cargas” + - + - - + m2 m1

5 1/9/07184 Lecture 25 Electrostatica  Cual es la fuerza entre un objeto eléctricamente cargado (q) y un objeto neutro (0)?  Nótese que siempre es atractiva !!  Por qué? +q 0 + + + + + - - - - - Polarización!

6 1/9/07184 Lecture 26 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

7 1/9/07184 Lecture 27 Carga del Electrón  Se puede definir la unidad de carga en terminos de la carga de un electr ó n**… Un electrón es una partícula elemental con carga q = -e, donde e = 1.602  10 -19 C Un protón es una partícula ‘elemental’ con q = +e e = 1.602 x 10 -19 C ** El ampere (André-Marie Ampère (1775 - 1836)]. es la 4ta unidad básica en el SI como el metro, el segundo y el kilogramo

8 1/9/07184 Lecture 28 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

9 1/9/07184 Lecture 29 El quantum de carga  La carga electrica esta’ cuantizada  La carga mas pequenha observable es la del electron  Robert Millikan (1868 - 1953) y el experimento de la gota de aceite Carga del electron =  e = 1.602  10 -19 C

10 1/9/07184 Lecture 210 La estructura atómica  Sistema planetario infinitesimal??!  Electricamente neutro: electrones ‘orbitando’ alrededor de un nucleo ‘en reposo’  Ejemplo: 12 C tiene 6 protones, 6 neutrones and 6 electrones  Isotopos

11 1/9/07184 Lecture 211 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

12 1/9/07184 Lecture 212 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

13 1/9/07184 Lecture 213 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.

14 1/9/07184 Lecture 214 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

15 1/9/07184 Lecture 215 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)

16 1/9/07184 Lecture 216 Dielectricos, Semiconductores, Conductores, Superconductores  Dielectricos = aislantes, no conducen o conducen muy mal la electricidad  En el conductor los electrones se mueven como si fuesen un fluido  Semiconductores tienen un comportamiento electrico como aislante y conductor  Superconductividad: solo a muy bajas temperaturas Replica del 1er transistor en 1947 Chip con millones de transistors

17 1/9/07184 Lecture 217 Ley de Coulomb  Fuerza eléctrica entre cargas puntuales…  dependencia 1/r 2 como en gravitación  K constante de Coulomb  0 permitividad del vacío

18 1/9/07184 Lecture 218 The Electric Field Field Theory The electric force is not “action at a distance” but is the action of a field. A field is a physical entity that extends throughout a volume of space and exerts forces. Electric field = E(x,t) Magnetic field = B(x,t)

19 1/9/07184 Lecture 219 Campo Eléctrico  Una carga crea o genera un campo alrededor de ella y otras cargas sienten ese campo Carga de prueba: carga puntual positiva muy pequenha tal que no modifica el campo original + Test charge q +

20 1/9/07184 Lecture 220 The Electric Field (3)  A field is not just an abstract concept that we use to describe forces. The field is real.  The electric field extends throughout space and exerts forces on charged particles.  If we place a positive point charge in an electric field, there will be a vector force on that charge in the direction of the electric field The magnitude of the force depends on the strength of the electric field. Field theory versus “action at a distance.”

21 1/9/07184 Lecture 221 Algunas propiedades de las lineas de campo  La intensidad del campo eléctrico está representada por la densidad de líneas  La dirección del campo eléctrico es tangente a las líneas de campo Strong Weak

22 1/9/07184 Lecture 222 Lineas de campo: carga puntual 2D 3D