“modern” physics means physics based on the two …...modern physics.notebook 1 may 14, 2019...
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Modern Physics.notebook
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May 14, 2019
Modern Physics
“Modern” physics means physics based on the two major breakthroughs of the early the twentieth century: relativity and quantum mechanics.
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WaveParticle Duality of Energy and MatterLight, a form of electromagnetic radiation can be represented as a wave propagated by a changing electric and magnetic field. Waves have frequencies, amplitudes, wavelengths and velocity. Light also experience interference and diffraction patterns.
The Particle Nature of LightThe wave model of light cannot explain other phenomena such as interactions between light and matter. In these interactions, electromagentic radiation acts as though it is a particle with kinetic energy and momentum. For example, when light strikes matter, some of the lights momentum is transferred to the matter.
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The Photoelectric EffectThe experiment that demonstrated the particle nature of light. Awarded Einstein the Nobel Prize in Physics.
Light incident of certain materials would cause the the material to release electrons. It was discovered that the frequency determined whether electrons were released and the amplitude determined the rate at which it was released.
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Quantum TheoryQuantum theory assumes electromagnetic energy is emitted from and absorbed by matter in discrete amounts or packets called a quantum (quanta) of energy. The amount of energy is directly proportional to the frequency of the EM radiation. The proportionality between them is known as Planck's Constant.(h)
E = hf
The quantum, or the basic unit, of EM energy is called the photon. Although the photon is a massless particle of light it behaves as though it has Kinetic Energy and Momentum.
Because these energy values will be relatively low, we will see the Electronvolt (eV) used a lot but remember we must convert eV into Joules.
ex: The energy of a photon is 2.11 eVs.
a. Determine the energy in Joules
b.Calculate the frequency of the photon.
c. Identify the color of the photon.
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PhotonParticle Collision The photoelectric effect demonstrates that when a photon in the visible light range is incident on a metal surface, the photon's energy is completely absorbed and transferred to the emitted electron. However, when Xray photons, which have a much higher frequencies and energies, strike a metal surface, not only are electrons ejected but EM of lower frequency is also given off.
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Particles have a wave nature!As it turns out all things have both particle and wave natures! That means you are also a particle and a wave. However the more massive the particle the less it will behave like a wave and the more it will behave like a particle.
Electrons, neutrons and protons all have wave like properties with electrons showing the most diffraction and interference pattern due to its low mass.
λ = h/p
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Models of the AtomThe atom is the smallest particle of an element that retains the characteristics of that element. Models of the atom have changed over the centuries as science has developed better ways to experiment on a particle too small to see with the unaided eye
Thomson's Model
Plum Pudding Model atom was a distribution of positive and negative charges.'
Rutherford Model
The Rutherford Gold Foil Experiment
Alpha particles were shot at thin gold foil, the idea being that if the atom were solid, very few particles should penetrate through the foil. However, most particles went through the foil and deflected at different angles. Rutherford postulates that the atom is mostly empty space with a dense positively charged nucleus.
The Bohr Model (incorrect but we use it for its simplicity)Made several key assumptions:
> All forms of energy is quantized, that is, electrons can only gain or lose energy in exact amounts.
>The electron in hydrogen can only exist in very specific radii, and no where else.
>The electron can only jump from one radius to another by absorbing exact energy from a photon.
>Each orbit corresponds to a different energy level, the one closest to the nucleus has the lowest energy while those further away have greater energy
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May 14, 2019The Energy Level Diagram for Hydrogen and Mercury (ALL OR NOTHING)
The different energy levels and their values are represented by the energy level diagrams above. In order for an electron to jump energy levels it must absorb an EXACT amount of energy in the form of a photon and jumps to discrete energy levels. The amount of energy required can be calculated by taking the difference between any two energy levels. When an electron drops, it will release energy in the form of a photon. The photon energy will be:
Ephoton = Efinal Einitial (eVs)
Examples:
1. What energy must an electron in the ground state of hydrogen absorb in order to go to the n = 4 energy level?
2. How much energy is required to ionize an electron from ground state in a hydrogen atom?
3. Can an electron in mercury accept a photon of energy .91 eVs? if so, where will it jump to?
4. An electron drops from the n=5 to the n=2 energy level in hydrogen. Determine the color of the photon.
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Emission and Absorption Spectrum
Every element has unique spectral lines!!! These distinct lines show that atoms can only absorb/emit discrete energies!!
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Emission Spectra (Bright Line Spectra) Different Frequencies are observed against a dark background
Absorption Spectra black lines on bright background
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Investigating the AtomThe Nucleus
The Rutherford experiment demonstrated that the nucleus has a positive charge. The nucleus is made up of at least one proton and some neutrons. The protons and neutrons that make up the nucleus are called nucleons.
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What keeps the nucleus together?
Strong Nuclear Force
The positively charged protons within the nucleus are extremely close together.(1015 m distance) Therefore there is a strong electrostatic force of repulsion between them. The gravitational force is not enough to keep two protons together therefore there must exist a force strong enough to keep the protons from violently separating. This force is known as the Strong Nuclear Force(Strong Force). It is 100x more powerful than the electrostatic force however it is only present in the nucleus and over extremely short distances.
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A new unit!!!! The universal mass unit.Just like we use eV to describe small amounts of energy instead of saying 1.6 * 10 19J we redefined it as 1eV. We will do the same for mass. Because atoms and their constituents have such small masses it makes sense to use another unit for the mass called the universal mass unit , u. 1u = 1.66 * 1027 kg.
Masses of subatomic particles in u, convert them into kg.
Proton: 1.0073u
Neutron: 1.0087u
Electron: .0005 u
REMEMBER, this is only for convenience, if we do calculations we must use kg!!!!!!!!!!!!!!!!
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EnergyMass Equivalence
Here's an example. If you could turn every one of the atoms in a paper clip into pure energy—leaving no mass whatsoever—the paper clip would yield 18 kilotons of TNT. That's roughly the size of the bomb that destroyed Hiroshima in 1945. On Earth, however, there is no practical way to convert a paper clip or any other object entirely to energy. It would require temperatures and pressures greater than those at the core of our sun.
Einstein showed that energy and mass are the same thing. That is, energy can be converted to mass and viceverse. As it turns out there is an equation that can tell us how much mass we get from the conversion of mass into energy.
E = mc2
example: convert 1 u into megaelectronvolts
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Applications of the energymass equivalence
Atomic Fusion the combination of two smaller nuclei to form a larger nuclei. Happens in the sun. Requires immense pressures and temperatures
When two nuclei fuse, the mass of the resulting nucleus is actually less than the mass of the individual parts added together. This is because part of the mass got converted into energy this is known as the mass defect.
Example: A helium nucleus is composed of two protons and two neutrons and has a mass of 4.0015u. If a proton has a mass of 1.0073u and a neutron has a mass of 1.0087u. Determine the mass defect. How much energy energy does that equal?
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Atomic Fission
Fission is the breaking down of large nuclei by collision with a neutron into smaller particles. This releases A LOT of energy which again could be calculated by E = mc2 the amount of energy released is the "missing mass"(mass defect) converted into energy.
ex: how much energy is released from converting 1kg of oil into energy?
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The beautiful but destructive nature of science.
“Now I am become Death, the destroyer of worlds” J. Robert Oppenheimer (originally from the Bhagavad Gita)
While we use nuclear energy today for our power plants. This energy was utilized and continued to be ultilized for it destructive power.
The atomic bomb