Flashcards
Special Relativity
Flashcards do conjunto (18)
- 1What was the Victorian idea of the aether (ether)?A fixed background of material that permeated the universe. It was a medium thjriugh which light travelled (by analogy with sound was that travel thirugh air). An extra consequence of the aether idea is provided an absolute reference frame for any motion: eg the earth travels through the stationary aether. It was thought that light travelled through this stationary at 3 x 10 8 ms-1. So did all moving objects.
- 2What was the aim of the Michelson-Morley experiment?To prove there was an ether material in which light travelled by showing that the measured speed of light was different if measured by an instrument oriented in different directions.
- 3What did the ether idea predict about the measured speed of light if an observer was moving towards a beam of light at speed v relative to the ether?The speed of light relative to the ether is c and the speed of the observer is -v relative to the ether. So, the measured speed between observer and light beam would be c --v = c + v
- 4What did Michelson and Morley expect to measure?When rotating the glass blocks and mirrors through 90 degrees, they expected the speed of light to be different and for this to change the interference pattern
- 5What was the actual result of the Michelson-Morley experiment and what conclusion was drawn?The interference pattern of light did not change when the kit was rotated through 90 degrees; thus the speed of light was constant regardless of observer's viewpoint. There was thus 1. no ether and no absolute motion 2. a constant value for the speed of light for all observers, regardless of their motion
- 6What is an inertial reference frame?an inertial frame is not accelerating. Thus, it will appear to be stationary although it may have a constant velocity relative to another observer outside the frame
- 7What are the two postulates of Einstein's Special Relativity?1. The laws of physics are identical in all inertial reference frames (e.g. a train going in a straight line at constant velocity relative to an observer; no experiment can be performed to prove that a passenger on the train is moving.) 2. The speed of light is constant in free space (it is invariant). This means that you could measure the speed of light of a beam on the train and also from a light shone from a train's platform - you would get the same speed.
- 8When an observer on a station platform observes a train moving by at constant velocity. The train traveller drops a ball and it falls at her feet. She measures the time interval to fall, as does the platform person observing trough the window. What is different about the platform person's time measurements compared with the train traveller's?The platform person measures a longer time interval. The platform observer's time is dilated. The train traveller measures a shorter time interval
- 9A spacecraft whizzes by an observer at a speed of 0.9c relative to an observer. The space traveller measures the time interval when he drops a ball which falls to the floor of the spacecraft. It is 0.5s. What time interval is measured by the outside observer?t = to/(√(1 - v2/c2) = 1.1 s
- 10What is time dilation?For a stationary observer, a longer time interval for an event on a moving vehicle is measured than the time interval measured by a traveller on the "vehicle" itself.
- 11How does muon decay provide evidence for time dilation?Muon's decay into electrons (and neutrinos) with a fixed half life in proper time. When observed moving through the atmosphere, fewer are measured to decay than expected. This is because the muons moving towards Earth have a shorter elapsed time than the dilated time measured by an observer standing on Earth. The Earth observer measures a long time interval and expects lots of muon decay; the muons experience proper time which is a much shorter time and so very few actually decay in their reference frame
- 12What happens to the length of an object measured by an observer measuring the length of a vehicle moving in the same direction as its lengthObserver measures a shorter length
- 13A space traveller measures the length of her spacecraft as 5 m. She is moving at 0.9c relative to an observer on Earth and the speed is the same direction as the length. What length s measured by the Earth observer?L = L0√(1-v2/c2) = 5 x 0.436 = 2.2m
- 14Why can ½ mv2 not be used to accurately calculate the speed given to an electron accelerated by an extremely large potential difference?A large gain in energy leads to a large gain in mass, which is not accounted for in the formula. At rest the electron has its lowest mass, i.e. the rest mass. When KE is increased there is only a small gain in mas at first, so the equation is approximately correct when speeds are very small compared to the speed of light. For larger energies, gain of KE gives rise to a large gain in mass and only a very small increase in speed.
- 15An electron is accelerated by a p.d. of 3.0 x 10 9 V. Calculate the gain in energy in Joule.E = qV = 1.6 x 10-19 C x 3.0 x 10 9 J/C = 4.8 x 10 -10 J
- 16For an electron accelerated by a p.d. of 3.0 x 10 9 V, calculate the new mass and show that it is over 5000 times its rest massE = qV = 1.6 x 10-19 C x 3.0 x 10 9 J/C = 4.8 x 10 -10 J. E = mc2. So, m = E/c2 = 4.8 x 10-10 /(3 x 108)2 = 5.3 x 10 -27 kg. Then the fractional increase is: m/m0 = 5.3 x 10-27/(9.1 x 10 -31) = 5900
- 17The fractional increase in the mass of an accelerated particle is 2. Calculate the speed.m = m0/(√(1 – v2/c2). So, 2 = 1/(√(1 – v2/c2). Thus 1 – v2/c2 = ¼. So, v2/c2 = 1 - ¼. Thus, v = 2.6 x 10 8 m/s
- 18In the 1960s Bertozzi set up an experiment to show that electrons accelerated by increasing Voltages did not gain energy according to 1/2mv2. Explain how he checked the energy input to the electrons and what he measured and plotted.He accelerated electrons in a linear accelerator using voltages of 0.5Mv to 15MV. They travelled about 8 m before colliding with an aluminium disc. He calculated the KE from E = qV and also checked it by measuring the temperature rise of the aluminium (E = m x SHC x ∆T). He calculated the speed by measuring the time taken for a pulse of electrons to travel 8m. He then plotted KE against speed and obtained a curve showing that speed did not increase according to E = 1/2mv2. He obtained a curve that had an asymptote at 3x108 ms-1 As energy increased, mass increased and E increased according to E = m0/√(1 - v2/c2)
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- GCSE Physics FlashCards sobre Special Relativity, criado por Rohan Badman em 15-09-2022.
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- Rohan Badman
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