Unit 4 Flashcards
What is an anti-particle?
A subatomic particle having the same mass as a given particle but opposite electric or magnetic properties.
Every kind of subatomic particle has a corresponding antiparticle, e.g. the positron has the same mass as the electron but an equal and opposite charge.
Describe the twin paradox.
Imagine a pair of twins, one of whom is an astronaut that travels to a distant star. The stay-at-home twin will see his brother age more slowly than him. You’d expect the space-faring twin to see the same happen to his Earth-bound counterpart since they’re moving at the same speed relative to one another. But if the twins are re-united, Einstein said that the space-faring twin will have aged less than the one on Earth, which is odd given that they’ve both performed identical journeys relative to each other.
Einstein’s prediction has been confirmed in experiments with atomic clocks, so what resolves this paradox? The answer lies in the fact that the twins don’t undertake identical journeys. To get back to Earth, the travelling twin experiences a force in order to slow down and reverse direction (acceleration). The stay-at-home twin doesn’t, making their journeys fundamentally different. Not surprisingly, so are the relative travel times of the twins, thus one of them ages more.
Describe Simultaneity.
Two flash lamps with observer A midway between them are on a rail car that moves to the right relative to observer B. The light flashes are emitted just as A passes B, so that both A and B are equidistant from the lamps when the light is emitted. Observer B measures the time interval between the arrival of the light flashes. According to postulate 2, the speed of light is not affected by the motion of the lamps relative to B. Therefore, light travels equal distances to him at equal speeds. Thus observer B measures the flashes to be simultaneous.
A girl as observer A is sitting down midway on a rail car with two flash lamps at opposite sides equidistant from her. Multiple light rays that are emitted from respective flash lamps towards observer A . A male observer B standing on the platform is facing her. Now observer A moves with the lamps on a rail car that is as the rail car moves towards the right of observer B. Observer B receives the light flashes simultaneously, but he notes that observer A receives the flash from the right first. B observes the flashes to be simultaneous to him but not to A.
Now consider what observer B sees happen to observer A. She receives the light from the right first, because she has moved towards that flash lamp, lessening the distance the light must travel and reducing the time it takes to get to her. Light travels at speed c relative to both observers, but observer B remains equidistant between the points where the flashes were emitted, while A gets closer to the emission point on the right. From observer B’s point of view, then, there is a time interval between the arrival of the flashes to observer A. Observer B measures the flashes to be simultaneous relative to him but not relative to A. Here a relative velocity between observers affects whether two events are observed to be simultaneous. Simultaneity is not absolute.
Describe Youngs Double Slit experiment
Young’s original double-slit experiments were in fact the first to demonstrate the phenomenon of interference.
When he shone light through two narrow slits and observed the pattern created on a distant screen, Young didn’t find two bright regions corresponding to the slits, but instead saw bright and dark fringes.
He explained this unexpected observation by proposing that light is a wave, in opposition to Newton’s idea that light is made of particles. Bright fringes were created by constructive interference and dark fringes were created by destructive interference.
Describe the Bohr model of the atom.
In the Bohr model of the atom, electrons travel in defined circular orbits around the nucleus. The orbits are labelled by an integer, the quantum number n. Electrons can jump from one orbit to another by emitting or absorbing a specific amount of energy (specific frequency of light).
Describe Rutherford’s Model.
The Rutherford model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
Describe Rutherford’s Gold Foil experiment and its results
In 1911, Rutherford bombarded very thin sheets of gold foil with fast moving alpha particles. Alpha particles are positively charged particles with a mass about four times that of a hydrogen atom.
According to the accepted atomic model, in which an atom’s mass and charge are uniformly distributed throughout the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all.
Surprisingly, while most of the alpha particles were indeed undeflected, a very small percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Some were even redirected back toward the source.
Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom’s interior, which he called the nucleus. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons.
What were the problems with Rutherford’s atomic model?
Although very useful, the planetary model of the atom as proposed by Ernest Rutherford had two very important flaws.
Firstly, the planetary model of the atom failed to explain why individual atoms produce discrete line spectra. In fact, according to Rutherford’s model, each individual atom should produce a continuous line spectrum.
The second flaw to his model was the fact that electrons orbit the nucleus in a circular fashion. If this was in fact true, then as electrons accelerate around the nucleus, the acceleration would cause the electron to radiate energy, which would imply that the electron would continually lose energy as per the conservation of energy law. Therefore, as the electron orbits, it continually loses kinetic energy and its velocity would decrease, causing it to spiral and eventually smash into the nucleus. This would imply that atoms are naturally unstable. Since atoms are generally stable, this means that the model does not correctly describe the structure of the atom.
What is a black body?
A Blackbody in physics is a surface that absorbs all radiant energy falling on it. The term arises because incident visible light will be absorbed rather than reflected, and therefore the surface will appear black.
Describe the Ultraviolet catastrophe.
The ultraviolet catastrophe is the name given to a conflict between theory and the observation in classical physics.
An ideal black body at thermal equilibrium emits radiation in all frequency ranges. It emits more energy as the frequency increases. By calculating the total amount of radiated energy, it can be shown that a blackbody would release an infinite amount of energy.
This contradicted the principles of conservation of energy and showed a new model was needed for the behaviour of blackbodies.
It led theorists like Max Planck to come up with quantum theory in which energy leaves the body in distinct packets called quanta rather than in continuous waves.
Discuss how light intensity and frequency would impact the photoelectric effect on a sheet of metal.
The remarkable aspects of the photoelectric effect when it was first observed were:
1. The electrons were emitted immediately - no time lag! 2. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy! 3. Red light will not cause the ejection of electrons, no matter what the intensity! 4. A weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths!
Analysis of data from the photoelectric experiment showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron!
What are the six types of quarks?
- Up
- Down
- Top
- Bottom
- Strange
- Charm
What is a Baryon?
Baryon, any member of one of two classes of hadrons (particles built from quarks and thus experiencing the strong nuclear force).
Baryons are heavy subatomic particles that are made up of three quarks.
Both protons and neutrons, as well as other particles, are baryons.
What is a Meson?
Mesons are a group of fundamental particles (such as the pion and kaon) made up of a quark and an antiquark that are subject to the strong force
What are the four gauge bosons?
- Gluon (Strong Force)
- Photon (Electromagnetic Force)
- W Boson (Weak Force)
- Z Boson (Weak Force)
