Turning points

Discovery of an electron

  • In 1838 Faraday performed a series of experiements attempting to pass an electric current through a glass tube with some of the air pumped out.
  • discovered that gases at sufficiently low pressure in these tubes conduct electricity and emit light of a certain colour. a glow is seen at the discharge tube
    • 1) discharge tube contains low pressure gas and a p.d across it
    • 2) electrons are pulled out, ionising some of the gas atoms to create positive ions (which accelerate towards the cathode)
    • 3) Electrons are accelerated towards the anode. Beams are called Cathode rays. (Recomination)
    • 4)  OR Electrons collide with other gas atoms, exciting them to a higher energy levels or ionising them, When they de-excite, they emit photons. (de-excitation)
    • Cathode rays are beams of electrons emitted from a metal cathode when an electric current passes through a vacuum
    • in 1897 J.J. Thomson proved that electrons were negatively charged particles which were fundamental to all matter.
    • proved that electrons had the biggest specific charge
    • concluded that these cathode rays were streams of tiny, negatively charged particles called "corpuscles" which was later renamed electrons
    • cathode rays were then proved not to be the new form of electromagnetic waves since the charges could not be separated from the particles. 
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Thermionic emission

  • Thermionic emission- emission of electrons from a hot metal filament cathode.
    • current passes through the filament, heating it up and electrons gain more kinetic energy-eventually escaping the surface of the wire
    • insde the vacuum tube, there are no air particles able to interact with the emitted electrons. hence, they are repelled from the metal filament and attracted towards the opposing anode. 
    • the anode with the high p.d accelerates the electrons to it
    • W=QV                   1/2MV^2=eV
    • the queations assume that each electron has negligilbe kinetic energy in comparision with the work done by accelerating p.d.
    • the higher thhe p.d, the greater the speed as the electrons are attracted towards the anode with a greater acceleration. Gain more Kinetic energy in crossing a greater p.d
    • The speed of the electrons is also much less than the speed of light. 

 

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3 methods to determine specific charge

Deflection by a magnetic field

  • uniform magentic field into the plane, electrons fired from the electron gun and an electron beam is visible
  •  magnetic force is always perpendicular to the velocity (so it is centripetal), Magnitude of magnetic field is constant as speed is constant. no work is done on each electron by the magnetic force so Kinetic energy is constant. 
  • glass tube filled with gas at low pressure
  • The measurement of e/m using a fine beam tube was desgined to make the beam visible as a result of collisions between some electrons in the beam and a small amount of gas in the tube. 

Balanced forces due to crossed fields

  • 2 deflecting parallel plates, with the electron in the middle 
  • measure the radius first when only the magnetic field is on
  • turn on, adjust electric field to straighten out the path.
  • equation forces to get an equatio for e/m

Deflection by a unfiorm electric field

  • electric uniform field. beam of electrons produced from an electron gun with a voltage supply between 3000v-5000v. Electron beam enters tube and is subject to the vertical force due to an electric field. Force causes electrons to move upwards in a parabolic arc. 
  • turn on magentic field to straighten out the path, and find time by (length of plates/speed).
  • use suvat to find a. the vertical distance would be the vertical deflection of the beam
  • so e/m=ad/V

The value found by using fields on an electric beam to determine e/m ws found to be 1800x higher than the smallest known ion, hydrogen. Thomson's expermient provided evidence for subatomic particles. 

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Millikan's Oil drop experiment

  • in 1913
  • discovered he could control the motion of charged oil drops fom an oil spray using an electric field between oppositely charged parallel metal plates
  • he was able to make any charged droplet stay in a stationary position by adjusting the p.d between the plates until the electric force on the droplet (acting vertically) is equal and opposite to the weight. 

so Electric force= mg= stokes law ( at terminal velocity)

  • he then used x-rays to ionise the oil drops which he sprayed into the chamber using an atomiser. 
  • realised that the charges of the oil droplets came in multiples of 1.6x10^-19- therefore stating that the charge is quantisied with the smallest quantum of charge being the charge of the electron. 
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Newton's theory of light

  • in 1690
  • believed that light was made up of coloured corpuscles- which are now called photons. 
  • in reflection - the component of their velocity perpendicular to the surface was reversed by a repulsive force.
  • in refraction- the corpuscles accelerated in the direction perpendicular to the surface (due to an attractive force), causing them to change direction while the parallel component stays the same. 
  • However, his theory could not prove diffraction and was based on the idea that light travelled faster in denser mediums (which is false)

His theory was preferred to Huygen's due to him being already well-established in the scientific world already and since the speed of light could not actually be proved until 150 years later

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Huygen's Theory of light

  • believed that light was a longitudinal wave and since they need a medium to travel through, the "space" between was an Aether, which was transparent and had no mass. 
  • in this theory, waves move through different materials by the propagation of wavefronts, which spread out forwards in the direction of the wave. 
  • This theory could explain reflection, refraction, and the dispersion of light
  • However, his theory was not accpeted even after his young's double slit experiment sicne Newton was more credible and that Huygen's thought light was a longitudinal wave, even though it was able to be polarised. He also could not explain the formation of sharp shadows. 

His theory was only accepted when Fizeau and Focault measured thespeed of light in water and air, and found that it was slower in water (hence providing evidence for Huygen's Theory (1851). Newton thought light travelled faster in water

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Significance of Young's Double slit experiment

  • In 1804
  • Observed the "shadows" produced by a very narrow beam of sunlight as it passed through small holes/slits/fine-edged objects.
  • Observed a series of "fringes", especially when he passed the beam through a pair of very close fine slits (young's double slit experiment).
  • Observed that the spacing of the fringes depended on (ƛD/s)

His observations could not be explained by Newton's corpuscular theory.

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Electromagnetic waves- Fizeau's Experiment

  • To determine the speed of light. the distance from the toothed wheel and the mirror was 8.6km
  • He measured the speed of light in air using a rotating toothed wheel which had 220 teeth and 220 gaps. it span at such a speed that a pulse of light leaving through one gap returned to the wheel after relfecting off a mirror at the instant that the next tooth blocked its passage back through. 
  • the combined flashes produced a series of maximum and minimum intensity flashes as the speed of the toothed wheel altered slightly. 
  • They then conducted the experiment using interference effects to measure the speed of light in moving water and observed that Light travelled faster when emitted in the direction of flow rather than against- known as the Aether drift.
  • the value of light was found to be lower than the expected and was confirmed later by einstein's theory of relativity- evidence of Huygen's theory

C=4ndf where d is the distance from the toothed wheel to the mirror

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Electromagnetic waves- Maxwell's Experiment

  • In electromagnetic induction, an emf is induced in a conductor when placed in a changing magnetic field. (there is an electric field assciated with this induced emf)
  • In 1864, by James clark Maxwell
  • He suggested that Electromagnetic radiation propagates as a wave of oscillating electric and magnetic fields at right angles to each other.
  • Therefore the magnetic and electric fields are perpendicular to each other
  • The electric field will be created whenever the magnetic field changes (even in absence of a conductor)
  • Maxwell implied the opposite is ostensibly ture; a magnetic field could be created by changing electric fields. 
  • Predicted that an oscillating charge could produce this- a disturbance consisting of time-varying electric and magnetic fields propagating through space in the form of transverse waves. (Varying electric and Magentic fields are coherent)
  • there was a difference of less than 1% in values of the speed of light between Maxwell's and Fizeau's.
  • Maxwell's equations also predicted the existence of other forms of electromagnetic radiation. 
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Radiowaves- Hertz

  • In 1887
  • discovered radio waves. 
  • used an induction coil and a capactior to produce a high voltage that caused sparks, emitting radiowaves. 
  • He then used a wire loop to show that they had a magnetic field component as they induced an emf. The elctric field could then be detected using a dipole receiver. 
  • the radiowaves induced a p.d. across a loop of wire, causing a spark to cross the gap.
  • He noted that the radiowaves generated by the first spark were absorbed by the metal plate but propagated through non-metals. He was able to focus the radio waves back onto the loop  of wire by using a metal parabolic reflector, increasing the intensity of the sparks. 
  • using the loop, he meausred the distance between adjacent positions of maximum intensity and determined the wavelenght of the radio waves. using the frequency of the radiowave generating apparatus, he calculated the speed of the radio waves to the similar to the value predicted by Maxwell. 
  • His value showed that Light was an EM wave and that EM waves actually exist.
  • also used UV and infrared to confirm this
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Photoelectricity

  • CLassical wave theory predicted that the energy emitted by a black body would continue to increase as the wavelength decreased- contradicts the observations shown and is known as the Ultraviolent Catastrophe
  • Ultraviolent catastrophe- lack of emission of UV radiation at high temperatures, predicted by classical wave theory. First noticed by Hertz when he was investigating radiowaves. The sparks emitted were ostensibly stronger/more intense when UV radiation was used instead of light. Further investigations showed that for any metal:
  • Phtoemission will not occur if the frequency of light is below threshold
  • Start/stop
  • rate of photoemission is proportional to the intensity. 
  • The electrons which are emitted have a range of Ek which depend on the type of material and the frequency of the incident light.
  • Photon- a quantum of electromagnetic radiation. 
  • Photoelectirc effect- emission of photoelectrons by a metal surface when photons are incident on the metal surface than the work function of the metal surface.
  • The wave theory of light fails to explain threshold frequency and the instaneous emissions. the theory predicted lowering the frequency would slow done the electrons escaping. 
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Wave-particle duality

  • De Brogile hypothesised that, just like waves can demonstrate particle properties, matter can demonstrate wave properties. derived the equation ƛ=h/mv

Wave particle duality- term used to describe the behavious of quamtum scale objects havinf wave like and particle like properties, depending on the nature of the experiment used to observe them. 

  • the experimental discovery of the diffraction of electrons confirmed his hypothesis. A beam of electrons passing through a graphite grid will undergo diffraction, producing a characteristic pattern on the screen (concentric circles)
  • When the speed of the electrons was altered, the angles by which the waves were diffracted by changed. they realized that the angles increased when the electrons vellocity was lower. 
  • electrons were prduced by Thermionic emission, where the velocity depends on the anode potential, V. 
  • eV=1/2mv^2    increasing the voltage descreases the wavelength, which means the elctrons diffract less and the diameter of the rings is smaller. 

Therefore, due to a breakdown of classical wave theory, a new branch of physics began- the photon theory of light

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Electron Microscopes TEM

  • 1933 Ruska produced the first working electron microscope.
  • Electron microscope- device for imaging microscopic objects at resolutions greater than a light microscope by using the wave-particle duality of a beam of electrons. the wavelength of the electron "wavicles" is less than the wavelength of visible light.
  • TEM- transmission electron microscopes
  • a beam of electrons is prodcued by an electron gun and the beam is focused onto a thin target sample using magnets. the electron waves are scattered by the sample, and then refocused by two further magnetic "lenses", and a magnified image forms on a fluorescent screen or an electronic detecter. 
  • o.1nm smaller than an atom
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Electron Microscopes STM

  • STM- scanning tunnelling microscopes
  • use a very fine-tipped needle probe which moves across/scans a small surface area of a specimen. the needle tip is held close to the surface by a series of piezoelectric transducers.  At this short distance, electrons "tunnel" across the gap cuasing a tunneling current to flow- changes as the tip moves up & down, increases as it gets closer to the surface, and decreases as it moves away. 
  • piezoelectric transducer- small crystal of a material that changes its sharp when a p.d. is applied across it. the amount of deformation  of the crystal depends on the size of the p.d across it.

STMS are used in 

  • constant current mode- current is constant and the needle tip is moved up and down relative to the surface. distance moved by the tip is mapped by the machine
  • constant height mode- the distance is constant and the machine maps the tunneling current that is relative to the distance
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The Michelson- Morley experiment

Absolute motion- the movement of an object compared to a fixed frame of reference

  • White light from a source is sent 2 ways through the apparatus, tehn recombinied to produce an interference pattern
  • 1st direction; beam of light moves with and against the aether
  • 2nd direction; beam of light ,oves perpendicular to the aether
  • propostition was that if there was an aether, there would be a differnce in the speed of light in both directions, and that the beam traveling with/against the aether would be slower than the perpendicular one. 
  • the interference pattern should change as the whole apparatus is rotated horizontally (mirror 1&2 would change positions)

there was no noticeable changes in the interference pattern as the apparatus was rotated through 90 degrees- therefore the aether did not exist and the speed of light was constant.

this experiment requires the motion of the universe to be relative- there is no origin/fixed reference frame that motion can be compared to and this required a new theory for motion of objects moving through space- especially for objects travelling 50% faster than C

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Einstein's theory of special relativity

  • 1905
  • his theory refers to the motion of objects, notably at high speeds, moving at constant velocities relative to each other. 
  • his theory could be used to explain the increase in half-life of muons when they are travelling close to the speed of light compared to when they are static in the la

Inertial frame of reference: all objects within the frame are moving at the same velocity- they are all stationary relative to each other. He proposed 2 postulates that:

  • Physical laws have the same form in all inertial frames
  • the speed of light in free space is invariant
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