Turning points

Physics Flash Cards: Section 12 Turning Points

What was the cathode ray? The glow observed at the cathode of a discharge tube when a potential difference is applied across it. How was a discharge tube made to glow? A high potential difference across the tube pulls electrons off gas atoms, the resulting ions are accelerated towards the cathode, releasing more electrons, the free electrons collide with gas atoms exciting them, the atoms de-excite releasing photons of light. What is thermionic emission? The emission of electrons from the surface of a metal when it is heated. How does an electron gun work? It uses thermionic emission to release electrons from a heated cathode, which are then accelerated towards an anode by a potential difference. What is the specific charge of an electron? The ratio of the charge of an electron to its mass, which is approximately 1.76 x 10^11 Ckg-1. Why was Thomson's determination of the specific charge of the electron significant? It showed that the specific charge was constant regardless of the gas used to produce the electrons, demonstrating that all atoms contain electrons. How did Millikan determine the charge of an electron? He used an oil drop experiment where the weight of a charged oil droplet was balanced by an electric force, and by measuring the terminal velocity of the droplet when the electric field was removed, he could calculate its charge. What did Millikan's results show? The charge of all oil droplets was an integer multiple of 1.60 x 10^-19 C, showing that charge is quantised and exists in discrete packets. What was Newton's corpuscular theory of light? Light is made up of tiny particles called corpuscles. How did Newton explain reflection and refraction using his corpuscular theory? Reflection - corpuscles collide with a surface and are repelled back, changing their velocity perpendicular to the surface. Refraction - corpuscles are attracted to a denser medium, increasing their velocity perpendicular to the surface and causing the light to bend towards the normal. What was Huygens' wave theory of light? Light is a wave and every point on a wavefront is a source of secondary wavelets that spread out to form the next wavefront. What was the significance of Young's double slit experiment? It demonstrated the diffraction and interference of light, supporting Huygens' wave theory and contradicting Newton's corpuscular theory. Why wasn't Huygens' theory widely accepted initially? Newton's reputation and the lack of measurements of the speed of light in different mediums. What are electromagnetic waves? Transverse waves formed of alternating electric and magnetic fields travelling in phase and at right angles to each other. What is the formula for the speed of electromagnetic waves in a vacuum? c = 1/√(μ0ε0) where μ0 is the permeability of free space and ε0 is the permittivity of free space. How did Hertz discover radio waves? By using an apparatus that produced high voltage sparks across a gap of air, generating radio waves that could be detected with a dipole receiver or a loop of wire. How did Hertz confirm that radio waves were EM waves? He measured their speed using stationary waves and found it to be the same as Maxwell's predicted value for the speed of light. How did Fizeau measure the speed of light? By using a rotating toothed wheel to create pulses of light that travelled to a distant mirror and back, and by measuring the speed of rotation required for the returning pulses to be blocked by a tooth, he could calculate the speed of light. What was the significance of Fizeau's result? It was very close to the value predicted by Maxwell, supporting the idea that light is an EM wave. What was the ultraviolet catastrophe? The prediction of classical wave theory that the intensity of radiation emitted by a black body would increase infinitely as the wavelength decreased, which contradicted experimental observations. How did Planck resolve the ultraviolet catastrophe? He proposed that EM waves travel in discrete packets called quanta with energy directly proportional to their frequency (E = hf). What aspects of the photoelectric effect could wave theory not explain? 1. The existence of a threshold frequency. 2. The immediacy of photoelectron emission. 3. The independence of photoelectron speed on light intensity. 4. The range of kinetic energies of photoelectrons. How did Einstein explain the photoelectric effect? He proposed that EM waves are released as discrete packets called photons that have particle-like interactions, and the energy of a photon is transferred completely to an electron upon interaction. What is stopping potential? The potential difference required to stop the photoelectrons with the maximum kinetic energy from being emitted. How can a graph of stopping potential against frequency confirm Einstein's photon theory? It produces a straight line with a gradient of h/e, a y-intercept of -Φ/e, and an x-intercept equal to the threshold frequency. What is the de Broglie hypothesis? All particles have both wave-like and particle-like nature, and the wavelength of a particle is given by λ = h/mv. What experimental evidence supports the de Broglie hypothesis? Electron diffraction, where electrons exhibit wave-like behaviour by diffracting through a crystal lattice. How does the accelerating voltage affect electron diffraction? Increasing voltage decreases the de Broglie wavelength and fringe spacing, while decreasing voltage increases wavelength and spacing, consistent with wave theory. What is the resolving power of a microscope? Its ability to distinguish between closely spaced structures. Why do electron microscopes have higher resolving power than light microscopes? The wavelength of an electron beam is much smaller than that of light. What are the two types of electron microscopes? Transmission electron microscope (TEM) and scanning tunnelling microscope (STM). How does a TEM work? It uses an electron gun and magnetic lenses to focus a beam of electrons that pass through a thin sample, forming a magnified image on a fluorescent screen. What factors limit the resolving power of a TEM? Sample thickness and electrons travelling at a range of speeds. How does an STM work? It uses quantum tunnelling of electrons between a fine-tipped probe and the surface of an object to image the surface. What are the two modes of operation for an STM? Constant height mode and constant current mode. What was the Michelson-Morley experiment designed to measure? The absolute speed of the Earth through the ether, a hypothetical substance thought to permeate the universe. What did the Michelson-Morley experiment show? There was no shift in the interference pattern regardless of the orientation of the apparatus, contradicting the idea of an ether. What are the postulates of Einstein's theory of special relativity? 1. The speed of light in free space is invariant. 2. The laws of physics have the same form in all inertial frames of reference. What is time dilation? A consequence of special relativity where time runs at different speeds depending on the relative motion of the observer. What is proper time? The time measured by an observer who is stationary relative to the event being measured. What experimental evidence supports time dilation? Muon decay, where muons travelling at high speeds decay slower due to time dilation. What is length contraction? Another consequence of special relativity where the length of an object moving at high speed appears shorter to an external observer. What is proper length? The length of an object as measured by an observer who is at rest relative to the object. How does muon decay provide evidence for length contraction? Muons travelling at high speeds experience length contraction, making the distance they travel appear shorter, which affects their decay rate. What is the relationship between mass and energy according to special relativity? E = mc^2, meaning mass and energy are interchangeable. What is relativistic mass? The mass of an object as it increases with its speed. Why does the classical formula for kinetic energy not apply at relativistic speeds? The mass of the object changes significantly. How did Bertozzi's experiment provide evidence for the increase in mass with speed? He measured the kinetic energy of electrons at various speeds by measuring the heat they transferred to an aluminium target and found his results matched Einstein's predictions. Can an object reach the speed of light according to special relativity? No, because as its speed approaches the speed of light, its mass and energy approach infinity.