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Unit 5D Turning Points in Physics Chapter 1 The electron

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Part 1 The discovery of the electron Allesandro Volta invented the first battery in 1800, but the nature of electricity wasnt really understood It was found that, by making and breaking a circuit and using coil of wire, high voltages could be generated, producing sparks (1850s) But could electricity flow even without air?

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Gas Discharge Tubes Developments in glass blowing and vacuum pump technology were key to making progress Around 1857 Heinrich Geissler produced the first working gas discharge tube

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Gas Discharge Tubes It was found that, for low pressure gas, electricity was conducted and glowing regions appeared in the tube. This was attributed to rays coming from the electrodes, but there was much debate about their source and nature. Different gases in the tube produced different colours of light.

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Gas Discharge Tubes

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Soon decorative Geissler tubes were being sold as amusements, like todays plasma balls.

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Gas Discharge Tubes The details of the glowing regions also depended on the applied voltage and the pressure of the gas

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FF: What causes the glow? Near the cathode: The strong electric field ionizes some gas atoms. Nearby positive ions hit the cathode and release more of the free electrons Electron-ion pairs recombining release energy as emitted photons (negative glow) Near the anode: Some electrons get further and collide with atoms to excite them, then the atoms emit photons when they drop back to their ground states.

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Crookes Tube At lower gas pressures the glass itself glowed Moving the anode around the corner suggested the cathode was the source of the rays Casting of shadows suggested rays travelled in straight lines (Roentgen went on to discover x-rays using the same apparatus)

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A flurry of activity... William Crookes, Johann Hittorf, Juliusz Plcker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, J.J. Thomson and others investigated the nature of cathode rays William CrookesJ. J. ThomsonEugen Goldstein

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Key Experimental Findings A paddle wheel is pushed from to + when a voltage is applied. Shining light on it doesnt move it. The rays are deflected by magnetic and electric fields.

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Pulling it all together J.J. Thomson ended the debate in 1897. He demonstrated that cathode rays: 1.have energy, momentum and mass, 2.are not electromagnetic waves, 3.have a negative charge, 4.have the same properties even if you change the gas or the material of the cathode, 5.have a specific charge much higher than that of hydrogen atoms, 6.travel in straight lines. The electron was discovered.

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Thermionic Emission In discharge tubes electrons are produced by ionising gas atoms with an electric field A more efficient way of producing them is to use thermionic emission and accelerate them with an electric field in a vacuum. Used in Valves (Fleming, 1904) and enabled the birth of electronics and computers Still used in some esoteric applications (eg iTube amplifier and dock)

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Thermionic Emission Increasing temperature increases the electron energy So at higher temperature more electrons have enough energy to leave the surface of the metal Like a crowded fish pond! With some encouragement, electrons can leave permanently

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The Electron Gun

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Electrons are released through thermionic emission are accelerated towards the anode Some escape through a hole in the anode No field beyond the anode, so travel on with constant velocity Need a vacuum to avoid electrons colliding with other particles Filament current adjusts number of electrons (intensity of beam) Accelerating voltage adjusts speed of electrons (energy of beam) (usually inside of tube has a conductive coating to provide a return path and keep the tube itself neutral)

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Speed of Electrons...provided we are well away from the speed of light!

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Electron Deflection E field The faster the electrons are travelling, the less they are deflected

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Electron Deflection E field Constant vertical acceleration and x velocity, so path here is PARABOLIC (remember SUVAT) Constant vertical force of eV/d experienced by electrons between plates p p p p So maximum deflection Y is proprtional to V p Y

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Electron Deflection B field Helmholtz coils give a (calculable) uniform field over most of the volume in between the coils If we provide a magnetic field perpendicular to the e-beam we induce a circular path

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Electron Deflection B field Magnetic field produces a force always perpendicular to direction of electron travel Circular motion Fine beam tube contains a horizontally mounted e- gun and is filled with low pressure gas to show electron path The faster the electrons are travelling, the less they are deflected

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Specific Charge of the Electron (1) Balance opposite forces from E and B fields If e-beam is undeflected: This gives us the velocity. Now switch off B-field to get deflection y: All directly measurable quantities

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Specific Charge of the Electron (1) Alternatively: we still have Now switch off E-field to get circular motion, measure radius r: All directly measurable quantities You dont need to memorise this, but you do need to know which equations apply (on formula sheet) and be able to do similar calculations

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Specific Charge of the Electron (2) Deflect with a B field, calculate centripetal force (fine beam tube) All directly measurable quantities

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Specific Charge of the Electron (2) For a better value can measure values of r and B. Plotting r against 1/B gives a straight line with gradient k e/m is then calculated: Again, you dont need to memorise this, but should be prepared to answer questions on the experiment

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Significance of e/m e/m=1.76x10 11 Ckg -1 This was 1860 times larger than the Hydrogen ion, the largest known value so far. But is the charge much bigger, the mass much smaller, or a bit of both? In 1895 Thomson could not draw any further conclusions, as neither e or m were known independently.

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Millikans Oil Drop Experiment Very difficult to do! But allowed him to determine the charge on the electron in 1913. Robert Millikan

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Millikans Oil Drop Experiment Drops of oil fall through the hole in the top plate By measuring the size of the drop and the speed at which it falls you can calculate its mass Some become ionised by the radiation source By adjusting the pd between the two plates weight and electrostatic force can be equalised: the drop is stationary You can then calculate the charge on the drop

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Millikans Oil Drop Experiment For a stationary drop: So if we know m we can calculate Q Note that the top plate must have the opposite charge to the oil drop

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Finding the mass of an oil drop With no electric field, the oil drops fall at their terminal speed. Millikan could measure the speed of the drop with his microscope Stokes Law gives the drag force: 6 rv At terminal velocity: We can measure the density of the oil and write: Now we know r we can turn on the field and write The only unknown is Q... is viscosity of air

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Millikans Results After making many measurements on many charged oil droplets, he found that the charge Q was always an integer multiple of 1.6 10 19 C. In other words, he showed that electric charge is quantised in whole number multiples of 1.6 10 19 C. He concluded that the charge of the electron is 1.6 10 19 C the whole number n corresponds to how many electrons on the droplet are responsible for its charge. The mass of the electron was very small, so there must be other things in the atom providing the mass He was within 2% of the currently accepted value What might have been the sources of experimental error he had to deal with?

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Something to note You might be asked about the motion of drops with the field on, be careful: Tiny oil drops reach terminal velocity (up and down forces equal) in a fraction of a second They then move at constant velocity The velocity depends on the drag force, weight and electric force: =0 d) Electric force equal to weight, drop is stationary, so no drag force

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Checklist What are cathode rays and how were they discovered? Why does the gas in a discharge tube emit light of a certain colour? How is a beam of electrons produced in a vacuum tube? How can electron beams be controlled and deflected? What happens to the deflection of an electron beam if the speed of the electrons is increased? How can we determine the speed of the electrons in a beam? How can e/m be measured? What measurements are needed to determine e/m? What was the significance of the first accurate determination of e/m? How can e be measured? What measurements are needed to determine e? Why was Millikans determination of e important?

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Do all the Chapter 1 summary questions! Lets look at some exam questions www.Exampro.co.uk www.Exampro.co.uk