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Phy107Spr06Lect35 - Particles and fields W e h a v e t a lk...

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1 Fri. Apr. 21, 2006 Phy107 Lect. 35 1 Particles and fields We have talked about several particles Electron, photon, proton, neutron, quark Many particles have internal constituents Not fundamental: proton and neutron We have talked about various forces Electromagnetic, strong, weak, and gravity And some fields… Electric field Magnetic field Fri. Apr. 21, 2006 Phy107 Lect. 35 2 Modern view is that particles, fields, and forces are intertwined. The electromagnetic field is quantized. It can be in many different energy states Ground state (lowest energy) First excited state (next highest energy) Fri. Apr. 21, 2006 Phy107 Lect. 35 3 Interactions between charges The positively charged rod attracts negative charges to the top of the electroscope. This leaves positive charges on the leaves. The like-charges on the leaves repel each other. attractive force between positive and negative charges. repulsive force between two positive or two negative charge Why did the electrons flow? Fri. Apr. 21, 2006 Phy107 Lect. 35 4 Force between charges Opposite charges attract Like charges repel. Other than the polarity, they interact much like masses interact gravitationally. Force is along the line joining the particles. Gravitational force : F G =GM 1 M 2 / r 2 G =6.7x10 -11 Nm 2 / kg 2 Electrostatic force: F E = k Q 1 Q 2 / r 2 k = 9x10 9 Nm 2 /C 2 + Force on positive particle due to negative particle Fri. Apr. 21, 2006 Phy107 Lect. 35 5 Electric field lines Faraday invented the idea of field lines following the force to visualize the electric field. Field lines emanate from positive charge and terminate on negative charge. Local electric field is same direction as field lines. Force is parallel or antiparallel to field lines. Charged particle will move along these field lines. Fri. Apr. 21, 2006 Phy107 Lect. 35 6 Magnetism: Permanent magnets North Pole and South Pole This is the elementary magnetic particle Called magnetic dipole (North pole and south pole) There are no magnetic ‘charges’ N S N S N S S N Likes attract Unlikes repel
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2 Fri. Apr. 21, 2006 Phy107 Lect. 35 7 Field lines of a magnet Field lines indicate direction of force Density indicates strength of force Similar to electrostatic force, but force is felt by magnetic dipole Fri. Apr. 21, 2006 Phy107 Lect. 35 8 Particles and fields But what are particles and forces? Is there are way to describe the two at once? An answer lies in considering everything as fields. Particles are quanta of a corresponding field. What does this mean? Think about photons. One photon means the electromagnetic field has (Planck’s const)x(frequency) = h f of energy. Two photons means 2h f of energy. Fri. Apr. 21, 2006 Phy107 Lect. 35 9 Quanta of the EM field Possible energies for green light ( λ =500 nm) E = hf E =2 hf E =3 hf E =4 hf One quantum of energy: one photon Two quanta of energy two photons – etc A photon is an excitation of the EM field.
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