Electromagnetic Radiation and Atomic Physics

Transcription

1 Electromagnetic Radiation and Atomic Physics Properties of Electrons, Protons, and Neutrons (The Main Constituents of Ordinary Matter) Mass Electrons have a mass of kg. The mass of a proton is 1836 times the mass of an electron. The mass of the neutron is 1839 times the mass of the electron. Charge Electrons are negatively charged. Protons are positively charged. Neutrons are uncharged. 1

2 How do electrically-charged particles influence each other? There are two kinds of electrical charge positive and negative. Charges with the same sign repel each other. Charges with opposite signs attract each other. Uncharged particles exert no electrical force on each other or on charged particles kq1q F = Coulomb's Law r ( ) An electric field is the property of space by means of which one electrically - charged particle exerts a force on another electrically-charged particle. The charge of particle 1 changes the space around it, giving it the property we call the electric field. The electric field of particle 1 exerts a force on particle. The electric force on a positively charged particle is in the direction of the electric field. A magnetic field is the property of space by means of which one moving charged particle exerts a magnetic force on another moving charged particle. The motion of particle 1 produces a magnetic field in space. That magnetic field exerts a magnetic force on moving particle. The magnetic force on a moving charged particle is in the direction perpendicular to the magnetic field and to the particle s velocity. Because of this, charged particles tend to spiral around magnetic field lines.

3 A wave is something that transfers energy and momentum from one point to another without the motion of mass between the two points. A wave transfers a disturbance rather than matter. Wt Water waves A pebble dropped into some water disturbs the water, causing ripples to move away from the point of impact. The disturbance is an up and down motion of the water. Sound Waves The vibration of the head of a drum alternately compresses and rarifies the air in contact with it. The series of compressions and rarefactions moves away from the drumhead. When these compressions and rarefactions reach an eardrum, the eardrum moves back and forth with the same frequency as the drumhead. Electromagnetic Waves and Photons An electromagnetic wave consists of oscillating electric and magnetic fields. E Field B Field c f = λ c = speed of light in vacuum λ = wavelength λ Direction of Wave Motion n = index of refraction v = speed of light in matter c v = n An electromagnetic wave consists of a stream of photons. E energy of a photon hc E = hf = λ = 34 h = J s 3

4 The Electromagnetic Spectrum Electromagnetic radiation with wavelengths between 400 nm and 700 nm is visible. In order of decreasing wavelength (increasing frequency), the colors are red (700 nm), orange, yellow, green, blue, violet (400 nm). γ ray X-ray UV Infrared μ wave radio Visible o 4000A o 7000A Types of Electromagnetic Radiation Visible (400 nm 700 nm) Visible (400 nm 700 nm) Near infrared (700 nm nm) Far infrared (more than nm) Near Ultraviolet (400 nm 90 nm) Far ultraviolet (90 nm 10 nm) X rays ( 10 nm 0.01 nm) y ( ) Gamma rays (less than 0.01 nm) 4

5 Atoms A neutral atom consists of a nucleus, made of protons and neutrons, and enough electrons to neutralize the positive charge of the protons. The diameter of the nucleus is about 1 femtometer (1 fm = m). The cloud of electrons, on the other hand, occupies a volume with a diameter of about 0.1 nm (1 nm = 10-9 m). Since m p = 1836 m e and m n = 1839 m e, most of the mass of an atom is concentrated in its nucleus. An ion is an atom that has either a deficit or a surplus of electrons. The process of removing an electron from an atom is called ionization. The number of protons in the nucleus determines the chemical identity of the atom. The Bohr Model of the Hydrogen Atom n = 3 n = n = eV E n = n = 1,,... n 19 1 ev = J ( ) The electron in a hydrogen atom moves in a circular orbit. The radius can only have one of a set of discrete values, r n (n = 1,, 3, ). n = 1, the lowest possible orbit, is called the ground state of the atom. The states above the ground state are called excited states. The electron can transfer to a higher orbit if it absorbs a photon of energy equal to the energy difference between the two orbits. When an electron transfers to a lower orbit, it emits a photon with energy equal to the energy difference between the two orbits. A transfer from one orbit to another is called a transition. 5

6 n = 5 4 n = 3 n = An energy level diagram is a graphical representation of the energies that are available to the electron in the atom. The following is an energy level diagram for the hydrogen atom. n = 1 Paschen Series - infrared The higher energy levels are increasingly close together and Balmer Series - visible blend into the continuum at an αβγ energy of 13.6 ev above the ground state. When a hydrogen Lyman Series - ultraviolet atom absorbs a photon with energy greater than or equal to 13.6 ev, the electron is removed from the atom ev is the ionization potential of the hydrogen atom. Any atom can be described in terms of its energy levels. He, for example, has twice as many protons as H. Its energy levels are closer together, and the ionization potential for removing one electron from a neutral He atom in its ground state is 4.5 ev. λ Hα = 656.3nm λ Hβ = 486.1nm λ = 434.0nm Hγ Kirchoff s Rules I. A solid, liquid, or dense gas has a continuous spectrum. II. A hot, low-density gas has an emission (bright line) spectrum. III. When light with a continuous spectrum passes through a cool gas, the result is an absorption spectrum. 6

7 The dominant component of a star s spectrum is a set of absorption lines. The visible lines in the absorption spectrum of the Sun are shown below. 7