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The electron.  An indivisible quantity of charge that orbits the nucleus of the atom.

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Presentation on theme: "The electron.  An indivisible quantity of charge that orbits the nucleus of the atom."— Presentation transcript:

1 The electron

2  An indivisible quantity of charge that orbits the nucleus of the atom

3  The idea that there is a natural unit of electricity inside the atom was suggested in 1874 by Irishman G.J. Stoney.  He named the unit the ‘Electron’ in 1891  Subatomic Particles make up the atom.


5  Orbits the nucleus  Very small mass  Negatively charged  The charge on the electron is the smallest amount of charge found in nature

6  You don’t have to know the actual charge (1.6 × 10 -19 C), but you do need to know that the man responsible for first measuring this charge was Robert Millikan.  Discovering the charge of the electron Discovering the charge of the electron

7  Dr. Quantum Dr. Quantum

8  Because an electron has mass it has kinetic energy when moving  The amount of energy depends on how fast it moves.

9  An electronʼs potential energy can be converted to kinetic energy  E = ½ mv 2 (Kinetic energy)  E=qV or for an electron E=eV (q = charge, e=charge of an electron, V= voltage) Therefore  eV= ½ mv 2

10  The Electron Volt (eV) is the energy lost or gained by an electron when it accelerates through a potential difference of one volt.  From the formula E = eV, we can calculate that the energy lost or gained is (1.6 × 10 –19 ) multiplied by (1), or 1.6 x 10 –19 Joules, Therefore  1eV =1.6 x 10 –19 Joules

11  What is 5.6 MeV in joules?  Energy (joules) = Energy (eV) x (1.6 × 10 –19 )  = (5.6 x 10 6 ) (1.6 × 10 –19 )  =8.98 x10 -13 J

12 Thermionic Emission  The emission of electrons from the surface of a hot metal  Metals are made up of atoms, when the metal is heated the electrons gain enough energy to leave the metal

13  The minimum energy required to remove the loosest electrons from the surface of the metal is called the work function of the metal Φ  For thermionic emission the metal usually needs to be heated to a temperature in excess of 800ºC

14  The main component in the oscilloscope is the Cathode Ray Tube  It consists of a glass evacuated tube, a thin filament, two electrodes and a fluorescent screen


16  1. A current is passed through the heating coil, causing it to heat the cathode which in turn causes electrons to be emitted (by thermionic emission).  2. Because of the high potential difference between the cathode and anode the electrons are accelerated across the tube towards the anode. As a result a current flows in the circuit.  3. Electrons which pass through the hole in the middle of the anode continue on until they hit the fluorescent screen.

17  4. The stream of electrons can be deflected by electric or magnetic fields, which are generated from the X and Y plates.  5. The voltage on the Y-plates is adjusted to make the cathode rays (or stream of electrons) move up or down.  6. The voltage on the X-plates is adjusted to make the cathode rays move left or right. The result is that the rays can move across the computer or television screen.

18  They travel from the cathode in straight lines.  They cause certain substances to fluoresce  They can be deflected in electric and magnetic fields  They can produce x-rays when they strike heavy metals

19  Old TV screens and computer monitors  Cathode ray oscilloscope  ECG  EEG

20  A beam of electrons moving at right angles to a magnetic field moves in a circle  Force on a charged particle moving in a magnetic field  F = q v B (F=force, q=charge, v=velocity and B=magnetic flux density)  Also:  Where m= mass, v=velocity, r=radius, F= centripetal force




24  X-rays are electromagnetic radiation of extremely high frequencies. They are produced when high energy electrons collide with a metal target (tungsten).  The x-ray tube consists of a cathode heated by a filament, a vacuum, a tungsten target, a high anode voltage which accelerates the electrons.


26  1. The low voltage supplies power to a filament which in turn heats the cathode.  2. Electrons are emitted from the hot cathode due to Thermionic Emission.  3. They get accelerated across the vacuum due to the very high voltage and smash into the high-density anode (usually tungsten).  4. Most of the kinetic energy gets converted to heat, which must be removed with a coolant.

27  5. Some inner electrons in the tungsten get bumped up to a high orbital, then quickly fall back down to a lower level, emitting X-rays in the process.  6. These X-rays are emitted in all directions.  7. Most of these get absorbed by the lead shielding, but some exit through a narrow window, where they are then used for the required purpose.

28  They are Electromagnetic Waves  They cause ionisation of atoms  They have high penetration powers

29 Uses of X-rays  Medicine: To detect broken bones  Industry: To detect breaks in industrial pipes Hazards  They can ionise atoms in the body, causing them to become abnormal, which can lead to cancer.

30  The photoelectric effect is the emission of electrons from the surface of a metal due to electromagnetic radiation of suitable frequency hitting the metal.  This is the opposite of X-rays. In x-rays light is emitted when electrons hit the metal target. In the photoelectric effect electrons are emitted when light of a suitable frequency hits the metal target.


32  Light must be considered as a bundle of energy called a photon. The energy of the photon is proportional to the frequency of the light (electromagnetic wave)

33  E = energy of the photon,  h = plankʼs constant,  f = frequency of the EM radiation

34  The energy of the fastest electron emitted is the difference between the energy of the photon and the work function of the metal.  Work Function φ = h f o where f o is the threshold frequency.

35 Procedure:  Charge the electroscope negatively.  Shine ultraviolet light on the zinc plate. Result:  The leaves fall together Observation:  Shining UV light on the zinc plate liberates electrons from the zinc and therefore the legs become neutralised and fall back together.


37  Operation  Light of a suitable frequency shines on the photocathode  This releases electrons (by the Photoelectric Effect).  The electrons are attracted to the anode and from there they flow around the circuit, where they can be detected by a galvanometer, or alternatively they can be used to activate an electronic device.

38 Photo current is directly proportional to the intensity of the light

39  Burglar alarms  Automatic doors  Control of burners in central heating  Sound track in films









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