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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

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Charge (2)

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  1. 184 Lecture 2

  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 184 Lecture 2

  3. “Ley de las cargas” - - - + + + m1 m2 • Cargas del mismo signo se repelen y de signo opuesto se atraen • Nótese que la electricidad es algo diferente de la gravitación …. 184 Lecture 2

  4. Electrostatica +q - - - 0 - + - Polarización! + + + + • Cual es la fuerza entre un objeto eléctricamente cargado (q) y un objeto neutro (0)? • Nótese que siempre es atractiva !! • Por qué? 184 Lecture 2

  5. Carga del Electrón • Se puede definir la unidad de carga en terminos de la carga de un electrón**… • 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 184 Lecture 2

  6. 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

  7. 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 184 Lecture 2

  8. La estructura atómica • Sistema planetario infinitesimal??! • Electricamente neutro: electrones ‘orbitando’ alrededor de un nucleo ‘en reposo’ • Ejemplo: 12C tiene 6 protones, 6 neutrones and 6 electrones • Isotopos 184 Lecture 2

  9. 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

  10. Dielectricos, Semiconductores, Conductores, Superconductores Chip con millones de transistors Replica del 1er transistor en 1947 • 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 184 Lecture 2

  11. Ley de Coulomb • Fuerza eléctrica entre cargas puntuales… • dependencia 1/r2 como en gravitación • K constante de Coulomb 0 permitividad del vacío 184 Lecture 2

  12. 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) 184 Lecture 2

  13. Campo Eléctrico + + Test charge q • 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 184 Lecture 2

  14. Algunas propiedades de las lineas de campo Weak Strong • La intensidad del campo eléctrico está representada por la densidad de líneas • La dirección del campo eléctrico es tangente a las líneas de campo 184 Lecture 2

  15. Lineas de campo: carga puntual 3D 2D 184 Lecture 2

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