AQA Physics A-level Section 12: Turning Points in Physics Key Points
Discovery of electrons A discharge tube contains a low-pressure gas with a high potential difference across it. Electrons are pulled out, ionising some of the gas atoms creating positive ions which accelerate towards the cathode. The electrons are accelerated towards the anode - these beams are called cathode rays. They collide with other gas atoms, exciting them to higher energy levels or ionising them; when they de-excite, they emit photons. A heated filament causes thermionic emission of electrons when current passes through it and heats it up. An anode with a high potential difference accelerates the electrons towards it in what is known as an electron gun. One electron volt is the work done on an electron accelerating it through a potential difference of of one volt.
Specific charge of an electron Significance of Thomson's findings The electron has a much higher specific charge than a H + ion. He concluded that electrons were negatively charged particles which were fundamental to all matter and that electrons had a much lower mass.
Millikan s oil-drop experiment When the p.d was turned on, the droplets were stationary: At terminal velocity: Using the results, Millikan identified that the charges of the oil droplets came in multiples of 1.6 x10-19 C. The Oil droplets were ionised by friction. He therefore concluded that the charge is quantised with the smallest quantum of charge being 1.6 x10-19 C. This is the charge carried by one electron.
Newton s theory of light Newton believed light was made up of coloured corpuscles. He explained reflection by saying that the component of their velocity perpendicular to the surface was reversed by a repulsive force. He explained refraction by saying that corpuscles accelerated in the direction perpendicular to the surface due to an attractive force, causing them to change direction as the parallel component stayed the same. Newton s theory of light was preferred as he was well respected, so his ideas were easily accepted without rigorous debate from the scientific community. Newton s theory couldn't explain diffraction and was based on the idea that light traveled faster in denser mediums, something which is now proved to be false.
Huygens theory of light Huygens believed light was a wave made from wavelets, that each spread out forwards in the direction of the wave. Despite the evidence from Young's double slit experiment, his theory was not yet accepted. This is partly because Newton's theory was seen to be more credible, and that Huygens thought of light as a longitudinal wave, despite it being able to be polarised. He also could not explain the formation of sharp shadows. The wave theory was only accepted when Fizeau and Foucault measured the speed of light in air and water and found that it was slower in water, providing evidence for the theory because Newton predicted light travelled faster in water.
Electromagnetic waves μ 0 - Permeability of free space ε 0 - Permittivity of free space Fizeau measured the speed of light in air using a rotating toothed wheel. It span at such a speed that the pulse of light leaving through one gap returned to the wheel after reflecting off a mirror at the instant that the next tooth blocked its passage back through. Hertz used an induction coil and a capacitor to produce a high voltage that caused sparks, emitting radio waves. He then used a wire loop to show that they had a magnetic field component as they induced an emf. The electric field could be detected using a dipole receiver.
The photoelectric effect The photoelectric effect is the process by which a metal with photons incident on it emits electrons (in the form of photoelectrons). No photoelectrons are released if the radiation s frequency isn't greater than the threshold frequency, which is the frequency at which the photon has energy equal to the work function, the minimum amount of energy needed for an electron to be emitted. Photoelectrons are released with different kinetic energies up to a maximum. The value of the maximum kinetic energy increases with the frequency of the incident photons. Intensity of radiation is the total power delivered per unit area. The number of photoelectrons emitted per second increases as intensity increases. This goes against the original wave model of light, which predicts there would be no such thing as a threshold frequency because the metal would gradually accumulate enough energy to release electrons. However this doesn't happen, meaning it must be described in terms of particles where one electron absorbs one photon.
The photoelectric effect can be demonstrated with a UV lamp and a negatively charged zinc plate with a gold leaf. UV light is shone on it and the gold leaf returns to the vertical position. Demonstrations The photocell also shows the photoelectric effect. UV radiation is incident on the negatively charged plate which allows electrons to flow to the positive cathode. The stopping potential lets us measure the maximum kinetic energy. The circuit is set up but the power supply is reversed. The p.d causes a force on the electrons back towards the plate. Eventually, when no current flows then the product of the stopping potential and electron charge equals the max Ek of the electrons.
Ultraviolet catastrophe A black body is a body that absorbs all electromagnetic radiation and can emit all wavelengths of electromagnetic radiation. The wave theory of light predicted that the power radiated was proportional to λ -4, which meant that the power would tend to infinity as the wavelength approached the UV region. This was both mathematically and experimentally shown to be incorrect. Planck's solution was that the energy of electromagnetic waves was quantised, not continuous, and that the size of a quantum of electromagnetic energy is given by E=hf.
Wave particle duality Young's double slit experiment and the photoelectric effect show that light behaves as a particle and a wave. Electron diffraction and electrons being deflected by an electric field show that matter behaves as both a particle and a wave. High p.d. Low p.d. Increasing the voltage decreases the de Broglie wavelength which means the electrons diffract less and the diameter of the rings is smaller.
Electron microscopes Transmission Electron Microscope (TEM) Scanning Tunnelling Microscope (STM) Condenser lens - focuses electrons Objective lens - magnifies image Magnifying lens - magnifies and focuses image The smaller the wavelength, the better the resolution. The wavelength to see atoms needs to be 0.1 nm and therefore the anode potential needs to be about 150V. A STM uses quantum tunnelling to map the shape of a surface. Electrons transfer from the negative probe to the positive surface due to their wavelike nature. To map the surface, the STM either fixes the height and measures the current (constant height mode), or fixes the current and measures the probe height (constant current mode).
Michelson-Morley experiment The ether was postulated as the medium for light and as a fixed background against which absolute motion could be measured. The Michelson-Morley interferometer was designed to detect differences in the speed of light when travelling parallel and perpendicular to the Earth's motion, by creating a shifting interference pattern when it was rotated. Rotating the apparatus should have meant that one beam would take longer than the other to travel. This would have changed the phase difference between them causing a shift in the interference pattern. However no shift was observed and it was concluded that the ether doesn't exist, and the speed of light is invariant for all observers.
Special relativity An inertial frame of reference is one which is not accelerating. Special relativity only works in inertial reference frames and has two postulates - that the laws of physics have the same form in all inertial reference frames and the speed of light in free space is invariant and does not depend on the motion of the source or of the observer. Muon decay: Muons have a half life of around 2µs so you would expect to see very few muons at the earth's surface. However, because they travel close to the speed of light and special relativity is considered, many more are observed. Twin Paradox: A twin who traveled around the universe at close to the speed of light would have aged less than her sister who stayed on earth.
Other effects of relativity Length Contraction A stationary observer will see the proper length, l 0. An observer moving at a constant velocity will measure a shorter length, l. Mass Increase An object with rest mass m 0 moving at a constant velocity has a greater, relativistic mass, m. Energy As an object approaches the speed of light, additional energy increases its mass more than its speed. length Electrons with different kinetic energies were fired at an aluminium disk that could measure their energy by the heat dissipated. The speed increased asymptotically to the speed of light, meaning their mass must have increased.